II. OVERVIEW OF FACILITY

A. Introduction

B. Site Description

1. Climate

2. Vegetation

C. Field Station Description

III. PROJECT DESCRIPTION

A. Current Status of the CSU Research Facility

B. Transportation

C. Computing Facilities

D. Building Renovations

IV. UTILIZATION OF FACILITY FOR RESEARCH AND EDUCATION

A. Research Utilization

B. Educational Utilization

C. Other Utilization

V. SIGNIFICANT RESEARCH ACCOMPLISHMENTS 1992-1997

A. Introduction

B. Populations and Processes

C. Biogeochemical Dynamics

D. Paleoecology/Paleopedology

E. Water and Energy Dynamics

F. Disturbances

Natural disturbances

Human-induced disturbances

G. Synthesis

VI. SELECTED BIBLIOGRAPHY

A. Significant Research Publications 1992-1997

A. Introduction

The Shortgrass Steppe Long Term Ecological Research (SGS LTER) project represents the continuing development of a research tradition that began with the US/IBP Grassland Biome project in the late 1960s, the time at which ecosystem science was formally recognized as a sub-discipline in ecology. All of this research has focused on the Central Plains Experimental Range(CPER), a 6,280 hectare research site owned by the Agricultural Research Service (ARS) that is adjacent to the 78,100 ha Pawnee National Grasslands (PNG). Research at the CPER over the past 20 years has had an important interactive relationshipwith the development of ecosystem science. The Grassland Biome project focused on the issue of productivity of natural ecosystems. Grasslands were conceptualized as homogeneous entities, appropriately described by an average square meter.The transition from the IBP project in the early 1970s to the LTER project in the early 1980s involved a change in thinkingabout the importance of spatial variability.

Our involvement in the LTER program (LTER I 1982-1986) began with spatially explicit ideas and questions about the importance of landscape structure, particularly the classic soil catena model, in the long-term development and maintenance of shortgrass steppe ecosystems. In the second phase of the project (LTER II 1987-1990) we expanded our concept of long-term processes to include the origin and persistence of spatial patterns at a range of spatial scales. This work included substantial questioning
of the generality of the catena model at the CPER and in the shortgrass steppe region. Our work for LTER III (1990-1996) built upon LTER I and II and expanded the depth of our investigations into interactions between spatial and temporal patterns in ecosystem structure and function. LTER IV (1996-2002) adds new dimensions by focusing on the keystone species prairie dogs and plains prickly pear, population genetics of Bouteloua gracilis, and atmosphere-biosphere interactions.

The CSU Headquarters was built at the CPER in the late 1960's, when it was the core of the NSF-funded International Biological Program (IBP) Grassland Biome Project. Hundreds of scientists, graduate students, and research technicians were on-site each day, collecting data, processing samples in the laboratories, having meetings in the classroom, being served from the large kitchen at that time, doing heavy work in the workshop/garage, and staying in the dormitories. The site manager lived in the on-site house to manage the bison and pronghorn herds, and to manage the huge logistics of the research program. This intensive field program lasted until 1974; from 1974 until 1982 the site was used in a slightly less intensive fashion by the large number of ecological research projects that followed the IBP. In 1982, the Shortgrass Steppe Long Term Ecological Research Program was funded by NSF. Since then, the site has continually increased in use, with 18 projects currently studying the shortgrass steppe and using the CSU facilities at the CPER, and another 9 projects pending in proposals that would focus entirely on the CPER.

The rangeland research conducted by the CSU faculty at the CPER is recognized worldwide as one of the most important sources of new ideas and important results in grassland ecology in the world. Our research has had major implications for range management in the region.

B. Site Description

From 1982 until 1996 , the Shortgrass Steppe (SGS) LTER was located on the Central Plains Experimental Range (CPER), a 6280-ha tract of shortgrass rangeland located in the piedmont of north central Colorado approximately 61 km northeast of Fort Collins and the campus of Colorado State University (lat. 40x49'N; long.104x46'W; elevation 1650 m). The CPER is administered by the USDA, Agricultural Research Service (ARS). In 1996, we increased the spatial extent of our LTER site to include both the CPER and the Pawnee National Grasslands (PNG). The PNG represents 78,100 ha of public lands administered by the U.S. Forest Service (USFS) adjacent to the CPER, and extending 90 km to the east. The PNG is discontinuously distributed across northeastern CO because these lands are the result of acquisitions of private land beginning in the Dust Bowl era. By expanding to our newly defined site, we increase our realm of inference to 23% of the SGS.

1. Climate

The climate of the SGS is typical of mid-continental semiarid regions in the temperate zone except for the strong influence of the Rocky Mountains approximately 60 km to the west. Mean monthly temperatures range from -4 to 22 degrees C seasonally and have a daily average max-min range of 17 degrees C. Annual precipitation at the CPER averaged 322 mm over the past 51 years and ranged between 107 and 588 mm. Approximately 70% of the mean annual precipitation occurs during the April to September
growing season. (For further information please see our summary of climate data at http://lternet.edu/im/climate/climdes/sgs/sgsclim.htm)

2. Vegetation

The vegetation of the SGS is dominated by shortgrasses (64%), forbs (7%), succulents (21%), and half-shrubs (8%). The key species of these groups are Bouteloua gracilis and Buchloe dactyloides; Sphaeralcea coccinea; Opuntia polyacantha; and Chrysothamnus nauseosus, Gutierrezia sarothrae, and Eriogonum effusum, respectively. Average aboveground net primary production is 125 g/m² and ranges from 60 to 180 g/m² depending on available soil water. Major differences in vegetation structure occur in saltgrass meadows dominated by Distichlis stricta and Sporobolus asper, and on floodplains
where the shrub Atriplex canescens is an important component.

C. Field Station Description

The CSU field station consists of the headquarters area located on a 1.6 ha plot, a residence for the site manager, and a corral and pasture area of 53 ha. Three buildings are located in the headquarters area: a dormitory, a storage/ workshed building, and a office/laboratory building.

The main SGS LTER headquarters building (214 m²) has offices, laboratories, a dining/meeting room, and a kitchen. This multi-purpose building serves as the focal point for all activities at the site including conferences, meetings, and classes. The laboratory has workbenches, a digital balance, a ph meter, a conductivity meter, and two grieve drying ovens which enable an investigator to process field samples to a finished or nearly-finished state. This laboratory space is used by researchers for weighing and sorting soil and vegetation samples, national atmospheric deposition program sample processing, and preparing mammal traps. Adjacent to the headquarters is a storage/sample processing building (134 m²) with facilities for washing, drying, and storing samples. This building also serves as the workshop/garage for field station heavy equipment. The dormitory has six rooms; five capable of double occupancy and one with four beds.

A. Current Status of the CSU Research Facility

The CSU field station has enormous potential as a resource for research, education, and retreats/meetings. However, essentially no renovations or improvements have been made at the site since it was built in the late 1960's, since regular research grants do not generally support this kind of work. The structures and utilities need renovating, and there are enormous opportunities to build the site into the kind of field station that NSF envisions - a place that has up-to-date facilities and technologies and supports even more and better research and education.

Our long-term vision for the CSU field station includes major renovations and improvements to our buildings and equipment. These future improvements include plans for structural renovations to the headquarters building, remodeling the kitchen area for better utilization of workspace, development of office space reserved for visiting scientists, major renovations to the dormitory, development of better sample archive storage facilities, and the development of an on-site research library. We hope to use this proposal to fund critical renovations and necessary improvements to our facility and form the starting point for future cooperative work with Colorado State University and NSF. In this proposal we will address what we feel are our highest priority items: transportation, computing facilities, and building renovations.

B. Transportation

We are requesting funds to purchase two vehicles for the field station: a four-wheel drive, extended cab, 3/4 ton pick-up truck for use by the site manager and a two-wheel drive light duty pickup truck. The four-wheel drive truck will replace the site manager’s current vehicle, a 1983 Dodge pickup truck which has nearly 150,000 miles and has been repaired over ninety times in the last two years at a cost of over $4,000. With a recent repair record like this, we feel it is time to stop pouring money and time into this truck and request funding for a new site manager’s truck. Aside from the monetary issue, we are
concerned for the safety of our site manager and those who use this vehicle. It is conceivable that this vehicle could break down miles from the nearest help and subject the driver and passengers to the unpredictable weather of the shortgrass steppe which includes lightning storms, tornadoes, and blizzards.

We are also requesting funds for a second truck, a light duty, two-wheel drive, extended cab pick-up with a covered bed. This multi-purpose vehicle will be used heavily from April through October by the summer field crews, visiting scientists, resident scientists, and graduate students. The logistics of moving 10-30 people around the SGS-LTER site have always been a nightmare during the field season and have created unsafe circumstances for field crew members located far distances from one another and subject to the weather conditions mentioned above. In addition to hauling people, the covered bed will give
researchers the option of water proof transport of samples and equipment. From November through March this vehicle will be used as the primary mode of transportation from CSU to the SGS-LTER site, reducing the number of times we will need to rent vehicles from the University.

C. Computing Facilities

The current computer equipment at the SGS-LTER field station consists of a 286-based PC, with 5.25" and 3.5" floppy disk drives and an Advanced Logic Research 386-based PC, with 5.25" and 3.5" floppy disk drives. The printing capabilities of our field station are limited to a 10 year old dot-matrix printer. This computing equipment is quickly becoming outdated for all but a few applications. The site manager makes extensive use of the 386 computer to data enter rain gauge charts, small mammal data, and arthropod data into a spreadsheet and for storage of those data. This computer is also a vital communication tool that the site manager uses to dial into the Colorado State University network system for email and transferring files, for writing correspondence, guidelines, and protocols, and for managing and coordinating projects. In addition, visiting scientists must share this computer with the site manager as time permits. These limited computing facilities have caused considerable logistical problems during the field season, when use by visiting scientists is highest.

We are requesting funds to purchase a 200Mhz Pentium computer, with a 3GB hard drive and high speed modem for use by the site manager and visiting scientists. Such a computer will allow the site manager to move data into a database program, have high speed access to the Internet, and allow the site manager and other researchers to conduct statistical analyses on site. This will also free the 386-based PC for use by visiting scientists or other researchers. We feel that this is a critical first step in developing a technologically advanced field station. In addition, we are requesting funds to purchase an all-in-one HP
OfficeJet 350. This machine combines a black and white laser printer, a copier, a plain paper FAX machine, and a scanner in one piece of equipment. Such a machine is a highly cost-effective way to complete the site manager’s field office and give visiting scientists and researchers all of the equipment they need to conduct business communications from the field station.

D. Building Renovations

As mentioned above, very little renovations or improvements have been made in the past thirty years. This lack of attention had led to a situation where we must make critical renovations to the headquarters buildings or risk closing rooms, due to safety hazards. The renovations that we are requesting funds for represent the highest priority items.

Highest priority goes to the dormitories which are in serious need of renovations and improvements to make several of the rooms useable. Water damage in one of the rooms has resulted in severe damage to a section of the ceiling, which is at risk of collapsing. In addition, many of the tile floor squares are peeling loose, due to water damage. We are requesting funds to replace this section of the ceiling, to retile many of the floors, and to replace the storm doors with heavy duty doors built to withstand the wind and weather at the CPER. By installing proper storm doors on each of the dorm rooms we expect to eliminate the tile floor problems.

A. Research Utilization

There are currently 18 ecological CSU projects utilizing the site (Table 1) with 9 submitted proposals focusing entirely on the CPER site (Table 2). The LTER project is only one of the currently funded projects, but it represents the core piece. The scientists of the SGS-LTER are currently conducting 46 long-term experiments, numerous short-term experiments and several simulation modeling projects (Table 3). To a very large extent, the other grants are only made possible because of the presence of the LTER project: the proposals justify the research to the agencies on the basis of the LTER experiments and long term datasets we maintain, the project reputation for high productivity, and the presence and security of an operating field headquarters. Because the LTER is the most consistent user of the site, we have assumed responsibility for managing the headquarters and building there.

Projects that utilize the site headquarters do so for many reasons. Many use the conference room facility as a place to work indoors between sampling times, and as a good field meeting room for interagency meetings. There is significant use of the laboratory, for sorting and weighing samples, and at times there has been considerable chemical work done out there. The electricity is used by a number of projects to maintain data loggers and other experimental manipulations (timers, instruments, automatic shelters, weather stations). Water use at the site ranges from bathroom use to irrigation experiments and laboratory
work such as washing plant material. The dormitories are used by intensive field crews that are working long days or early mornings/nights. The garage is very heavily used for storing field equipment and archiving samples. The site manager’s house is of course used by the site manager, but his presence is quite important to all of the projects using the CPER in securing their experiments and helping with site logistics. At noon during the weekdays in summer, you are likely to see as many as 30 people at the site headquarters, representing numerous projects.

Table 1 shows only the projects that utilize the site itself, and not those that utilize primarily the data and models generated by the LTER project, since the issue at hand is the field facility. At least 10 scientists at CSU have very large modeling and spatial analysis programs that utilize our data and models, and that are funded in large part because of their relationship with our LTER. Most important of all, from our perspective, is that the CSU collective effort to study shortgrass steppe systems is a world-recognized program because of the scientific quality, diversity, and productivity of our program.

B. Educational Utilization

C. Other Utilization

Professional groups have large field trips to the site, including just in the last several years the Ecological Society of America, the Soil Science Society of America, and the International Landscape Ecology Society. Distinguished visiting scientists from the U.S. National Academy of Science, the French National Academy, the Mongolian National Academy, the Chinese National Academy, the Indian National Academy, and a delegation of Argentinean scientists have visited the site in recent years. Visiting scientists conduct research at the site and live in our dormitories, including faculty from Duke University, University of Chicago, the University of California, the University of Michigan, and many many others.

Other public groups visit the site frequently, including the Nature Conservancy, Sierra Club, and the Crow Valley Grazing Association. The Audubon Society has a special interest in the site, since it is part of the limited habitat of two rare birds. The Audubon Society runs public participation field trips and a Christmas bird count at the site, and all of their birding guides list the CPER as a major birding site; many individual birders visit the site each week. Finally, we are initiating a new major research program into the biology of prairie dogs, which have been exterminated from grasslands but are known to be key
species. We hope that the site will become a focal area for the public interested in grasslands in general and in some of the particularly important species.

A. Introduction

The SGS LTER project focuses on the processes that account for the origin and maintenance of structure and function in shortgrass steppe (SGS) ecosystems. The key questions that continue to organize and guide our research are:

1. How are the distribution and abundance of biotic components of the SGS maintained through time and over space?

2. To what factors are the distribution and abundance of biotic components vulnerable?

3. How do changes brought about by these factors influence biological interactions and ecosystem structure and function?

We have made significant progress toward answering these questions through research efforts that include long­ and short­term experiments, monitoring, survey, simulation analyses, and spatial analyses.

In the past five years we have produced 131 papers in refereed journals, 38 book chapters, 19 dissertations and theses, and 81 abstracts from national and international meetings. We supported a large number of graduate (36) and undergraduate (~160) students and post doctoral fellows (5). Scientists at our site are involved in a number of LTER network activities, through comparative modeling studies, international collaborations, and development of new cross-site experiments. This section is organized by a modified version of the LTER core areas. Because of space constraints, comprehensive and more detailed results from our experiments and monitoring can be found on our World Wide Web site (http://sgs.cnr.colostate.edu/sgshome.html).

B. Populations and Processes

Bouteloua gracilis (blue grama) is the most drought and grazing tolerant grass species and it is also the dominant plant species of the shortgrass steppe (SGS) (Milchunas et al. 1990, Lauenroth and Milchunas 1992). Similar to many other dominant perennial grasses in North American grasslands, blue grama recruits infrequently (Lauenroth et al. 1994) and has a long life span (Coffin and Lauenroth 1989, Lauenroth et al. 1996). Key processes for recruitment are seed production (Coffin and Lauenroth 1992), dispersal, establishment (Lauenroth et al. 1994), and competition from established individuals (Aguilera
and Lauenroth 1993, 1995). Loss of this species results in more diverse but unstable systems.

Our research on consumer populations ranges from swift foxes to nematodes. Fifty-four of 68 captured swift foxes, a CategoryII species, have been radio-collared and ear tagged. Comparison with data from the late 1970's suggests a population increase over the past 20 years and that the (CPER) has a higher density of foxes than the surrounding area (Fitzgerald, in prep). Coyote predation is the most important source of fox mortality.

Diets of great horned owls are composed mostly of lagomorphs, although most individual prey items taken are rodents (Zimmerman et. al., submitted). Thirteen-lined ground squirrels and other rodents are most abundant in shrublands in association with high orthopteran density (Higgens and Stapp 1995). The spatial distributions of shrubs and orthopterans concomitantly increase along a soil texture gradient from grassland (sandy loam) to shrubland (loamy sand) (Stapp 1995a, Stapp 1995b, Stapp 1994, Stapp et al. 1994, Stapp and Van Horne 1995). The shrubby, sandy lowlands in SGS appear very important to landscape-level biodiversity, even though the area of this habitat is small. Preliminary work on
landscape-level plant diversity suggests a conceptually similar relationship; most of the diversity is concentrated in habitat types that make up a small proportion of the total landscape (Kotanen, submitted).

Nematodes at the CPER occur within 6 orders, 23 families and 40 genera and fall into five trophic groups (i.e. bacterial feeders, fungivores, omnivores, predators, and plant parasites). Populations are approximately twice as large in soil under individuals of blue grama than in the interspaces between individuals and appear to be unaffected by grazing.

Our work on patterns in detrital food webs supports the "dynamics hypothesis" that food chain length is a function of the limitations that increased length places on the likelihood of the system recovering from a minor disturbance (Moore et al. 1993a, b, Moore and De Ruiter 1993, Moore et al. accepted, De Ruiter et al. submitted). A second important result is that the feasibility of food chains (ability to maintain positive population densities at steady state) is a function of productivity and detritus inputs. Higher levels of productivity sustain longer food chains. The conclusion of this work is that the energetic organization of communities forms the basis of ecosystem stability.

C. Biogeochemical Dynamics

In addition to long-term monitoring of net primary production, we have been very active in issues concerning patterns and controls on primary production, as well as in assessing methodological problems, developing new techniques, and spanning spatial scale issues (Biondini et al. 1991, Milchunas and Lauenroth 1992, Lauenroth and Sala 1992, Todd et. al 1993, McNaughton et al. 1996). Early modeling and analytical work showed that traditional means of estimating NPP could lead to serious errors (Singh et al. 1984, Lauenroth et al. 1986, Sala et al. 1988). A long-term 14C experiment substantiated these findings, and provided realistic estimates of NPP and an alternative to error-prone methods (Milchunas and Lauenroth 1992).

Spatial patterns of aboveground net primary production (ANPP) at the CPER are controlled by location on catenas at a toposequence scale, and soil texture at the larger landscape scale (Liang et al. 1988). Soil texture is also an important control on plant functional types with different rooting distributions (Milchunas et al. 1992, Lee and Lauenroth 1994). Analyses of temporal patterns of ANPP showed a positive relationship with precipitation, but that ANPP was more variable than precipitation (Lauenroth and Sala 1992). An important result from comparing these data to a regional data set was that spatial and temporal relationships between ANPP and precipitation are not interchangeable. Grazing has negative but small effects on ANPP and variability in cool-season precipitation has a large effect on ANPP (Milchunas et al. 1994).

The SGS is characterized by patchy plant cover and associated spatial heterogeneity of soil resources (Hook et al. 1991). Heterogeneity in soil organic matter (SOM) at this scale is as strong as that induced by topography, and is tightly associated with root biomass (Hook et al. 1994, Kelly et al. 1996). Enrichment of C and N under plants is associated with perennial bunchgrasses (Vinton and Burke 1995), and develops at a rate that corresponds with plant community successional dynamics (Burke et al. 1995). Death in individual bunchgrasses results in rapid depletion (3 years) of the enriched zone with respect to mineralizable N and labile SOM pools, but heterogeneity of total C and N persists for decades (Kelly and Burke, submitted).

Additional factors in small-scale nutrient distributions involves the influence of aboveground structures on erosion processes, and on the quality of inputs by different plant species. There is substantial accumulation of wind- and water-transported material under blue grama and plains prickly-pear (Opuntia polyacantha). Grazing increases the movement from interspaces to under plants, and these effects are greatest in summit and midslope topographic positions. Plant species differ in quantity and quality of biomass, but differences in nutrient cycling under their canopies are slight (Vinton 1994). Soils under
bunchgrasses had more microbial biomass and activity than under a rhizomatous grass, and soils under annuals had the highest mineralization rates. Comparing the importance of plant cover pattern to that of plant species effects across a gradient from the CPER to Konza, we found that plant cover pattern is most important in SGS, and plant species differences are most important in causing spatial pattern in tallgrass prairie (Vinton 1995).

SOM and nutrient availability are also strongly influenced by both soil texture and topographic position (Hook and Burke in prep). In two long-term 15N studies (Delgado et al. 1995 and Hook et al. in prep), we have found that toeslope areas tend to retain up to 90% of 15N added 10 years ago, while summits and midslopes retain half or less. Grazed pastures retain less 15N than ungrazed pastures.

Our trace gas flux studies (Mosier et al. 1991, 1993, 1994a,b, 1995, Bronson and Mosier 1994, Parton et al. 1994, Scholes et al. 1994) are confirming that aerobic SGS soils are important sinks for atmospheric CH⁴, and that gaseous N losses may be important to long-term N balance and productivity. Calculations using IPCC global warming potential indicate that the SGS has a net global warming capacity of -1100 (sink for greenhouse gases), and N gas loss rates are similar to atmospheric inputs. Fertilization has a large effect on N2O emissions and CH4 uptake .

D. Paleoecology/Paleopedology

In our investigation of paleosol geochemistry at the CPER, we proposed a Holocene paleoclimatic scenario (Kelly et al, 1993). A number of factors indicated that the early Holocene was cooler than either the mid-Holocene or current soil forming intervals, and that temperature has not increased from the mid-Holocene to the present (Blecker et al., in press). The 13C/12C ratios of paleosol SOM, phytoliths and CaCO3 established the dominance of C3 vegetation in the soil forming interval spanning 10,000-8,000 y.b.p., and the dominance of C4 vegetation in both the 5,000-3,000 y.b.p. and contemporary soil forming intervals (Kelly et al, 1993). Organic C and phytolith data suggest that both the early- and mid-Holocene climatic conditions were more favorable for plant productivity than the present climate (Kelly et al. submitted). This suggests cooler, moister conditions than presently occur. Pedon development in both the early and mid Holocene paleosols suggests wetter soil moisture regimes than in the present (Blecker, 1993).

E. Water and Energy Dynamics

Soil water monitoring data emphasize the interannual and seasonal variability in soil water as well as the critical role that soil texture plays in mediating storage and losses (Singh et al. in prep). Soil water availability is critical in controlling recruitment of blue grama (and probably other species) by influencing seed production, germination and establishment (Lauenroth et al. 1994). Soil water availability is closely related to ANPP (Singh et al. in prep). Relationships between ANPP and precipitation assume that precipitation is a good surrogate for soil water. At large temporal (growing season or larger) and spatial scales (km² or larger) this is a good assumption.

Water losses (evaporation and transpiration) are largely controlled by water availability. Energy to evaporate water is almost always far in excess of the supply as indicated by the annual or seasonal ratio of precipitation to potential evapotranspiration (Parton et al. 1981). Sala et al (1992) found that the probability of a ratio of precipitation to potential evapotranspiration >1 was only 0.1 at the daily scale and decreased exponentially to 0.01 at the monthly scale. Quantifying bare soil evaporation and understanding how it is affected by soil texture are critical issues in understanding water loss. Results from an experiment with mini-lysimeters confirm a 2-stage water loss pattern with a short period of
high water loss followed by a long period in which losses decrease exponentially (Wythers et al. in prep). By contrast, the mini-lysimeter results contradict the idea that the depth to which evaporation can remove water from a soil decreases as coarseness of soil texture increases. We found the greatest depth of loss for a coarse sand.

Many of our questions about structure and function of SGS ecosystems require an understanding of long-term patterns of soil water dynamics. We used a simulation model to generate 30 years of soil water data for the CPER to provide this long-term view (Sala et al. 1992). Several key results were identified by this work. First, the general temporal pattern in soil water at the daily scale is not identical to the pattern in precipitation because of an interacting pattern of atmospheric demand. The peak in average soil water precedes the peak in average precipitation by almost a month. Second, the layer that most frequently had available water was very shallow (4-15 cm). Layers above this were drier because of bare soil
evaporation and layers below because most precipitation events are too small to move water that deep in the soil. Third, on average water penetrated to 100 cm. In wet years, it reached 135 cm but in dry years it only reached 45 cm. These data provide important information for understanding vegetation structure in the SGS.

F. Disturbances

Our work in this area is extensive. We divide this section into natural and human-induced disturbances. Currently landcover in the SGS is 60% cropland and 40% native grassland grazed by cattle (Burke et al. 1993).

Natural disturbances

The majority of natural disturbances are small (<100 m2) and there is an inverse relationship between size and frequency (Coffin and Lauenroth 1988). The most frequent disturbances (cattle fecal pats, harvester ant nest sites, root feeding invertebrates, and burrows from small animals) produce patches that range in size from 5-40 cm diameter (Hook et al. 1994) and are important in shaping plant community structure and diversity across the landscape (Coffin and Lauenroth 1990). An important focus of our natural disturbance work has been the effect of disturbances on individuals of blue grama and its subsequent recolonization of disturbed patches. Previous studies had reported that blue grama can not recover after disturbance. In a study designed to look at the effects of patch size and neighbors on blue grama seedling growth and survival, we found that in openings <30 cm diameter, established neighbors could preempt resources and inhibit seedling establishment (Aguilera and Lauenroth 1993). Openings >30 cm were large enough to result in seedling establishment. Hook et al. (1994) reached a similar conclusion about the size of a regeneration gap based upon how the root systems of neighbors explored patches created by disturbances. Simulation analyses and assessments of recovery on abandoned fields (see below) confirm that blue grama does reestablish following a disturbance although in some situations the rate may be quite slow (Coffin and Lauenroth 1990, Coffin et al. 1996, Lauenroth et al. 1994). Recovery rates are dependent upon the characteristics of the disturbance, and in particular size and soil texture (Coffin and Lauenroth 1994, Coffin et al. 1993).

Human-induced disturbances

We are continuing our research on the recovery of plants and soils on abandoned agricultural fields along precipitation and temperature gradients in Pawnee National Grasslands and soil texture gradients at the CPER (Burke et al. 1995, Coffin et al. 1996). We found a large variability in recovery of the vegetation that could not be explained by climatic factors. Recovery may be related to soil texture, since the ability of blue grama to recover through seedling establishment is affected by silt content of the soil (Lauenroth et al. 1994). Net N mineralization and other indicators of active SOM were lowest on cultivated fields, but were not significantly different between abandoned and native fields, suggesting slow recovery rates.
Higher N mineralization and turnover in cultivated fields may make them more susceptible to N losses (Ihori et al. 1995a), and recovery of N cycling in abandoned fields appears to involve a return to slower N turnover and tighter N cycling similar to native SGS. Although variation in native soil C and N correlated with climate and soil texture (Ihori et al. 1995b), soil losses due to cultivation were not explained by these variables.

We are also assessing the long-term recovery on nutrient-enrichment treatments (Lauenroth et al. 1978). Invasions by exotic species and the development of characteristics of highly disturbed plant communities did not occur until several years after treatments were terminated (Milchunas and Lauenroth 1995). The tendency of existing plant populations to continue to occupy a site when conditions become unfavorable, can create time-lags in response that represent important challenges for environmental monitoring. The existence of time-lags means that an ecosystem can pass through a threshold to an alternate state before it is detectable in species composition. These results have important implications for such things as global climate change and atmospheric nitrogen deposition which also have the potential to act as enrichment stressors.

The SGS has a long evolutionary history of grazing by large herbivores. The importance of this force in shaping current-day structure and function of this system, and the importance of cattle grazing on both public and private land, have been reasons for the emphasis in research at this site in plant-animal interactions and long-term effects of grazing. Research in this area spans all five major areas. Long-term ungrazed SGS plant communities are more similar to disturbed than were long-term grazed communities (Milchunas et al. 1990). Ungrazed compared to heavily grazed communities were found to be more
susceptible to invasion by "weed" species (Milchunas et al. 1992). Current-year defoliation does have important effects on individual plants and on nutrient uptake by plants (Milchunas et al. 1995, Varnamkhasti et al. 1995). Increased forage quality in response to defoliation suggests a positive feedback between plants and grazers.

Although plant population changes with grazing are minor, other populations display a wide range of responses to grazing (Milchunas et al. in prep.). Groups such as aboveground arthropods, birds, and lagomorphs show large changes in abundance, dominance, or diversity in response to grazing, whereas groups such as microarthropods and nematodes show very little change. Endemic birds associated with SGS prefer to nest in heavily rather than lightly grazed treatments. Changes in biodiversity do not relate to changes in other structural or functional characteristics of the ecosystem.

Little or no effects of grazing on root and crown biomass have been observed (Milchunas and Lauenroth 1989). On average, ANPP declines slightly with grazing (Milchunas et al. 1994), but differences between treatments can shift depending upon levels of water resource and current-year defoliation (Varnamkhasti et al. 1995). Soil nitrogen and carbon pools are not affected at moderate intensities of grazing, but have lower values than ungrazed treatments in heavily grazed lowlands (Milchunas and Lauenroth 1993).

In a grazing experiment initiated in 1991, we are examining long-term grazed and protected and newly ungrazed and protected SGS at 6 sites at the CPER. Key results to date focus on C and N dynamics. Total soil C and N pools were unaffected by moderate grazing or exclosure following both 2 and 53 years of treatment. Pools representing recent belowground litter inputs and substrate available for decomposition (particulate SOM and microbial biomass) were also unaffected by grazing treatments. However, mineralizable C and N, representing the most active pools of SOM, were significantly higher under long-term exclosure than long-term grazing, but only in bare soil areas between plants. Small decreases in mineralization may reflect slight erosion due to reduced canopy and litter cover, but the overall small
effects of grazing may be due to increased plant basal cover that ameliorates thermal effects of reduced litter.

G. Synthesis

We have produced many synthesis products including cross-site studies and simulation modeling over the past 5 years. We describe our current activities in detail in on World Wide Web site (http://sgs.cnr.colostate.edu/sgshome.html).

A. Significant Research Publications 1992-1997

Burke, I.C., E.T. Elliot, and C.V. Cole. 1995. Influence of macroclimate, landscape position, and management on nutrient conservation and nutrient supply in agroecosystems. Ecological Applications. 5 (1): 124 - 131.

Burke, I.C., W.K. Lauenroth, and D.P. Coffin. 1995. Recovery of soil organic matter and N mineralization in semiarid grasslands: Implications for the Conservation Reserve Program. Ecological Applications. 5 (3) : 793 - 801.

Coffin, D.P., W.K. Lauenroth, and I.C. Burke. 1996. Recovery of vegetation in a semiarid grassland 53 years after disturbance. Ecological Applications. 6 (2): 538-555.

Lauenroth, W. K. and O. E. Sala. 1992. Long-term forage production of North American shortgrass steppe. Ecological Applications. 2 : 397 - 403.

Milchunas, D.G. and W.K. Lauenroth. 1992. Carbon dynamics and estimates of primary production by harvest, C14 dilution, and C14 turnover. Ecology. 73 (2) : 593 - 607.

Moore, J.C., P.C. de Ruiter, and H.W. Hunt. 1993. Influence of ecosystem productivity on the stability of real and model ecosystems. Science. 261 : 906 - 908.

Sala, O.E., W.K. Lauenroth, and W.J. Parton. 1992. Long-term soil water dynamics in the shortgrass steppe. Ecology. 73 (4) : 1175 - 1181.

Vinton, M.A and I.C. Burke,. 1995. Interactions between individual plant species and soil nutrient status in shortgrass steppe. Ecology. 76 : 1116 - 1133.

Wiens, J.A., T.O. Crist, K.A. With, and B.T. Milne. 1995. Fractal patterns of insect movement in microlandscape mosiacs. Ecology. 76 (2) : 663 - 666.

With, K.A. 1994. Ontogenetic shifts in how grasshoppers interact with landscape structure: an analysis of movement patterns. Functional Ecology. 8 : 477 - 485.