The shortgrass steppe (SGS) occupies the middle of the productivity gradient along which the LTER grassland sites lie. It is unique among North American grasslands for its long evolutionary history of intense selection by both drought and herbivory, leading to an ecosystem that is very well adapted to withstand grazing by domestic livestock. The distinctive features of the SGS are both its vegetation and the concentration of biological activity and organic matter belowground. The vegetation of the SGS is characterized by lowgrowing plants that are either tolerant or resistant to grazing and drought. The large concentration of biological activity belowground reflects the distribution of plant production (Milchunas and Lauenroth 1992) and the enhanced rates of energy flow through heterotrophs belowground (Lauenroth and Milchunas 1992). It is also explained in part by the fact that most biologically active elements in grasslands are protected from natural disturbances by being stored in soil organic matter (SOM).
The SGS LTER project has been in operation since 1982, and currently supports 46 longterm experiments, numerous shortterm experiments, and has a large emphasis in integrative simulation analysis. SGS LTER work is divided into 5 major areas: populations and processes, biogeochemical dynamics, paleoecology and paleopedology, water and energy dynamics, and disturbances.
Populations
and Processes:
Work in this area is organized by the idea that two kinds of populations are most
important in the longterm dynamics and sustainability of the SGS. The first are dominant
species, such as the shortgrass blue grama (Bouteloua gracilis), which
overwhelmingly dominates the SGS. The second kind of important population is those that
have a large effect on the ecosystem because of their unique traits; these are keystone
species and we propose that prairie dogs (Cynomys ludovicianus) and prickly pear cactus (Opuntia polyacantha) are keystone species
in the SGS. SGSLTER past and continuing work has focused a great deal on population
dynamics of blue grama and the factors that influence its role in the system. In addition,
population dynamics of other species across the trophic structures are also studied. New
work is proposed for the next 6 years to focus on two aspects: 1.) the biology of prairie
dogs and prickly pear cactus as keystone species, and 2.) population genetics of blue
grama as they influence its resistance to grazing.
Biogeochemical
Dynamics:
Our research in this area focuses on elucidating the key abiotic and biotic variables that
control biogeochemical dynamics. Continuing longterm experiments are designed to assess
how precipitation, temperature, topography, and soil texture interact to control spatial
and temporal patterns of primary productivity, nutrient cycling, and trace gas loss.
Proposed new work will address the importance of atmospheric inputs to ecosystem
processes.
Paleoecology and Paleopedology: Over evolutionary time scales, climatic variation has been the major force influencing the structure and function of SGS ecosystems. Our work in this area involves sampling the extensive paleosols across the SGS site, and evaluating their longterm vegetative and physiographic history by analyzing the stable C isotope signatures of soil organic matter, phytoliths, and CaCO3. These studies provide information on the distribution of C3 and C4 plants during the Holocene soilforming intervals.
Water and
Energy Dynamics:
The SGSLTER project identifies this area as important because water availability is the
key variable driving SGS ecosystem structure and function. Measurements of precipitation,
temperature, microclimate, and soil water are part of the longterm program. A weighing
lysimeter measures daily evapotranspiration. We propose to initiate a new study to
quantify the potential impact of current land use on the SGS by coupling a mesoscale
atmospheric model with an ecosystem dynamics model.
Disturbance:
Two key hypotheses organize SGS LTER work in the area of disturbance. The first is that
smallscale disturbances are the most important source of mortality for the dominant
plant, blue grama, and as such, these disturbances represent a major influence on the
sustainability of the shortgrass steppe. The second is that the distribution of biotic
components with a large bias towards the belowground portion of the system leads to a high
degree of resistance of the ecosystem to aboveground disturbances such as grazing or fire,
but a high vulnerability to disturbances such as cultivation that disturb the soil system.
In addition, the evolutionary adaptations of organisms in the SGS provide additional
resistance to grazing. Continuing SGSLTER experiments evaluate the long and
shortterm effects of smallscale disturbances, grazing, and recovery from cultivation
on plant communities, primary productivity, nutrient cycling, and belowground foodweb
dynamics.
In addition, investigators involved in the SGSLTER program continue to be involved in many synthesis activities. The SGSLTER project will produce a synthesis book this coming year. One of the areas that has made the SGSLTER unique is our emphasis on integrating our knowledge about SGS ecosystems into simulation models. Our models are some of the most widely used worldwide. We plan to continue to use them to encapsulate and test our knowledge about SGS ecosystems.
02/08/01