Table Of Contents
- A. Field Experiments
- B. Simulation Modeling
- STEPPE model
- Species recruitment across an environmental gradient
- The importance of soil water in the recruitment of Bouteloua gracilis in the shortgrass steppe
- Seedling recruitment potential of Bouteloua gracilis in the central grassland region
- CENTURY model
- Soil-atmosphere exchange of CH4, N20 and CO2 using field-flux measurements for development of process- based gas flux models for CH4 and N2O
- C. Regional Analysis
- Aboveground net primary productivity across a precipitation gradient in the Central Grassland region
- A regional analysis of N mineralization in coarse
and fine textured soils along a precipitation gradient in the
Central Grasslands of the U.S - N retention in the northern mixedgrass prairie and shortgrass steppe
- Regional patterns of NDVI in North American shrublands and grasslands
- The effects of grazing on ecosystems of the Great Plains
- ANPP estimates from NDVI for the Central Grassland region of the US
- Climatic controls of the relative abundance of
plant functional types in grasslands and shrublands of North
America - Cross LTERsite comparison of factors that control
decomposition: Effects of Bouteloua gracilis and Bouteloua
eriopoda and indigenous microbes on rates of decomposition and soil organic matter formation - Generating and testing daily weather data using a regional grassland data set
2. Synthesis Activities
A. Field Experiments
Consequences of patterns in
food webs on stability:
Data collected from the CPER has been used in conjunction with data
from other sites as part of an across site comparison in search of patterns in food webs
and the effects that disturbance has on these patterns and system stability. The work has
resulted in 6 publications. The more significant results of the project have been reached
by coupling the models traditionally used by community ecologists with those used by
ecosystem ecologists. The studies listed below have provided insight into the roles of
productivity and dynamics in structuring ecosystems, the calculation of interactions
strength using field data, and common features of agricultural practices on the structure
and function of soil communities.
We previously demonstrated that the resilience of simple food chains was directly related to the level of primary productivity and the rates of detritus inputs from external sources. The key to this study was in the development of Lotka Volterra based models that were expressed in terms of ecological energetics. This result would seem to support the "dynamics hypothesis" that proposes that food chain length was a function of the limitations that increased length placed on the likelihood of the system recovering from a minor disturbance. A second important result was the finding that the feasibility (ability of the system to maintain positive population densities at steady state) of the food chains was a function of the level of productivity and detritus inputs. Higher levels of productivity were able to sustain longer food chains. This finding had been hypothesized G.E. Hutchinson (1959 Am. Nat. 93:145)and subsequently termed the "energetics hypothesis." The conclusion of our work was that system dynamics and energetics were inextricably interrelated. Food chain length was a function of both energetics and dynamics.
Considerable progress has been made in the development of procedures used to estimate the interaction strength among organisms with a food web. Paine (1992 Nature 355:73) proposed measuring interaction strengths among species through a series of species removal experiments. As an alternative, we estimated interaction strength with models. The population dynamics of species within the community were described after Moore et al. (1993) and DeRuiter et al. (1994), and the field average population densities, were substituted for the theoretical equilibria. The elements of the community matrix (interaction strengths) are estimated by sampling physiological parameters, consumption coefficients and population densities, from ranges established by microcosm and field data.
An interesting pattern emerged when we estimated the interaction strengths (as above) and the trophic position of each consumerprey interaction within food webs in North America and Northern Europe. The negative effects of consumers on prey decreases with increased trophic position and the converse was true for the positive effects of prey on consumer. De Ruiter et al. (Science, in review)demonstrated that communities that possess this asymmetric patterning of interaction strength with increased trophic position are more likely to be stable than matrices depicting the same trophic structure but no pattern in the distribution of interaction strength.
The conclusion of our work with food chain length and interaction strength is that the energetic organization of communities forms the basis of ecosystem stability. In 19951996 the work will be directed towards empirical tests of these findings. (see Figure webs)
References: Moore et. al 1993, Moore et. al 1993, Moore and DeRuiter 1993, Moore 1994, Moore et. al 1995 (accepted), DeRuiter, et. al 1995 (in review)
Changes in species composition with grazing were primarily a function of ANPP and evolutionary history of grazing of the site, with sensitivity to level of consumption third in importance. Changes in species composition increased with increasing productivity and with longer, more intense evolutionary histories of grazing. These three variables together with latitude explained 50% of the variance in composition of grasslands, and precipitation, evolutionary history, and years of protection explained 72% of the variance for shrublands, even though methods of measurement and grazing systems varied between studies. The response of shrublands was different from that of grasslands, with a change in evolutionary history of grazing of shrublands resulting in less effect on change in species composition than in grasslands.
Similar variables entered models of change in the dominant species with grazing, but explanatory power was less than for species composition. Increases of the dominant species were more likely to occur in shrublands than grasslands, and decreases more likely among bunch grasses than other lifeforms and among perennials than annuals.
Percentage differences in ANPP between grazed and ungrazed sites decreased with increasingly long evolutionary histories of grazing and increased with increasing ANPP. Although most effects of grazing on ANPP were negative, the statistical models predicted increases in ANPP with grazing under conditions of long history, low consumption, low ANPP, and large range in annual temperature for grasslands plus shrublands. The data and the models support the controversial hypothesis that grazing can increase ANPP in some situations.
Counter to the common held view that grazing negatively impacts root systems, there was no relationship between difference in ANPP with grazing and difference in root mass; as many positive as negative differences occurred even though most ANPP differences were negative. Further, there was a weak relationship between change in species composition and change in ANPP, and no relationship with root mass, soil organic matter, or soil nitrogen. All three belowground variables displayed both positive and negative values in response to grazing. Current management of much of the worlds grazinglands based on species composition criteria may lead to erroneous conclusions concerning the longterm ability of a system to sustain productivity. (see Figure 2.20b)
References: Milchunas and Lauenroth 1993
STEPPE model:
We are continuing to use models of soil water dynamics (SOILWAT), plant community
structure (STEPPE), and production and turnover of soil organic matter (CENTURY) to better
understand various aspects of shortgrass steppe structure and function. We are continuing
to use these models both independently and as linked, interacting models to aid in the
formulation of questions and hypotheses, and in the interpretation of field and laboratory
data.
References: Parton et al. 1988, Coffin and Lauenroth 1990
Species recruitment across an environmental gradient:We evaluated the effects of precipitation and temperature on the recruitment of two dominant perennial C4 grasses along an environmental transect from the Central Plains Experimental Range in northern Colorado to southern New Mexico. Bouteloua gracilis ranges from Canada to Mexico, and dominates the shortgrass steppe. B. eriopoda is limited to the southern United States and Mexico, and dominates the desert grasslands. Successful sexual reproduction is rare in these species, but may be important in explaining the species geographic ranges. We evaluated the probabilities of recruitment for these two species at 16 sites in Colorado and New Mexico, including three LTER sites, the Central Plains Experimental Range, the Sevilleta, and the Jornada, using a daily time step, multi layer soil water model (SOILWAT). We used 30 years of climatic data from weather stations to produce 5000 years of weather data for each site using a Markov weather generator. Both the environmental factors of most importance to the two species and probability of recruitment along the transect differed. Precipitation was more important to the recruitment of B. gracilis while temperature was more important to the recruitment of B. eriopoda. Probability of recruitment decreased from north to south for B.gracilis while increasing from north to south for B. eriopoda. These results suggest that recruitment is important in the distribution and dominance of these two species.
References: Minnick, T.J. and D.P. Coffin 1995
The importance of soil water in the recruitment of Bouteloua gracilis in the shortgrass steppe:In the shortgrass steppe region of North America there is a controversy about the ability of the dominant species to recruit from seedlings. The prevailing view is that B. gracilis is incapable of recruitment form seedling in areas receiving <380 mm of annual precipitation. A common explanation for this situation is that environmental conditions permitting seedling establishment are infrequent. To assess the frequency of environmental conditions appropriate for the recruitment of B. gracilis we used a soil water simulation model and longterm climatic data in conjunction with detailed information about the ecophysiological requirements for seed germination and growth of seminal and adventitious roots.
We found that recruitment events occur as frequently as every 30 50 yr. on silty clay, silty clay loam, and silty loam soils, but less that once in 5000 yr. on sandy soils. Simulated frequencies of recruitment were sufficient to account for the observed abundance of B. gracilis in 7 or 11 soil textures evaluated. The differences in silt content and available water occurrence of recruitment events. Therefore, soil texture variability may explain the spatial pattern of recruitment and of population recovery after disturbance that occur at the soil type and microsite scales.
Annual precipitation explained a large fraction of the temporal variability in recruitment. On average, recruitment occurred in years when precipitation was above the mean. The occurrence of recruitment events in some dry years (precipitation < mean), and their absence during some wet years (precipitation > mean), emphasizes the importance of the intraseasonal distribution of precipitation.
The sensitivity of recruitment to soil water availability suggests that climate change, particularly changes that increase or decrease the amount or the effectiveness of soil water, could have important effects on the future of populations of B.gracilis. (see Figure pop_b)
Reference: Lauenroth et al. 1994
Simulations conducted with the STEPPE model also indicated the importance of soil texture to the recovery of B. gracilis after disturbances of various sizes. Average probabilities of seedling establishment from the SOILWAT simulations were used in STEPPE based on silt and content. Disturbances on the soil with a large silt content (silt loam) had large B. gracilis biomass through time compared to soils with low silt content (sandy clay loam, loamy sand). Additionally, recovery was dependent upon disturbance size, where small disturbances recover faster than large disturbances. A nonlinear relationship was found between the proportion of plots recovered by B. gracilis in year 500 and silt content of the soil. Disturbance size was less important than soil texture in determining plant recovery. (see Figures 1.15 2.33)
References:
During the last year we have worked on improving the soil water and soil temperature model (SOWAT) and have developed a new model for simulating snow melt, sublimation and bare soil evaporation during the winter. The SOWAT model was tested using data from the PILPS soil water comparison workshop in Australia and compared to 12 other water flow models. A paper describing this work was written and submitted for publication. A paper describing the new winter water budget model has been written, however, it has not yet been submitted for publication. The winter water budget model has been developed using data from the CPER lysimeter and microclimate station. (see Figure 2.34)
References:
The CH4 uptake model is based on the observation that CH4 uptake rates are limited by diffusion, and can thus be modeled based on Fick's first law of diffusion in which flux is proportional to the concentration gradient with respect to soil depth. Diffusion coefficients are estimated as functions of pressure, temperature, soil bulk density, texture and moisture. A factor relating soil N turnover to CH4 uptake is also included.
Further refinement of these models and linkage to the CENTURY model is planned. Once that phase is accomplished, then the model package will be linked with GIS systems to develop regional flux models.
References:
C. Regional Analysis
Our regional analysis efforts are in conjunction with another NSFsupported project (BSR 9106183) that provides the funding for these efforts. Below we summarize a few of our important findings from that project that are relevant to our sitelevel research at the CPER.
Aboveground net primary productivity across a precipitation gradient in the Central Grassland region:Controls on aboveground net primary productivity (ANPP) and vegetation structure at the CPER and at 12 satellite sites were studied across a precipitation gradient through the Central Grassland region. Patterns of productivity across the gradient may be explained by the inverse texture hypothesis which predicts that in arid and semiarid regions, plant communities on coarsetextured soils should have higher aboveground net primary productivity (ANPP) than those on finetextured soils, while the reverse should occur in humid regions. The first objective was to test these predictions by measuring ANPP in 1993 and 1994 at 14 natural grassland sites (CPER + 12 satellite sites), equally divided between coarse and finetextured soils, along a regional precipitation gradient (333 mm/y to 759 mm/y) where mean annual temperature remains constant. Below 400 mm of mean annual precipitation (MAP), coarsetextured sites were significantly more productive than finetextured sites in 1993 but not in 1994. For all sites, ANPP was significantly correlated with both MAP and precipitation for the 12 month period preceding sampling. Soil texture explained a significant proportion of variability in ANPP of graminoids and forbs but not total ANPP.
The second objective was to quantify aboveground canopy structure of natural grasslands along the precipitation gradient to examine the consequences of increases in the importance of aboveground competition with increased productivity. We found abrupt shifts in maximum canopy height and in PAR transmission at the transitions between vegetation types. Transmission of PAR through the canopy decreased with increasing precipitation. With increasing precipitation, total biomass and LAI increased, but the proportion of biomass and leaf area near the soil surface decreased. Overall, these results revealed discontinuous changes in canopy structure between different vegetation types.
The third objective was to examine longterm precipitation dynamics and long term soil water dynamics for two contrasting soil textures to understand potential mechanisms for observed patterns in ANPP and canopy structure across the precipitation gradient. We used a daily time step multilayer soil water simulation model to examine these dynamics. With increasing total precipitation, the proportion of growing season (April 1 October 1) precipitation accounted for by large events (> 30 mm) increased significantly. Distribution of precipitation interacted with soil texture in determining transpiration and evaporation. Transpiration from the sandy soil was higher than from the clay loam at the four driest sites, while the reverse was true at the two wettest sites. For the clay loam, the deep 30100 cm soil layer represented a consistently important water resource only for the two wettest sites. Simulation results suggest that across this precipitation gradient the relative success of plant functional types with different rooting distributions and phenologies may vary according to soil texture.
Results presented here demonstrated that effects of abiotic driving variables on ecosystem dynamics may be mediated through vegetation structure. These results indicate the importance of including vegetation structure in regional analyses of ecosystem structure and function.
References:
A regional analysis of N mineralization in coarse and fine textured soils along a precipitation gradient in the Central Grasslands of the U.S:We estimated field and laboratory N mineralization rates for seven paired sites of fine and course textured soils along a precipitation gradient extending from the semiarid shortgrass steppe of Eastern Colorado through the mesic tallgrass prairie of Nebraska.
No clear trend in field or laboratory rates of N mineralization was evident along this precipitation gradient. Laboratory rates were generally higher than field rates, especially toward the dry end of the gradient. Fine textured soils generally supported higher rates of N mineralization than did coarse textured soils.
References: Barrett et al. 1995
N retention in the northern mixedgrass prairie and shortgrass steppe:Anthropogenic sources of fixed N contribute to a suite of environmental problems including acid deposition, groundwater pollution and accumulation of greenhouse and photochemically active grasses. Additions of fixed forms of N in excess of plant demand, may stress native terrestrial systems. We are developing an experiment to study the effects of elevated levels of N on native grassland systems along a temperature gradient of the Central Grasslands. We have collected soils form five sites along this gradient, extending form SE Montana, through Wyoming and Colorado, to the Panhandle of Texas. Preliminary analysis will include estimates of gross and net rates of N mineralization, potential C mineralization and SOM. Plans to initiate a fertilization experiment and monitor N trace gas flux are being developed.
References:
Regional patterns of NDVI in North American shrublands and grasslands:We characterized the seasonal pattern of the Normalized Difference Vegetation Index (NDVI) of 49 grassland and shrubland sites corresponding to areas with low human impact. Each site was characterized as a vector of 21 elements, where each element corresponded to the average NDVI for one date. We used the integral of the NDVI as an estimate of the total production, and the difference between the maximum and minimum NDVI over the year as a measure of the seasonality.
Based on a Principal Component Analysis, the integral of the NDVI accounted for 63% of the variability of the SITEXDATE matrix. The difference between the maximum and minimum NDVI throughout the year accounted for 19%. Mean annual precipitation explained 67% of the variability of the integral of the NDVI curves (p<0.01). The slope of the relationship between precipitation and integrated NDVI was significantly steeper for grasslands than for shrublands. The difference between the extreme values of NDVI increased with mean annual precipitation and the thermal amplitude, and decreased with mean annual temperature (p<0.01). (see Figure 2-20a)
References: Paruelo and Lauenroth 1995
The major westeast and southnorth climatic gradients provide the best explanations for differences in the responses of ecosystems to grazing. Experiments conducted in the western, driest portion of the region suggests that plant communities in these areas are the least susceptible to detrimental impacts of improper grazing management. These areas also have the lowest productivity both of plants and animals. By contrast, plant communities in the eastern, wettest part of the plains has the capacity to support the largest number of animals per unit area but they are most susceptible to changes in community composition, diversity, and physiognomy under well managed light to moderate grazing. These communities are also potentially the most vulnerable to the detrimental impacts of poor management. Aboveground net primary production generally declines with grazing, although increases in the wettest areas have been attributed to opening dense canopies and reducing litter layers.
Interactions between livestock grazing and wildlife is a complex issue in the Great Plains as it is every where else. The same grazing management system can have positive impacts for one wildlife species and negative for others. Furthermore, it is clear in may cases that the effects of one management system on a single species can be a mixture of positive and negative impacts.
Despite the fact that grazing by domestic livestock does not have simple relationships to the other elements of the structure and function of Great Plains ecosystems, management of grazing animals using current technology appears to be a sustainable resource use. The conversion of rangeland to cropland has many implications for both sustainable resource use and global environmental issues. (see Figure 2.20d)
References: Lauenroth et al. 1994
For grassland areas with mean annual precipitation between 280 mm and 1150 mm, and mean annual temperature between 4oC and 20oC, the NDVII and ANPPshowed a strong and statistically significant relationship (r=0.93, n=19, p<0.001). NDVII was positively related to the ANPP. The relationship found is based on NDVII and ANPP data averaged over time and aggregated at the same spatial scale.
References: Paruelo et al. submitted
References: Paruelo and Lauenroth 1995
References:
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