'89-'96 Progress: Section 1: Core Area Research

1. Core Area Research

A. Disturbances and Human Use

Work in this area is extensive. We divide this section into natural disturbances that create pattern in structure and influence processes in shortgrass steppe, and forces through past and present human landuse practices in this region that represent either exogenous or endogenous disturbances. Landuse in this region is currently 60% dryland wheat and 40% native rangeland or abandoned cropland grazed by cattle. Livestock production is a crucial part of the economic and social systems of the Great Plains, but has recently come under close scrutiny as an influence on ecosystems and their inherent biodiversity. Conversion to crop agriculture may be particularly unstable in this semiarid environment, as evidenced by the large abandonment of land during the 'dust bowl' period. Past and potential future shifts in landuse are perhaps one of the most critical factors affecting the integrity of the shortgrass steppe ecosystem.

1. Small Natural Disturbances

Recovery of shortgrass vegetation after smallscale disturbances:
Most studies of vegetation recovery in shortgrassdominated systems have focused on largescale disturbances, and in particular the cultivation and abandonment of agricultural fields. Small, patchproducing disturbances (ie., fecal pats of cattle, nest sites of western harvester ants, and burrows from small animals) are also important, but they have largely been ignored and have the largest potential effect on community structure. Our objective is to evaluate recovery of vegetation on naturallyoccurring and artificiallyproduced disturbances of different type, seasonality, size, and location by soil texture.

After one year of recovery, species composition on nest sites and animal burrows were similar to each other, yet different from artificially produced plots. High density and cover of perennials on nests and burrows indicates that perennial organs were not necessarily killed by the clipping activities of ants or burrowing activities of animals. Most cover on artificiallyproduced plots was annuals. One of the most important results from this study is the ability of Bouteloua gracilis to recover from seed on small disturbances. Previous studies had reported that this species can not recover after disturbance. Results from this field study as well as the old field study described below and our modeling exercises indicate that B. gracilis can recover after disturbance. Furthermore, recovery rates are dependent upon the characteristics of the disturbance, and in particular size and soil texture.

References: (Coffin and Lauenroth 1989)

Longterm recovery patterns of vegetation after patchy disturbances (1977 to present):
White grubs, including the larvae of June beetles (Phyllophaga fimbripes), are among the most destructive soildwelling insects, and have severely damaged large areas of grassland in North America. Our objective is to evaluate effects of white grubs on shortgrass steppe communities by determining the characteristics of patches of vegetation affected by grubs, and monitoring plant recovery on these patches through time. We are also evaluating the importance of spatial heterogeneity in survival of Bouteloua gracilis, and effects of grazing by cattle on plant recovery patterns through time.

At the start of the study, patches in grazed pastures had more complete mortality of B. gracilis than patches in ungrazed exclosures. This difference likely resulted from indirect effects of grazing by cattle on white grubs rather than direct effects of grazing on plants. Successional dynamics of functional type composition was similar on patches affected by white grubs and areas affected by other types of disturbances. Annuals dominated initially followed by shortlived perennial forbs and grasses, and finally dominance by longlived perennial shortgasses, and in particular B. gracilis. The rate of recovery was faster for areas affected by white grubs than for disturbance types of similar size, yet greater intensity. Spatial heterogeneity in survival of B. gracilis was only important to the recovery of this species. Linear relationships were found between spatial heterogeneity in survival of B. gracilis and cover of this species in each year of sampling. High r2 values for ungrazed patches throughout the sampling period indicate the importance of initial conditions to recovery of B. gracilis for as many as 13 years after the start of recovery. Low r2values after 1979 for grazed patches indicate the increasing importance of grazing and decreasing importance of initial conditions to recovery of B. gracilis on these patches. The infrequent importance of grazing to the recovery of other functional types is similar to the effects of grazing on other structural and functional aspects of shortgrass steppe ecosystem, and reflects the long evolutionary history of grazing by large herbivores in these systems.

The persistence and stability of shortgrass ecosystems, in spite of disturbances such as white grubs, is determined at least in part by the characteristics of the disturbance interacting with the ability of plants to respond, and in part by the evolutionary history of the system. Management of shortgrass ecosystems must account for aspects of the current systems as well as past history.

References: Coffin et. al. submitted

The effects of cattle fecal pats on plant mortality and recovery:
We also evaluated the effects of cattle fecal pats, the most frequentlyoccurring small disturbances of sufficient size (0.10.3 mdiameter) to kill B. gracilis plants, on plant mortality and recovery. The time required for pats to decompose indicates the cumulative effect of pats through time; both in terms of the probability of plant mortality and the length of time the area is disturbed before plant recovery can begin. Two years were required for most (90%) pats to decompose; slow decomposition occurred after that time. Plant recovery by B. gracilis on areas killed by pats occurs within two years; this species dominated plant cover on disturbed areas within three years for both types of plant communities where the study was conducted. Buchloe dactyloides (buffalograss) was also an important perennial grass to recover within two years after the disturbances occurred.

References:

Response of individual B. gracilis plants to small disturbances:
In 1991 we initiated a field study to evaluate the response of individual B.gracilis plants to small (0.1 to 0.3 mdiameter) disturbances. We selected six sites at the CPER to represent three soil textures (clay loam, silt loam, and sandy loam). At each site, a total of 100 B. gracilis plants were selected, half of which were protected from grazing by cattle and half were unprotected. Effects of small disturbances were simulated either by shading portions of each plant to represent cattle fecal pats or by removing above and belowground parts of each plant to represent digging and removal by small animals. Ten plants in each grazing treatment were randomly assigned to five mortality treatments: 0, 50, 75, 90, and 100% of each plant either shaded or removed. Treatments began in July (1991) when plant size and number of live tillers in the undisturbed part of each plant were recorded. Survival of each plant based on remaining number of live tillers was recorded in June and August (1992). We also established permanent plots at each site for demographic analyses. Plots will also be established at the sites representing the old and new grazing treatments. (see Figure 2.8)

References: Fair 1995 (thesis)

2. Human Use Grazing

Defoliation effects on plant morphology, aboveground biomass, tissueN, and phenology in longterm grazed and longterm protected pastures:
This study investigates how the morphology, biomass, and aboveground nitrogen dynamics of Pascopyrum smithii (western wheatgrass) and Bouteloua gracilis (blue grama) plants are affected by defoliation and grazing history. A field experiment was carried out in four grazing treatments (longand shortterm grazing and long and shortterm protection) during the 1992 and 1993 growing seasons. We conducted a parallel greenhouse study to examine whether morphological and chemical differentiation due to longterm grazing has occurred. We also compared phenological development between grazed and protected populations of functional groups of graminoids, forbs, and halfshrubs. Longterm protection has resulted in plants with taller tillers and longer leaf blades in both species. Defoliation enhanced tillering in western wheatgrass plants under moderate defoliation intensity (clipped at 6 cm height) in longterm grazed plants, and under severe defoliation (clipped at 3 cm height) in longterm protected plants. Tillering was enhanced in the greenhouse by defoliation only in protected populations. Defoliation also reduced tiller density of greenhousegrown blue grama plants in longterm grazed populations.

Although biomass of western wheatgrass and blue grama plants was reduced by defoliation in the field and in the greenhouse, aboveground tissue N concentration and N yield were increased. A similar inverse relationship was observed between biomass and N yield in the greenhouse. Some differences in chemical and morphological characteristics between shortterm grazing and shortterm protection (two years in both cases) were also observed. No biomass or tissueN differentiation was observed in both species as a result of longterm grazing. With little difference in graminoid phenology (mainly as a result of more vegetative growth of longterm grazed western wheatgrass and needle leaf sedge populations), there was no significant difference in growth within and between functional groups in grazed and protected populations. Some of our results are consistent with previous findings regarding plant morphology, biomass and tissue N dynamics response following defoliation. However, comparisons of morphology, biomass and N dynamics, and phenological development across grazing treatments and between tiller and plant organization has provided a broader view of defoliation and grazing history effects in the shortgrass steppe.

References:

Plant responses to defoliation and competition at two landscape positions:
Experiments were conducted in 1989 and 1990 to determine whether landscape position modifies the effects of defoliation and competition by Bouteloua gracilis(H.B.K.) Griffiths on established tillers of Pascopyrum smithii (ryd.) A. Love. During the first year, tiller survival, total growth in height, and number of green leaves per tiller were not significantly affected by topographic position. However, competition reduced those variables and defoliation reduced tiller survival and green leaves, but growth in height was increased almost five times by defoliation. Variables recorded in 1989 responded similarly to competition and defoliation between topographic positions. However, during 1990 with a drier growing season, individual tiller biomass, total growth in height, number of green leaves, total leaves produced, tiller survival, and density were greater on hillsides than in swales. Competition reduced all of these variables, but defoliation causes no response in aboveground biomass and total leaves produced, in spite of negative effects on tiller survival and, most of the time, negative effects on number of green leaves. This study suggests that the highly positive response to defoliation of growth in height and tiller N concentration partially contributed to the exact compensation of aboveground net primary production (ANPP) per tiller, and consequently almost doubled nitrogen yield. Furthermore, the responses of tiller survival, number of green leaves per tiller, and tiller density to competition were stronger in swales than on hillsides. Moreover, a significant second order interaction on the final sampling date on tiller survival and on total leaves produced indicated that the interactive effects of competition and defoliation did not vary in swales, but the negative effect of defoliation on those variables were exacerbated by competition.

Finally, the response of over winter variables suggest that competition and defoliation acted additively in reducing tiller density and the proportion of parent tillers producing daughter tillers, and that their effects are similar between topographic positions. Furthermore, a negative effect of defoliation on height of tillers growing the next spring suggests that P. smithii would be at a disadvantage in competition with surrounding vegetation dominated by B. gracilis.

References: Ibarra Gil 1992 (dissertation)

Longterm grazing in the shortgrass steppe: leaf photosynthetic characteristics and water relations:
Growing season gas exchange patterns were measured in two shortgrass steppe species growing on upland and lowland sites that were either protected or subjected to 57 years of heavy grazing. On average, net photosynthesis (A) and stomatal conductance (Gst) in Elymus smithii, a C3 grass, remained, respectively, 29% and 40% lower in plants growing on both upland and lowland ungrazed sites relative to grazed sites. Diurnal leaf water potentials were also lower, although infrequently, in plants from ungrazed sites. Conversely, A and Gst measured in Bouteloua gracilis, a C4 shortstatured grass, were not changed by grazing history or by topographic position. A clipping experiment with E. smithii revealed the sensitivity of this species to shortterm defoliation events as well, with A in clipped plants over 14% higher than unclipped plants during the week following clipping. Leaf age was also shown to be an important factor influencing plant carbon gain with 35% higher A in upper canopy leaves of E. smithii relative to lower canopy leaves and 35% higher A in leaf bases relative to leaf tips. These results have important implications for net carbon gain in plants subject to various levels and durations of herbivory. Moreover, the positive response of E. smithii to short and longterm defoliation may partially explain the persistence of this coolseason grass in shortgrass steppe dominated by B. gracilis.

References: Fahnestock and Detling

The role of Opuntia polycantha in providing a refuge for plant species under longterm heavy grazing:
We evaluated the role of Opuntia polyacantha in providing a refuge for plant species under longterm heavy grazing. In previous work, we found that heavy grazing resulted in a decrease in species richness compared to ungrazed areas. This summer, we tested the hypothesis that there is a greater species richness inside patches dominated by O. polyacantha that outside, in grazed pastures, due to the protection from grazing afforded by the spines. Our results supported the hypothesis. We found greater species richness in the patches, suggesting that O. polyacantha is important for sustaining plant species richness under grazing. (see
We evaluated the role of Opuntia polyacantha in providing a refuge for plant species under longterm heavy grazing. In previous work, we found that heavy grazing resulted in a decrease in species richness compared to ungrazed areas. This summer, we tested the hypothesis that there is a greater species richness inside patches dominated by O. polyacantha that outside, in grazed pastures, due to the protection from grazing afforded by the spines. Our results supported the hypothesis. We found greater species richness in the patches, suggesting that O. polyacantha is important for sustaining plant species richness under grazing. (see Figure 2.12)

References:

Effects of grazing, topography, and precipitation on the structure of plant communities:
Structural aspects of the shortgrass steppe plant community, functional groups,and species populations were examined in response to longterm heavy grazing and exclosure from grazing, contiguous wet or dry years, and an environmental gradient of topography. Of the three factors, relatively greater differences in community similarity were observed between catena positions, particularly on the ungrazed treatments. Grazing was intermediate between catena position and shortterm weather in shaping plant community structure. Grazed treatments and ridgetops had a less variable species composition through fluctuations in weather.

An increase with grazing of the dominant, heavily grazed species was observed. Basal cover and density of total species was also greater on grazed sites. The more uniform "grazing lawn" structure of the grazed plant communities had an influence on segregation of plant populations along topographical gradients. Segregation was less on grazed catenas, but diversity and the abundance of introduced and opportunisticcolonizer species was also less.

Although the shortgrass steppe community was relatively invariant, less abundant species were dynamic and interactions occurred with respect to grazing, weather, and catena position. The effects of grazing may be mitigated by favorable growing seasons but magnified in unfavorable years in populations that are adapted to favorable sites. Grazing can be considered a disturbance at the level of the individual but it may or may not be a disturbance at the level of the population, and it is not a disturbance at the level of the community in this particular grassland. (see Figure 2.6)

References: Milchunas et al. 1989

Community structure relationships across a perturbation gradient encompassing different types of disturbances:
We constructed a perturbation (response) gradient that encompassed several types of disturbances (causes) affecting shortgrass steppe communities and that was confounded by time scales of initiation and duration. The objectives were to (1) examine the type of disturbance and the relative magnitude of response in relation to the history of the shortgrass steppe, and (2) determine which attributes of the plant communities displayed relationships across the perturbation gradient, rather than across traditional successionaltime gradients.

Comparisons of disturbance types indicated that longterm heavy grazing by cattle resulted in plant communities differing less from communities resulting from other disturbances than were longterm ungrazed sites. The removal of grazers from this system promoted attributes of earlier seral stages. Water addition, belowground grazing by white grubs, and waterplusnitrogen enrichment had distinct and large impacts on community composition. Nitrogen enrichment resulted in additions but not losses of species. The shortgrass steppe has a high degree of adaptation to both shortterm drought and aboveground grazing. Belowground grazing and semiaridity may be considered antagonistic pressures on the community.

Community attributes that displayed a relationship with increasing level of perturbation were decreasing dominance and increasing diversity, and fluctuation in species composition during shortterm wet/dry cycles. The level of perturbation was related to negative impacts on the two primary species and a corresponding increase of other warmseason species, but was not related to densities of any other species, lifeform, or functional group. (see Figure 1.2)

References: Milchunas et al. 1990

Threedimensional distribution of plant biomass in relation to grazing and topography:
The horizontal and vertical distribution of plant biomass was examined on shortgrass steppe communities that were heavily grazed or protected from grazing for fortyseven years. Uplands and swales were sampled along the gently rolling topography. Threedimensional distributions of plant biomass were generated by direct sidebyside coring whereby 0.5m X 0.5m X 20cm deep volumes were completely sampled.

Longterm grazing had no effect on total biomass of surface crowns and only small effects on total biomass of roots at 010 cm and 1020 cm depths. The effect of grazing on the vertical distribution of crown and root biomass was also smaller than the difference between topographical positions. In contrast, grazing had a large influence on the horizontal distributions of all vertical components of the plant community by producing smoother more uniform horizontal distributions. This was most evident for the more heavily grazed swale communities. The grazinglawn concept was extended to the belowground plant community and discussed in terms of possible herbivore mediated plantplant interactions rather than as an aboveground grazing avoidance mechanism.

References: Milchunas and Lauenroth 1989

Abiotic and biotic control, and direct and indirect effects of large herbivores on demography of opportunistic species:
The initial emergence and subsequent survival and growth of five opportunistic 'weeds' after seed addition was examined in relation to indirect effects of longterm grazing treatments (heavily grazed vs protected), direct effects of currentyear defoliation, and removal treatments designed to eliminate plant competition while either leaving vegetation and soil structure unaltered or disturbed. The treatments were applied on both upland and lowland topographic positions to assess the relative influence of macroenvironment versus plant competition.

The indirect effects of large herbivores on the initial emergence of seedlings were so great that they prevented the potential for direct effects of the grazers to manifest to any large extent. Very few individuals emerged on the longterm grazed treatments that were either grazed or ungrazed during the current experiment. Numbers of individuals emerging on the longterm protected treatments were greater or equal to those emerging on the no competitionundisturbed treatments, but numbers were greatest on nocompetition disturbed treatments. The microenvironment amongst a livingplant canopy may in some cases increase emergence, but soil disturbance is of greater importance.

None of the seeded individuals on the longterm grazed, currently grazed treatments survived to the end of the growing season. There was a slightly greater endofseason biomass of seeded species and percentage of the total population reaching reproductive status on the longterm ungrazed compared with grazed nondefoliated treatments, and very high survival, biomass, and proportions of reproductives on both nocompetition treatments.

Equal compensation to currentyear herbivory occurred on longterm heavily grazed treatments even though aboveground production, and soil carbon and nitrogen, was much greater on longterm protected sites. Productivity and soil nutrients varied with topography, but very few topographical main effects or interactions occurred with demographic variables of seeded species, suggesting that macroenvironmental effects were of minor impotence compared with grazing and plant competition. (see Figure 1.18, Figure gz1.a)

References: Milchunas et al. 1992

Production and rain use efficiency in shortgrass steppe: grazing history, defoliation, and precipitation:
Grassland, subjected to fifty years of heavy, light, and no grazing intensity was clipped to simulate the natural pattern and intensities of defoliation by cattle or not clipped to simulate no grazing. A level of water resource treatment was superimposed upon the grazing and clipping treatments. Half of the plots were supplemented with additional water to simulate a wet year and half were not supplemented in a year of average precipitation. All three treatments interactively determined aboveground production. Water treatment had the largest overall effect on aboveground production. Currentyear defoliation had no direct significant effect on production, but mediated differences between both longterm grazing and watering treatments. Longterm ungrazed compared to grazed grassland was capable of responding to high amounts of precipitation, but was also most affected by low amounts of precipitation and, therefore, displayed greater variability in aboveground production and rain use efficiency. Only in the year of average precipitation, defoliation increased rain use efficiency in longterm lightly, but not heavily, grazed treatment. This suggests a water conservation mechanism of defoliation that is diminished with heavy grazing. (see Figure 2-8b)

References: Varnamkhasti 1991, Varnamkhasti et al. 1995, Milchunas et al. 1995

Forage quality in relation to longterm grazing history, currentyear defoliation, and water resource:
Forage nitrogen concentrations, nitrogen yields, and invitrodigestibilities were assessed in shortgrass steppe that had been ungrazed, lightly, or heavily grazed for 50 years. Caged plots were not defoliated or defoliated based upon removals observed in naturallygrazed reference plots. This was done in a year of average precipitation and with a supplemental water treatment to simulate a wet year. In general, currentyear defoliation had positive effects, and longterm grazing and supplemental water had negative effects, on forage nitrogen concentrations and digestibilities. However, defoliation interacted with longterm grazing in determining forage nitrogen concentrations, and with grazing and with watering in determining digestibilities. Nitrogen concentration and digestibility increased with defoliation in lightly, but not in heavily, grazed treatments. The dilution effect of supplemental water on digestibilities through increased plant growth was offset by defoliation. The negative effects of longterm grazing on forage quality were small, equally or more than compensated for by defoliation in a year of average precipitation, but more pronounced in the simulated wet year.

Nitrogen yields and digestible forage production were usually increased by defoliation, but this depended upon grazing and watering treatments. Increased nitrogen and digestible forage yields and concentrations in response to defoliation were greater than the biomass response in lightly grazed grassland. For both nitrogen and digestibility, yields were greater in grazed than ungrazed treatments in the year of average precipitation, but less in the simulated wet year. Optimizing quantity and yeartoyear stability of nitrogen and digestible forage yield may best be achieved with light grazing rather than no or heavy grazing.

Clipping was conducted in a manner closely resembling the natural pattern and intensity of defoliation by the cattle, and confirm the potential for a positive feedback of increased forage quality with defoliation observed in pot experiments. Longterm heavy grazing can diminish this response. Quantity (ANPP), quantity of quality (digestible and N yields), and quality (concentrations) do not necessarily respond similarly in interactions between currentyear defoliation, longterm grazing history, and level of water resource. (see Figure gz2a)

References: Milchunas et al. 1995

Aboveground primary production across fifty years of grazing intensity treatments:
Estimates of forage production for longterm ungrazed, lightly, moderately, and heavily grazed treatments (0, 20, 40, 60 % removal of annual forage production) established in 1939 in shortgrass steppe communities were subjected to multiple regression analyses to assess longterm temporal trends resulting from grazing and shortterm sensitivities to abiotic factors. Average production based upon all data from 19391990 was 75, 71, 68, and 57 g m2 yr1 for ungrazed, lightly, moderately,and heavily grazed treatments, respectively. Variability in forage production was explained mostly by coolseason precipitation, and magnitude of forage production was more sensitive to annual fluctuations in precipitation than to longterm grazing treatments. Production per unit increase of precipitation was greater for coolseason than warmseason precipitation, but only when coolseason precipitation was above average. This was attributed to differences in evaporative demand of the atmosphere resulting in different utilization efficiencies of small and large rainfall events in the two seasons. Based upon a regression model constructed using data from 1939through 1962, forage production was not affected by grazing at 20 to 35 % removal. For pastures of average relative productivity, grazing at 60 % level of consumption for 25 yrs resulted in a 3 % decrease in forage production in wet years and a 12 % decrease in dry years. Estimates of productivity after 50 years of heavy compared to light grazing treatment were 5 and 18 % for wet and average years of precipitation, respectively. (see Estimates of forage production for longterm ungrazed, lightly, moderately, and heavily grazed treatments (0, 20, 40, 60 % removal of annual forage production) established in 1939 in shortgrass steppe communities were subjected to multiple regression analyses to assess longterm temporal trends resulting from grazing and shortterm sensitivities to abiotic factors. Average production based upon all data from 19391990 was 75, 71, 68, and 57 g m2 yr1 for ungrazed, lightly, moderately,and heavily grazed treatments, respectively. Variability in forage production was explained mostly by coolseason precipitation, and magnitude of forage production was more sensitive to annual fluctuations in precipitation than to longterm grazing treatments. Production per unit increase of precipitation was greater for coolseason than warmseason precipitation, but only when coolseason precipitation was above average. This was attributed to differences in evaporative demand of the atmosphere resulting in different utilization efficiencies of small and large rainfall events in the two seasons. Based upon a regression model constructed using data from 1939through 1962, forage production was not affected by grazing at 20 to 35 % removal. For pastures of average relative productivity, grazing at 60 % level of consumption for 25 yrs resulted in a 3 % decrease in forage production in wet years and a 12 % decrease in dry years. Estimates of productivity after 50 years of heavy compared to light grazing treatment were 5 and 18 % for wet and average years of precipitation, respectively. (see Figure 1.20, Figure 2.26, Figure gz1)

References: Milchunas et al. 1994

Impact of cattle grazing on nematode communities:
Nematode populations were 41% greater in underplant soils than in inter plant soils, with any differences between the four grazing treatments having yet to be detected. In this study, 119 taxa (109 genera, eight subfamilies, one family and one unidentified nematode taxa) in 45 families and nine orders were identified from five sites across the CPER. Nematode communities were dominated by three genera, Acrobeles, Tylenchorhynchus and Helicotylenchus, which represented 51% of thepopulation. Another 16 taxa occupied 34% of the population, 37 taxa comprised 13%, and 63 other taxa 2%. Based on percentages, bacterial feeders were a greater proportion of communities in long term ungrazed (UU) than in long term grazed soils (GG), whereas plant parasites were a greater proportion of the community in long term grazed treatments. There were more omnivores in shortterm grazed (GU) than in short or longterm ungrazed treatments (UG and UU), and more fungal feeders in interplant (I) than in underplant (P) soil. Indices of diversity, evenness, richness, and dominance indicated no differences in nematode communities in the four grazing treatments (GG, GU, UG and UU). However, principle component analysis (PCA) using relative abundance data grouped the grazed treatments (GGP, GUP, GGI and GUI) separately from the ungrazed treatments (UUP, UUI, UGI, and UGP). Trophic group data in PCA clustered four grazed treatments over the zero level of second component, and four ungrazed ones below the level, and distinguished the interplant and underplant treatments.

References: Haung and Freckman 1995

Livestock grazing: consumer and plant biodiversity and the relationship to ecosystem function:
We synthesized published and unpublished data from longterm grazing treatments in the shortgrass steppe on the diversity and abundance of plants, lagomorphs, rodents, birds, aboveground and belowground macroarthropods, microarthropods, and nematodes. The relatively invariant nature of the shortgrass steppe plant community in response to grazing provides an opportunity to address some broad questions concerning relationships between responses of various structural and functional aspects of systems in general. Are there consistencies in diversity and abundance responses to grazing between groups of organisms? Are some groups more sensitive than others, or do responses mirror that of vegetation? Are the responses in terms of biodiversity related to functional responses?

Responses to longterm grazing intensity treatments in term of diversity, abundance, dominance, and dissimilarity were highly variable across classes of organisms. Some groups of consumers displayed large differences between grazing treatments even though differences in plant community attributes were relatively minor. Some responses were large even when comparing ungrazed to lightly grazed, or lightly to moderately grazed treatments. Birds appeared to be a particularly discriminating group to the grazing intensity treatments. Differences among grazing treatments in richness of groups other than plants and birds were relatively minor, especially when compared to large declines in abundance of some groups with increasing grazing intensity. For the wellstudied groups (plants and birds), shifts in species in terms of 'quality' factors, such as exotic, endemic, rare, generally suggest that livestock grazing may be more similar to conditions this particular system was exposed to in recent evolutionary time than would be the removal of the exotic, domestic grazers that functionally serve as a surrogate to bison.

Trophic structure composition did not vary greatly across grazing treatments. Further, large effects of grazing on some consumer groups did not translate into similarly large effects on ecosystem processes such as primary production or soil nutrient pools or cycling rates. (see Figure 1.19, Figure 2.8a)

References: Milchunas et al. 1995

3. Human Use Cultivation

Soil Organic matter recovery in semiarid grasslands: implications for the Conservation Reserve Program:
Although the effects of cultivation on soil organic matter and nutrient supply capacity are well understood, relatively little work has been done on the longterm recovery of soils from cultivation. We sampled soils from 12 locations within the Pawnee National Grasslands of northeastern Colorado, each having native fields and fields that were historically cultivated but abandoned 50 years ago. We also sampled fields that had been cultivated for at least 50 years at 5 of these locations.

Our results demonstrated that soil organic matter, silt content, microbial biomass, potentially mineralizable N, and potentially respirable C were significantly lower on cultivated fields than on native fields. Both cultivated and abandoned fields also had significantly lower soil organic matter and silt contents than native fields. Abandoned fields, however, were not significantly different form native fields with respect to microbial biomass, potentially mineralizable N, or respirable C. In addition, we found that the characteristic smallscale heterogeneity of the shortgrass steppe associated with individuals of the dominant plant, Bouteloua gracilis, had recovered on abandoned fields. Soil beneath plant canopies had an average of 200g/m2 more C than betweenplant locations.

We suggest that 50 years is an adequate time for recovery of active soil organic matter and nutrient availability, but recovery of total soil organic matter pools is a much slower process. Plant population dynamics may play an important role in the recovery of shortgrass steppe ecosystems form disturbance, such that establishment of perennial grasses determines the rate of organic matter recovery. (see Figure 1.16)

Reference: Burke et al. 1995

Recovery of soil organic matter and nutrient availability on Conservation Reserve Program fields:
Large reductions in soil organic matter (SOM) due to cultivation have been widely investigated, but the short term (<10 year) dynamics of SOM recovery following cultivation are far less clear. In two experiments, we measured the recovery of SOM pools (coarse particulate organic matter (coarse POM), fine POM and total SOM) and nutrient availability (mineralizable C and N) on 6 year old Conservation Reserve Program (CRP) fields relative to baseline SOM levels in adjacent, conventionallytilled, wheatfallow fields. We also tested plant life form effects on SOM recovery by measuring microsite (<10 cm) SOM patterns, where we expected labile SOM to be larger under plants than between plants and to be larger under plants that produced more labile tissue.

In the first study, CRP fields seeded with rhizomatous grasses contained higher fieldscale (~ 20 ha) rates of C and N mineralization than wheatfallow fields, but none of the other SOM pools were altered. At the microsite scale, only coarse POM N was higher under grasses than in plant interspaces, indicating that the soil heterogeneity associated with native arid grasslands was only weakly restored on these fields.

In the second study, we tested CRP management and plant life form effects on SOM recovery using 3 treatments: wheatfallow, 20% legume / 80% grass (LL CRP) and 80% legume / 20% grass (HL CRP). The net impact of increased plant inputs and cessation of tillage generally increased pools on mineralizable and coarse POM C and N by factors of 24 relative to wheatfallow fields, but had negligible effects on fine POM and total SOM pools. Recovery of soil heterogeneity was accelerated under legumes, which produced more labile tissue than grasses, in that soils under legumes contained larger pools of coarse POM C and higher net N mineralization rates than soils under grasses or in plant interspaces.

Grasses growing in HL CRP soils, which contained the highest rates of potential net N mineralization,produced more labile tissue than the grasses in the more nutrient depleted LL CRP soils, suggesting that plant/soil feedbacks were important. Recovery of labile soil and plant N was thus enhanced when the proportion of legumes was high, and this may lead to improved grain or animal N nutrition if these CRP fields are subsequently cropped or grazed.

References: Robles and Burke 1995, Robles 1995 (thesis), Robles and Burke 1995a (submitted), Robles and Burke 1995b (submitted)

Patterns of N availability among dominant land uses in the shortgrass steppe:
We conducted a set of in situ incubations to evaluate patterns of N availability among dominant land uses in the shortgrass steppe region of Colorado and to assess recovery of soil fertility in abandoned fields. Replicated 30 d incubations were performed in 3 sets of native (never cultivated), abandoned (cultivated through ca. 1937), and currently cultivated, fallow fields. Net N mineralization and the percentage of total N that was mineralized increased in the order: native, abandoned, cultivated. Higher soil water content in fallow fields is the most likely reason for greater mineralization in cultivated fields, while higher total organic C and C/N ratios in native and abandoned fields may explain differences between these land uses. Recovery of soil organic matter in abandoned fields appears to involve accumulation of soil C and N under perennial plants, but probable methodological artifacts complicate evaluation of the role of individual plants in recovery of N availability. Higher N mineralization and turnover in cultivated fields may make them more susceptible to N losses; recovery of N cycling in abandoned fields appears to involve a return to slower N turnover and tighter N cycling similar to native shortgrass steppe.

References:

Separation of the processes of cultivation that cause SOM loss:
Cultivation decreases soil organic matter (SOM) due to fewer plant residue inputs and greater outputs such as decomposition and erosion, but the relative effects of these processes are unknown. We designed a study to separate the effects of alteration of inputs and outputs on total and active SOM pools. We sampled four different SOM manipulations: high litter inputs (beneath live plants); low litter inputs (interspace); lack of litter inputs (antinduced bare area); and higherosion, highdecomposition, low litter inputs (wheatfallow agriculture)at two sites in and near the Pawnee National Grasslands. A cultivated system and a native system with 90% of all plant removal were simulated using the Century Ecosystem Model. Both active and total pools decreased in response to decreasing litter inputs, and the highest losses were found in cultivated treatments. Our study suggests that 1) depending on topographic position, erosion may both increase SOM inputs to and increase SOM outputs from a cultivated system, 2) plant absence in native areas confers comparable variability to landuse management practices, and 3) when comparing Century simulations to our field data, we found that Century overestimates the amount of loss in SOM due to increased erosion and decomposition and underestimates the amount of SOM loss due to reduced plant litter inputs. (see Figure SOM_loss)

References: Bisbee et. al 1995

Recovery of shortgrass vegetation on abandoned agricultural fields:
Plowing and subsequent abandonment of semiarid grasslands in the shortgrass steppe region of North America results in both short and longterm changes in plant community structure. The traditional Clementsian model of succession in which shortgrasses rapidly dominate the vegetation was modified for these grasslands in the 1970's to predict a prolonged stage characterized by the dominance by the bunchgrass, Aristida purpurea, followed by a very slow recovery of shortgrasses after largescale disturbances. Because neither the Clementsian nor the modified model was supported by results of recent scaledependent field experiments and simulation analyses, we designed a study to evaluate recovery of shortgrass communities on old fields abandoned for 53 years in the CPER and in the adjoining Pawnee National Grasslands of northeastern Colorado. Our objectives are: (1) to compare species composition on abandoned fields with adjacent, unplowed areas; (2) to compare vegetation on these fields with predictions from the prevailing conceptual models; and (3) to evaluate the relationship between recovery patterns within fields and distance from the source of propagules at the edge of a field. Dynamics of soil processes are also being studied on the same fields and are described elsewhere in this report.

For data collected in 1990, we reached different conclusions based upon the choice of indicator of recovery. For most fields (9 of 13), relative shortgrass cover did not fit predictions of either the Clementsian model or the modified model. High shortgrass cover on two of the remaining fields was similar to that expected by the Clementsian model, and low shortgrass cover on the remaining two fields was similar to that expected by the modified model. Two fields with high shortgrass cover were dominated by the less drought and grazingresistant species, Buchloe dactyloides,compared to Bouteloua gracilis, the dominant species in undisturbed communities. Uniformity in cover of other perennial graminoids and density of perennial forbs and annuals on and off fields indicated that these groups had recovered on most fields. However, differences in similarity in species composition on and off fields indicated that none of the fields had recovered. High variability in recovery of vegetation among fields with similar annual climatic variables and soil textures may be attributed to differences in initial conditions, management practices through time, finescale climate, and/or other site characteristics that were not measured in this study.

We found B gracilis on all fields sampled, and it dominated basal cover on two fields. Four groups of fields were distinguishable based on the relationship between B. gracilis cover and distance from the edge with unplowed vegetation: (1) fields with uniformly high cover of B. gracilis; (2) fields with a decrease in cover with distance,and cover dominated by B. gracilis; (3) fields with a decrease in cover with distance,and cover dominated by B dactyloides and (4) fields with uniformly low cover of B.gracilis and B. dactyloides, and dominated by other perennial graminoids indicating a mid to late successional stage had been reached.

Our results contrast with the conventional view of shortgrass community response to disturbances, and suggest an alternative view of the recovery process that focuses on interactions between individual plants and their environment to explain recovery patterns that vary in time or space. Accounting for this variability in recovery is critical to the management of these systems, especially under conditions of changing climate and land use.

Results from this study, and in particular the large variability in recovery rates and patterns, led us to expand our sampling of old fields at the CPER. Approximately 25% of the CPER was plowed and abandoned prior to 1937. These fields are of similar abandonment age, climate, and grazing regime since at least 1969, yet differ in soil texture and most likely in past management practice. In 1994, we sampled vegetation and soils for 6 of these fields. Our plan is to continue sampling old fields at the CPER until all fields have been sampled once. We will then repeat the sampling of all fields at a 5 year interval. In addition, we plan to resample the 13 fields in the PNG at approximately 5 to 10 year intervals. (see Figure 1.16, Figure 2.30, Figure 2.31)

References: Coffin et. al 1993, Burke et. al 1995, Ihori et. al 1995, Coffin et. al 1995 (in press)

Influence of plant presence and land management practices on nematode communities in shortgrass steppe:
Previous research in shortgrass steppe has shown that soil organic matter dynamics are extremely dependent upon both the spatial patterning of individual plants, and upon landuse management history. We conducted a study to evaluate the extent to which plant presence and landuse management control nematode community structure. We sampled soils in native shortgrass steppe, cultivated wheat fallow, and 8year old Conservation Reserve Program (CRP) land. We found that there were significantly more bacterial and fungalfeeding nematode under individual bunchgrasses than in the bare soil interspaces in both native and CRP fields. Total numbers of all nematodes were similar on CRP and native fields, demonstrating that 8 years was sufficient for recovery of these populations. Nematode populations were lowest on fallow wheat fields, and highest in currentyear wheat fields immediately following harvest. Our data suggest that nematode populations are highly responsive to litter inputs, resulting in strong spatial and seasonal patterning in shortgrass steppe.

References: Ortiz et. al 1995

Inertia in plant community structure: deflection after cessation of nutrient enrichment stress:
Water, nitrogen, and waterplusnitrogen at levels beyond the range normally experienced by shortgrass steppe communities were applied from 197175. Plant populations were sampled through 1977 and sampling was reestablished in 1982 to follow recovery. Although productivities increased, dissimilarities in plant species composition at the end of the five years of nutrient treatments were not significantly different from controls. Two years after cessation of the treatments exotic "weed" species were increasing in water plusnitrogen treated communities, and community dissimilarities were diverging in water and waterplusnitrogen treated communities. Seven years after cessation of treatments all communities were significantly different from controls. Exotics were more than ten times more abundant in waterplusnitrogen and nitrogen treated communities than they had been two years post treatment. A consistent trend in recovery of all treated communities was evident over the next five years. However, the trend towards recovery reversed over the next four consecutive years in the previously waterplusnitrogen and water treated communities. The fourtofive year cycles in species composition and abundance of exotics towards, and then away from conditions in undisturbed, control communities were not related to weather, but large accumulations of litter suggested biotic regulation.

Inertia of existing plant populations, or the tendency to continue to occupy a site when conditions become unfavorable, can mask future deterioration in ecosystem condition and unstable behavior resulting from environmental stressors. Timelags in initial response mean that an ecosystem can pass a threshold leading to transitions to alternate states before it is evident in structural characteristics such as specie composition. Global climate change and sulfur and nitrogen oxide pollutants also have the potential to act as enrichment-stressors with initial timelags and/or positive effects and cumulative, subsequent negative effects, rather than as disturbance forces with immediate negative impacts. Sociopolitical systems, however, often require change in biological variables or negative impacts before acting to ameliorate environmental problems. The manner in which conclusions changed at various periods in time, and the potential for timelags in responses of species populations raises questions about which variables are most useful for detection of stress and how long studies must last to be useful. (see Figure 1.17)

References: Milchunas and Lauenroth 1995

B. Biogeochemical Dynamics:

Our research in this area encompasses primary productivity , spatial patterns of carbon and nutrient cycling processes in unmanipulated grassland, responses to manipulations and landuse practices (see Disturbances/Human Use section), and fluxes of trace gasses. Because the inputs and movements of nutrients are closely tied to the inputs and fate of soil organic matter in semiarid regions, our approach is to deal with them together. We have focused on spatial heterogeneity and its causes at a range of scales, from individual plants to catenas, and physiographic units. We have also explicitly addressed the influence of recovery from disturbance on soil organic matter and nutrient dynamics.

1. Primary Production

The effects of increased temperature, water availability, and N availability on the relationship between ecosystem structure and function:
A prediction of our conceptual framework is that there are twoway interactions between the structure and function of shortgrass steppe ecosystems. Results from analyzing a longterm data set on aboveground net primary productions(ANPP) suggested that ecosystem structure constrained ANPP. Production was greater in dry years and less in wet years than expected by comparison with sites with mean annual precipitation corresponding to the wet and dry conditions. This leads us to the question: How does the relationship between ecosystem structure and function change under altered climate and resource availability?

We initiated a new longterm experiment that addresses questions about the effects of increased temperature, increased water availability, and increased N availability on the relationship between ecosystem structure and function. A portion of the experiment (temperature and water manipulations) is also being conducted in the Patagonian steppe in Argentina with our collaborator Dr. Osvaldo Sala. In each of two blocks, we have implemented 4 treatments that each cover 0.12 ha (1200 m2): irrigated, control, N fertilization, and N fertilization plus irrigation. In the control and irrigated treatments, we have installed 60 warming chambers, so that we can assess a total of 6 treatments (the prior 4 plus warmed, and warmed and irrigated). We are measuring the responses of vegetation structure (species composition and numbers of tillers), ANPP, and decomposition (leaf and root litterbags) to these treatments, which we plan to sample intensively for the next several years, and less intensively for 20 years or more. (see Figure 1.6)

References: Lauenroth and Sala 1992

Longterm monitoring of aboveground production:
We are continuing to monitor aboveground production on six sites selected to represent topographic positions and soil textures. Four sites have been sampled since 1983, and two since 1991. Production in the sandylowland site was consistently greater than a more loamylowland site, and the least productive site was the clay loamlowland. Grasses contributed nearly 100% of the production in the clayloam lowland, and shrubs and forbs were a relatively greater proportion of the production in ungrazed uplands compared to grazed uplands. In addition, nitrogen concentrations and yields are determined for all longterm ANPP sites. As a result of an experiment started in 1992 to study the effects of grazing and protection from grazing on shortgrass ecosystem structure and function, we are sampling net primary production in each of four treatments at five sites. These sites will be sampled as part of this longterm exclosure study, and will complement other data being collected at the same sites, as well as add to our longterm production data set. We are also continuing to evaluate belowground production using a radioisotope technique. (see Figures 1.5, NPP.a)

References: Milchunas and Lauenroth 1992, Zak et al. 1994, Singh et al. (1996)

Longterm 14C plots, belowground production, and root biomass dynamics:
Large areas of native shortgrass steppe were heavily labeled with 14C for the purpose of assessing the implications of short and longterm carbon dynamics on estimates of aboveground, crown, and root production using 14C dilution, 14C turnover, and traditional harvest methods. Stabilization of plant labile 14C via translocation, incorporation into structural tissue, and respiration and exudation required one growing season. Respiration was 73% of initial uptake, and exudation was 17% of plant 14C after stabilization of labile 14C. Turnover estimates for leaves, crowns, and roots by 14C turnover were 3, 5, and 8 years, yielding estimates of belowground production that were much lower than previously thought. Estimates of aboveground production by 14C turnover were close to those obtained by harvest of peakstanding crop, but lower than reported values obtained by harvest maximaminima. Estimates of root production by harvest maximaminima were zero in 2 of 4 years. 14C turnover appeared to provide reliable estimates of above-ground, crown, and root production, although they are an integration over many years. Annual estimates of production by 14C turnover are biased by the difference in decomposition during a particular year from the average decomposition rate over the complete turnover period.

Anomalous estimates of root production by 14C dilution were attributed to a nonuniform label resulting in differential decomposition of 14C:12C through time, as well as movement and loss of labile 14C through the first growing season. Based on 14C turnover, eight years of labeling would be required to uniformly label the rootmass with 14C. Isotopedilution methodologies may be unreliable for any estimate of pool turnover when the labeling period is not as long as poolturnover time. This does not similarly apply to isotope turnover methodologies when the labeled portion of the pool temporally progresses through all states, but assumes the proportion labeled within a defined pool, but not the quantity, at a pulse labeling is the same that would be labeled through all potential times of pulsing during the pool turnover time.

This spring was the 11th year of sampling on our longterm C14 plots. Several interesting developments have occurred since the initial publication of results from this experiment. First, the amount of C14 activity in aboveground, crown, and root tissue has held constant for several years, after a previous steady, linear decline. Second, all plant tissuetypes appear to be converging to similar activities. Third, live roots (only those obviously live) were separated from dead detrital material in the previous year's sample, and no difference in activity between the two were found. These results suggest some type of internal cycling of carbon in this system. We took separate samples of new, green leaves in 1995 in addition to the usual aboveground leaf plus litter samples in order to further assess these unusual phenomena.

A minirhizotron was obtained through an Agricultural Experiment Station equipment grant. This year we began installing tubes adjacent to the C14 plots and the rootharvest plots, where we have 11yrs of root biomass data collected through the growing season. Minirhizotron tubes will also be installed at our new sixsite, fourtreatment grazing experiment. (see Figures A minirhizotron was obtained through an Agricultural Experiment Station equipment grant. This year we began installing tubes adjacent to the C14 plots and the rootharvest plots, where we have 11yrs of root biomass data collected through the growing season. Minirhizotron tubes will also be installed at our new sixsite, fourtreatment grazing experiment. (see Figures 2.17, NPP.b, NPP.c)

Reference: Milchunas and Lauenroth 1992

Errors in estimating production:
Simulation modeling has demonstrated the potential for large errors of estimating production when using traditional methods. We developed an analytical solution to the problem of calculating production. Random errors associated with estimates of biomass used in calculation of net production always have the effect of overestimating net production. The overestimation was related to sampling effort in such a way that the more times biomass was estimated, the higher the overestimate. A method was also developed to correct net production values for the overestimation. This problem has been addressed by developing isotopeturnover methods of estimating ANPP, BNPP, and crown production.

References: Boindini et al. 1991, Milchunas and Lauenroth 1992

How can net primary productivity be measured in grazing ecosystems?:
The majority of studies that measure grassland aboveground production use peak biomass in yearlong temporary caged exclosures as estimates of production, and calculate consumption as the difference between standing crop in grazed plots. However, this method does not account for compensatory responses by grazed plants; production and consumption values may be systematically biased. We review four alternative methods: (1) animal metabolic models, (2) moveable exclosures, (3)clipping inside exclosures to simulate grazing, and (4) moving herbivores onto and off of pastures. Methods 23 are designed to account for compensatory responses; however, they too may be subject to errors and biases. Selecting an appropriate method to measure grassland production and consumption requires understanding the limitations of the available methods.

References: McNaughton et al. 1995

2. Spatial Heterogeneity

Spatial patterns of root biomass and plant cover:
We quantified spatial patterns of root biomass and plant cover in 10 late successional, shortgrass steppe communities in which a large proportion of soil is bare and regeneration is frequently limited by soil water. Our main objectives were to evaluate patterns of root density associated with previously documented variation in recruitment in canopy openings of different sizes and to estimate the abundance of openings with low root density. Root biomass in the top 30 cm of soil was much lower in openings of all sizes than under plants and declined steeply as opening size increased. Biomass of light roots presumed to be functional was 62, 33 and 4% as much as under plants in centers of 10, 20, and 60 cm openings, respectively. Openings more than 5 cm across made up 34% of the surface. Most were small: 86% of openings were <20 cm across, a size at which strong interference between established plants and seedlings has been demonstrated. Openings without signs of disturbance were 88% of the openings. Only 2% of openings, equivalent to 2% of the area, were more than 50 cm across, a size supporting enhanced regeneration and having low root density. Nearly all of these large openings were caused by disturbance. However, many openings caused by disturbance were 3050 cm across, a size range of transition form strong to weak interference, or smaller. Less than 0.5% was beyond B. gracilis root systems. We infer that most openings large enough to support enhanced recruitment are explored by roots of dominant bunchgrasses and that gap dynamics in shortgrass steppe involves constraints on water use in B.gracilis root systems. Because large openings are rare, variation in belowground competition in abundant, smaller openings may be important to regeneration.

References: Hook et al. 1994

Evaluating spatial heterogeneity in aboveground biomass:
Our remote sensing research activities are evaluating spatial heterogeneity in aboveground biomass using the CENTURY model for comparison. Our objectives were to 1) compare the spatial heterogeneity of remote sensing indices and models and CENTURY models of aboveground biomass estimates. 2) to test the spatial independence of modeled estimates, and 3) to determine if spatial information could improve model estimates. Multiple regression models of the tasseledcap soil brightness index used in conjunction with vegetation indices has improved site level biomass estimation relative to univariate regression models. Soil texture, precipitation, and temperature were used as driving variables for aboveground biomass estimates in the CENTURY model. Holding weather variables constant, soil texture drives the heterogeneity of CENTURY model estimates at this site. An appropriate textural and spatial resolution for model and remote sensing data comparison were determined from canonical discriminant analysis. Ten soil texture groups were formed from the original sixteen soil texture classes. We compared linear, quadratic, and cubic regression models of modeled biomass as a function of remote sensing soil brightness index (SBI), soil wetness index (SWI), and green vegetation index (GVI) as well as elevation, stream proximity, and slope for two dates. Similar patterns of remote sensing indices as a function for percent sand were associated with texture group means for both years. Results indicated a poor correlation between model estimates and remote sensing, elevation, and 1st and 2nd order, 3rd and 4th order, and 1st through 4th order stream proximity. The mean remote sensing biomass estimates for two soil groups with relatively low sand content were higher than CENTURY estimates. High negative residuals (TM estimates higher than CENTURY) were associated with some but not all stream drainages.

References: Todd et. al 1993

Soil heterogeneity following death of individual plants:
The shortgrass steppe of northern Colorado is characterized by patchy plant cover and associated spatial heterogeneity of soil resources. Plantassociated zones of relatively high soil organic matter (SOM) and nutrient availability are likely the result of direct organic inputs through root death and exudation, as well as wind induced redistribution of soil. We studied the duration of plantassociated enrichment following plant death in labile, intermediate, and total pools of SOM. We sampled plantassociated microsites 0, 1, 9, and 36 months following plant death, and compared these values to betweenplant microsites. Soils associated with live plants and dead plants of all ages were enriched in total C and N relative to bare microsites. Labile and intermediate pools of SOM, however, were not enriched relative to bare microsites 36 months after plant death. We found a general pattern following plant death that is characterized by an initial phase of increased SOM and nutrient availability due to greater litterfall than decomposition. Shortly thereafter, there is a decrease in SOM and nutrient availability when substrate supplies decline and decomposition continues. Though decomposition and nutrient release provide important resources to maintain plantassociated zones of enhanced SOM and nutrient availability for the first several months following plant death, our results suggest that enriched nutrient supply zones under dead plants do not persist beyond several months to provide resources to subsequentlycolonizing individuals. (see Figure 1.8)

References: Kelly and Burke 1995 (submitted), Kelly 1995 (thesis)

Smallscale spatial heterogeneity in soil nutrients associated with the presence of B. gracilis individuals:
During 1990, we conducted a study on the smallscale spatial heterogeneity in soil nutrients associated with the presence of B. gracilis individuals. Field and laboratory analysis suggest that 1) plantassociated C and N are distributed concomitantly with the presence of B. gracilis individuals, 2) total soil C and N are higher under individual B. gracilis plants than between, and 3) available and potentially mineralizable C and N are higher under than between individual B.gracilis plants. These results have a great deal of significance for our understanding of shortgrass steppe ecosystems because they suggest that semiarid grasslands are subject to the same kind of plantinduced heterogeneity that is often recognized as occurring in semiarid shrublands.

References: Hook

Soil organic matter and nutrient availability responses to reduced plant inputs: an assessment of turnover characteristics:
It is difficult to obtain an understanding of the temporal dynamics of soil organic matter (SOM) because of the multiple pools and processes that interact to determine SOM content. Many simulation models subdivide SOM into pools based on turnover characteristics. Common to most of these models is the separation of an active pool responsible for nutrient supply, an intermediate pool, and a pool of stable SOM with an extremely long turnover time. Almost all of these separations are theoretically based on kinetic characterization, not empirically based on measurable pools. Through a finer separation and better understanding of decay characteristics of conceptual pools, we may be able to improve our ability to predict ecosystem response to disturbance.

Many studies attribute levels of SOM to levels of root biomass, but few, if any, go beyond a theoretical relationship to a quantitative measurement of the connection between root biomass and SOM. By examining this relationship over time, we will better understand the temporal dynamics of SOM in the region. We designed an experiment to assess the effects of reduced plant inputs on SOM based upon naturallyoccurring zones of plant removal across a spatial gradient in root biomass and a temporal gradient of disturbance age. In addition, we compared our field estimates to simulation modeling results, cultivation studies, and theoretical concepts of SOM pool size and turnover.

We found a tight connection between root biomass and SOM, especially in the active pools of microbial biomass and mineralizable C and N. Over time, a reduction in plant inputs led to large initial decreases in active and total SOM, but loss rates leveled off following the initial period of approximately 10 years. Based upon our temporal measurements, we calculated turnover rates (kvalues) that were somewhat similar to theoretical abstractions of SOM turnover dynamics, except that we found passive SOM to be more tightly coupled to environmental changes than previously reported in the literature.

References: Kelly 1995 (thesis), Kelly et. al submitted

Influence of grazing on soil organic matter in U.S. shortgrass steppe:
Grazinginduced soil degradation has been cited as a major cause of ecological changes and reduced primary and secondary productivity in the world's dry regions. However, few controlled studies have been conducted to evaluate the effects of grazing on soil biogeochemistry. The shortgrass steppe of eastern Colorado provides an excellent location to evaluate the biological and physical effects of livestock grazing on soils. The composition of the dominant native bunchgrass community is not altered significantly by grazing , even under high levels of biomass removal.

We initiated an experiment to determine short(2y) and longterm (53y) effects of grazing on C and N dynamics at the Central Plains Experimental Range (CPER). Total soil C and N pools were unaffected by moderate grazing or exclosure following both 2 and 53 years of treatment. Particulate organic matter (POM) and microbial biomass were also unaffected; these pools represent recent belowground litter inputs and substrate available for decomposition. However, mineralizable C and N, representing the most active pools of soil organic matter, were significantly higher under longterm exclosure than longterm grazing, but only in bare soil areas between plants. Previous work reported increases in N mineralization in response to herbivory due to decreases in litter, decreases in immobilization potential, faster recycling of nutrients via feces and urine, higher ammonia volatilization, and increased soil temperatures. Our animal densities were lower than those reported upon due to lower primary productivity, thus, cycling rates are not as accelerated. Additionally, the basal cover of the dominant grass, B. gracilis, increases with grazing, ameliorating the thermal effects of reduced aboveground litter.

Bulk density was significantly higher in grazed treatments compared to the longterm ungrazed treatment (UU). Soil compaction is commonly cited as a factor that promotes runoff and, therefore, erosion on grazed lands, but soil C and N data suggest that erosion did not differ substantially among treatments. The small decreases in mineralizable C and N in bare areas may reflect slight erosion due to reduced canopy and litter cover or compaction by cattle.

Effects of grazing on soils were much smaller than localized differences imposed by individual plants. Individual bunchgrasses influenced all indices of soil organic matter strongly , indicating important effects of plants on organic matter accumulation and protection from erosion. This result is consistent with recent work on shortgrass steppe and other semiarid regions. Our results suggest that individual plants impose greater variation and are more important to system function than grazing management.

Grasslands are widely recognized for the large proportion of total carbon and nutrient capital that is stored in relatively recalcitrant organic matter pools. Carbon distribution and turnover to a 1 m depth at the CPER demonstrates that less than 1% of the C is present aboveground, and only 12% is present as roots. Previous work at the CPER indicated that grazing has a minor effect on roots, although work in other areas has shown grazing induced reductions in root biomass sufficient to increase net N mineralization. In our experiment, longterm removal of plant biomass by cattle did not reduce soil organic matter pools or processes, indicating that most organic matter originates from root litter. While our grazing intensities were not as high as may occur in heavily overgrazed areas, plant basal cover and rooting patterns in a pasture that has been grazed heavily for 53 years (50% higher forage removal than this study) provide no evidence that a threshold response in soils would occur under heavy grazing, short of denudation. (see Figure 1.21)

Reference: Burke et al. 1994, Burke et al. in prep

The role of Opuntia polycantha and Bouteloua gracilis on conserving SOM in grazed and ungrazed pastures:
We are measuring microtopographic variation as an estimate of net erosion rates, and sampling material that accumulates under these species to assess the relative roles of organic and mineral deposition. We plan to regularly sample, at decadal intervals, to assess the actual rates of accumulation of material. Thus far, we have analyzed data on net erosion. Our data demonstrate that 1) there is substantial accumulation of material under both B. gracilis and O. polycantha, with the highest accumulation under O. polycantha; 2) grazing increases the redistribution of materials from interspaces to under these plants; and 3) the highest levels of redistribution occur in summit and midslope positions. (see Figures 2.13 2.21)

References: Brannen

Long term 15N studies in a catena of the shortgrass steppe:
A set of long term 15N studies was initiated during the summers of 1981 and 1982 on the backslope and footslope, respectively, of a catena in the CPER. Microplots labeled with 15N urea were sampled for 15N and total N content in 1981and 1982 and again in 1992. In November, 1982, 100% of the added N was recovered in the soilplant system of the finertextured footslope, compared to 39% in the coarsertextured backslope microplots. Ten years later, 15N recovery of the applied N decreased at both topographic positions to 85% in the footslope and 29% in the backslope. Average losses since the time of application were 3.5g N m2yr1 in the back slope and 0.8g N m2yr1 in the footslope. In 1992, soil organic matter was physically fractionated into particulate (POM) and mineral associated (MAON) fractions and 21day mineralization incubations were conducted to assess the relative amounts of 15N that were in the slow, passive and active soil organic matter pools, respectively, of the two soils. Our findings confirm the assumptions that POM represents a large portion of the slow organic compartment and that the MAON represents a large fraction of the passive compartment defined in the Century model. The N located in the MAON had the lowest availability for plant uptake. Isotopic data were consistent with textural effects and with the Century model compartmentalization of soil organic N based on the residence time of the organic N.

References: Delgado et. al (in press)

Controls of soil carbon and nitrogen dynamics in semiarid grasslands:
Geomorphic processes, effects of individual plants, and seasonal variation in weather are thought to be important controls of soil carbon and nitrogen dynamics in semiarid grasslands, but their relative importance and interactions are not known. We quantified C and N pools and monitored seasonal patterns of N mineralization in 8 paired upland and lowland plots in shortgrass steppe. Plots represented diverse geomorphic settings, with lowlands ranging form minor swales to alluvial surfaces of ephemeral streams. Total C and N and mineralizable C in the top 15 cm of soil varied relatively little among upland sites but were 23 times greater in the 3 alluvial lowlands than on adjacent uplands; C and N in nonalluvial lowlands ranged from no greater to 1.8 X greater than adjacent uplands, with no clear pattern related to landscape position. Soil C and N pool were consistently greater under than between bunchgrass plants, and these differences were generally similar to or greater than topographic differences. Neither large nor small scale spatial patterns of N mineralization were as clear as C and N pool results. For most of the landscape studied, topographic variation in C and N appears to be less consistent than microscale heterogeneity related to patterns of plant cover, probably because runoff and associated material transport is limited. (see Figure 1.7, Figure 2.5)

References:

The influence of individual plants on soil nutrient dynamics in the central grassland region of the United States:
The extent to which plant community structure influences ecosystem nutrient cycling is an important but poorly understood element of ecosystem ecology. We studied the effects of two aspects of vegetation structure, plant cover patterns and plant species composition, on nutrient cycling in soils of shortgrass steppe, mid and tallgrass prairie, and desert grassland in the Great Plains. The general objective was to identify the importance of plant cover patterns and species composition, especially in the context of other environmental variables, to soil nutrient dynamics in these grasslands.

In the dry shortgrasssteppe and desert grasslands, plant cover patterns were very important in determining patterns of soil nutrient dynamics. Soils under plants had generally higher rates of carbon and nitrogen pool sizes and turnover rates than soils from adjacent bare ground areas between plants. Individual plant characteristics, such as lifespan and growth form, explained the degree of soil heterogeneity in some cases, with the most longlived, productive species fostering the most plantinterspace soil heterogeneity. Also, abiotic environmental variables explained patterns in plantinduced soil heterogeneity. The desert grassland with the largest proportion of bare ground, and thus possibly the most soil erosion, had the largest plantinterspace soil heterogeneity. The wet grasslands, the mid and tallgrass prairies, had more continuous plant cover; thus plant cover did not impose strong control over soil nutrient patterns in these ecosystems.

Plant litter quantity and quality of tissue for decomposers differed between species and grassland ecosystems and, in some cases, affected soil nutrient cycling. Kochia scoparia, an introduced species in shortgrass steppe, had high quality tissue(low carbon:nitrogen and lignin:nitrogen) and had relatively high rates of nitrogen and carbon mineralization in its soils. Precipitation affected plant tissue quality, with a general decrease in average quality and increase in interspecies variation in quality from dry to wet grasslands.

Vegetation structure, and its interaction with sitebased abiotic variables such as precipitation, had important effects on soil carbon and nitrogen dynamics in these grassland ecosystems. Results indicate that information about plant community structure may be critical to largescale estimates of ecosystem function. (see Figure 2.20c)

References: Vinton et al. submitted, Vinton 1994 (dissertation)

3. Gas Flux

Nitrogen biogeochemistry and N gas emissions in the shortgrass steppe:
Recent studies in grasslands have found that gaseous N losses may be a keystone process regulating biogeochemistry of these ecosystems. Results from our studies at the CPER and data from other research indicate that rates of Ngas loss from grasslands are similar to rates of atmospheric N inputs, suggesting that exports of N in gaseous form may well be the principal control on longterm grassland N balance and productivity. Along with its role in regulating primary productivity, the N cycle of terrestrial ecosystems interacts with atmospheric trace gas chemistry in important ways. Keys to understanding the dynamics and magnitudes of Ngas fluxes are the relative and absolute magnitude of processes and biological properties which regulate N turnover.

From the 24 sites where tracegas studies have been or are being conducted in the CPER five were selected with differing textures, landscape positions and management. Within these sites simulated rainfall intensity was varied and the effect of various rainfall episodes on nitric oxide (NO) and N2O, CH4 and CO2 fluxes were assessed. Concurrently, soils were analyzed for microbial biomass, net and gross N mineralization and immobilization estimates were made using 15N dilution and other soil parameters.

In the grassland sites, simulated light and heavy rain storms resulted in large and rapid responses in NO (10200 UG N m2 hr1) and N2O (150 UG N m2 hr1) flux rates. Maximum rates were obtained in 30 minutes to 4 hours after wetting. NO flux rates were typically >10 times higher than N2O flux rates in coursetextured soils but were lower in finertextured soils. The magnitude of the NO flux appeared to be related to the substrate availability and was well correlated with soil CO2 fluxes. The duration of the flux was more closely controlled by the soil water filled pore space (WFPS). The NO flux rate peaked at approximately 35% WFPS for coursetextured soils but were relatively lower and showed less marked response to WFPS in finer textured soils. N2O fluxes continued to increase with increased WFPS. NO and N2O fluxes are highly correlated following moderate size wetting events but are not correlated following large rain events. Microbial biomass values for soils collected in early June show highest correlation with the anaerobic N mineralization and NO fluxes.

References:

Soil atmosphere exchange of CH4, N2O and CO2: importance of yearround measurements:
Most tracegas budgets assume that tracegas exchange between the soil and the atmosphere stops when soil is snow covered or soil temperatures drop to about 0oC. In systems where soils are not typically snow covered, like the grasslands and cultivated fields of eastern CO and western NE, the soilatmosphere exchange of CH4,N2O and CO2 were measured yearround at 2030 locations using closed chamber techniques. In subalpine systems gas fluxes were measured by measuring gas concentration gradients within the snow and calculating the fluxes using diffusion equations.

In the subalpine systems, soils beneath the snow pack emit CO2 and N2O and take up atmospheric CH4 throughout the snowcovered period. Soil respiration under the snow represents oxidation of more than 25% of the estimated carbon fixed in the ecosystem during the growing season. About 30% of the annual CH4 consumption and 32% of the N2O emission in the subalpine system occurred during the snowcovered period. In the grasslands, Nov.Feb. fluxes of NO and N2O comprise 1050% of the annual mean flux. There are times during the winter when surface soils thaw and moisture content of the thawed surface is high when N2O fluxes are at the peak for the year at some sites. These data clearly show that gas fluxes that are controlled by soil microbial processes cannot be assumed to be negligible even when soil temperatures are near zero. Fluxes of N2O, CH4 and CO2 during the four months of Nov.Feb. comprised 1556% of these gas fluxes. Using the assumption that Nov.Feb. CH4 uptake rates were zero underestimate the mean annual CH4 uptake rate by 2530%. (see Figure 1.9)

References:

Soilatmosphere exchange of CH4, N2O and CO2: agricultural systems in northeastern Colorado:
The influence of agriculture and agricultural management practices on N2O emissions and CH4 consumption in soils are not well known. We measured fluxes in native grassland (NG), grassland that had been N fertilized from 1976 to 1989 (FG), grassland that was first plowed in 1992 (PG), wheatfallow fields (one placed in the conservation reserve program in 1985 (CRP), one planted to wheat in September, 1993 (WHT), and one planted to wheat in September, 1994 (FAL)), and an irrigated wheat field (where no fertilizer was added (IWC) and another where 90 kg N ha1urea N was applied (IWN))

We observed the following trends: (1) in NG there is a longterm effect of N addition and plowing on both N2O emission and CH4 uptake (N2O emissions increased 160% and CH4 uptake decreased 25%); (2) in CRP N2O fluxes remain 170% higher and CH4 uptake 30% lower than NG; (3) N2O emissions were 50% lower from the WHT than from the NG or the FAL while CH4 uptake rates averaged 35 and 44%,respectively; (4) N2O emissions in IWC and IWN were 304 and 945% that of the NG while CH4 uptake was 6 and 4% of the NG, respectively. Generally, as cropping intensity increased N2O flux increased and CH,sub>4 consumption decreased.

References:

Importance of aerobic soil consumption of CH4 on the global CH4 budget and relative effect on the global warming potential of trace gases in shortgrass steppe:
Considerations of global budgets of atmospheric methane have typically focused on production sources. Consumption of CH4, other than through hydroxyl radical reaction in the atmosphere, has been given little attention in spite of a few measurements which indicated that aerobic soils potentially served as sinks for atmospheric methane. Our research was initiated partially to determine the soil sink strength for atmospheric CH4 in a large variety of ecosystems. Longterm field campaigns for trace gas flux measurement were initiated two to four years ago in managed grasslands in AK and PR, in native and managed agricultural systems in CO, in subalpine forests and a managed grassland in WY, and in a long term tillage experiment in western NE. Weekly or more frequent flux measurements were made at most sites.

These studies are providing information which confirms that aerobic soils are an important sink for atmospheric CH4. These and other studies around the globe led to a reassessment of the soilsink strength by the latest IPCC assessment, which concluded that the total global soil sink for atmospheric CH4 is about 40 Tg C yr1. The soil CH4 sink appears to be intimately tied to soil nitrogen dynamics and is strongly inhibited by fertilization or cultivation in some systems. This sink is similar in magnitude to other much more intensively studied terms of the global CH4 budget,including emissions from rice paddies and enteric fermentation, and the rate of increase in the atmospheric CH4 pool. Although it is clear that aerobic soil consumption of CH4 is an important part of the global budget, how does a land area like the CO shortgrass steppe for example, impact the net green house gas flux? Since the grasses fix CO2 we assume a net balance between fixation and respiration. The grasslands produce N2O and are used for cattle grazing part of the year. The cattle produce CH4 through enteric fermentation. Based on CPER stocking rates, animal size and diet, cattle produce about 0.16 kg CH4 day1 cow1. A pasture stocked at the moderate rate of 20 animals for a 130 ha pasture and 130 stocking days emits about 420 kg of CH4 yr1. The pasture produces about 40 kg of N2O yr1 and consumes about 610 kg1 of CH4 yr1. Using IPCC1994 global warming potential factors for CH4 and N2O, calculations indicate that because of the soil CH4 sink, the shortgrass steppe has a net global warming capacity of 1100, ie is a net sink for greenhouse gases. Long term studies at selected sites will continue.

References:

C. Population and Trophic Dynamics:

Work in this area encompasses short and longterm population dynamics studies, food web structure, as well as competition, ecophysiological, demographic and phenological, and genetic structure studies. Some of this research is reported in the Disturbance/Human Use or Biogeochemical Dynamics sections of this report.

1. Plant Populations

Competition between dominant species within a guild of short warm season grasses:
Bouteloua gracilis and Buchloë dactyloides are dominant and subdominant species in the shortgrass steppe and together constitute a guild of warmseasonperennial shortgrasses. Our general objective was to study the competition between these two species when resources are added. Our results indicated that competition can be very intense and important under both low and high availability of resources. Addition of water and nitrogen did not change the intensity of either intra or interspecific competition. Addition of resources did not reduce the intensity of belowground competition or significantly increase the shading (aboveground competition) among plants.

We also studied factors other than competition to explain the difference in the relative importance of these two species: recruitment differences, disturbance regime and drought tolerance. Longterm data suggest that B. dactyloides is less tolerantto drought than B. gracilis. The results indicated that differences in drought tolerance and recruitment probabilities along with their interactions with disturbance regime exert a major control on the biomass of the species. Biomass of each species increased with the relative superiority in recruitment. Disturbance favored B.dactyloides, while the intolerance to drought in B. dactyloides favored B. gracilis. Inabsence or with intermediate disturbance, a higher recruitment probability and greater tolerance to drought of B. gracilis than B. dactyloides yielded relative abundance of these species similar to the patterns observed in the shortgrass steppe.

References: Aguiar et. al 1994, Aguiar et. al submitted, Aguiar 1995 (dissertation)

The ability of B. gracilis roots to exploit resources:
We evaluated the ability of B. gracilis to exploit resources in central and outer portions of root systems. Morphological constraints were hypothesized to explain scales of strong neighbor interaction and gapdynamics observed in shortgrass steppe. Water or water plus nitrogen were supplied at horizontal distances greater or less than 10 cm from tillers in containers. Growth was significantly less when water was supplied only to the outer root system than when water was supplied only to the central root system. Reduced growth was associated with asymmetrical distribution of roots relative to direction of tiller growth. N addition did not affect plant growth. Although adventitious roots were roots were mostly <10 cm from crowns, root length density and water use were similar in central portions of root systems and at distances >10 cm from leading ends of tillers, which corresponded to distances >10 cm from intact plants. Given typical plant spacings in shortgrass steppe (<15 cm), established B. gracilis individuals are likely to compete strongly and preempt resources in most openings. Results suggest that enhanced resource availability and colonization in small gaps (=50 cm) in shortgrass communities do not reflect strong morphological constraints on the outer root system of B. gracilis plants.

References: Hook

Plant species effects on temporal patterns of nutrient cycling:
We conducted an experiment to examine the impact of species with different phenologies on nutrient cycling patterns in the shortgrass steppe. Species having the C3 or C4 photosynthetic pathway have different optimum temperatures for photosynthesis and thus have periods of offset growth during a growing season. These differential growth patterns are expected to yield temporal differences in nutrient cycling for communities dominated by C3 species, C4 species or mixed communities. We conducted experiments at three time periods throughout a growing season; spring, summer and fall. During each time period we plan to examine nitrogen availability, nitrogen uptake by plants and movement through the soil profile, and atmospheric fluxes of NO, N2O, CO2 and CH4. We will analyze these response variables for C3, C4 and mixed plots at two locations at the CPER.

During each time period, initial soil cores were taken for ammonium and nitrate extractions, and cores with resin bags were left in the field to analyze ammonium and nitrate levels at the end of the monthlong experiments. These data are currently being analyzed. 15N was added to plots to trace nitrogen use by plants and movement through the soil profile. Soil samples in the 15N plots were taken to 50cm after one month. Lab work is currently underway for analyzing soil and biomass samples for 15N. Gas fluxes were sampled on a weekly basis. (see Figure 2.20)

Seed production in relation to soil texture and grazing:
We are continuing to sample seed production of individual B. gracilis plants each year on 10 sites selected to represent a range in soil textures as well as grazing intensities. Seed production as well as other morphological characteristics have been sampled from 108 plants at each site since 1989. Our data indicate that temporal and spatial variability in production of viable seeds are important constraints on the ability of B. gracilis to recover after disturbances. (see Figure 1.1)

References: Coffin and Lauenroth 1990

Vectors of seed dispersal for two important grass species:
Plant recovery following disturbance is affected by interactions between disturbance characteristics and life history traits of plants in the community. Seed dispersal into a disturbed area is an important method of plant colonization during recovery. This project investigates vectors of seed dispersal for two dominant grasses at the CPER: Bouteloua gracilis (blue grama) and Buchloe dactyloides (buffalo grass). These two species vary in life history traits associated with recruitment that may affect their dispersability and recovery after disturbance. Specifically, we are evaluating the importance of cattle as vectors of dispersal of B. gracilis and B.dactyloides seeds. We will also investigate the wind dispersal patterns of these two species. Results from this study will aid in our understanding of the persistence and dominance by B. gracilis and B. dactyloides in the presence of disturbances of different sizes, and the role of cattle in the plant recovery process.

Approximately 225 recently deposited cow fecal pats were located and mapped in each of four halfsection pastures at the CPER in late Sept to early Oct 1994 after both B. gracilis and B. dactyloides had produced mature seed. These pats will be used to test for the presence of viable seed and examine changes in seed viability over time. Collection of 15 soil samples in June, July, and September from each pasture will be used to evaluate the seed availability in pats as compared to the soil seedbank. Wind dispersal of seed will be estimated by marking seed of isolated B.gracilis and B. dactyloides plants with fluorescent dye in July and August/September. Dispersal distances of seeds will be measured to determine the relationship between seed density and distance as well as maximum dispersal distance under natural conditions. In addition, wind tunnel studies will be conducted to measure seed dispersal distances for both B. gracilis and B. dactyloides at several wind speeds and release heights to determine the pattern of seed sinking velocity of each species.

References:

Development of microtopographic relief in a semiarid grassland: effects of disturbance size and soil texture:
Our objective was to evaluate effects of disturbance size (50, 100, 150 cm diameter created in 1984 to 1985) and soil texture (sandy loam, clay loam) on development of microtopographic relief for a shortgrass plant community. Disturbed plots were not manipulated after being created, and recovery occurred until microtopographic relief was measured in 1993.

Crown heights of plants were significantly higher than bare soil openings both for the undisturbed landscape and inside disturbances. Differences between crown heights of plants and bare soil openings were similar for both locations, indicating that smallscale microtopographic relief had recovered within nine years after disturbance occurred. However, complete recovery to the undisturbed state had not occurred, because crown heights of plants and bare soil openings were significantly less on disturbed than undisturbed microsites. These differences between locations and between microsites may indicate a net loss of soil material from bare soil interspaces, and a net accumulation of soil material under plants on disturbed areas.

Differences in height between plants and bare soil openings on disturbed plots increased as disturbance size increased, indicating greater soil erosion with increasing plot size. Larger differences in height were also found on plots on fine than coarsetextured soil indicating the importance of soil particle size to the development of the microtopography.

Plant co

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