Central Plains Experimental Range

Understanding the origin and sustainability of shortgrass steppe ecosystems

Central Plains LTER results have led to the conclusion that shortgrass ecosystems are unique in their resistance to climatic variation and grazing

Pattern and process in shortgrass steppe ecosystems at the Central Plains Experimental Range (CPR), located in the eastern plains of northern Colorado, are shaped both by the past history of the site and by current events. In the past 10,000 years, soil and vegetation have developed in concert with a fluctuating climate resulting in changes in C3 and C4 species composition through time. Large herbivores and semiarid conditions have also been important over the same time period, resulting in an ecosystem that is both grazing- and drought- tolerant. In the past 100 years, the cultivation and subsequent abandonment of agricultural fields has had profound influences on current landscapes. In addition, interactions between plants and soil processes have been and continue to be important in generating and maintaining spatial and temporal system heterogeneity. The shortgrass ecosystems that we currently observe are a product of these past events, as well as current factors such as climate, disturbance, and landuse.

The focus of the shortgrass steppe LTER is to understand the factors affecting spatial and temporal variability in the structure and function of shortgrass steppe ecosystems in order to understand the origin and sustainability of these systems. It is only by knowing the role of both past and current events that we can make predictions about the future responses of the system to new events, such as global change.

Research Areas

Paleoenvironmental research: The evolutionary history of the shortgrass steppe is preserved in Holocene paleosoils, soils which formed in ancient landscapes and retain an imprint of the climate and ecosystems which prevailed during their formation.

Our research utilizes the stable C isotope composition of paleosol organic matter, carbonate and opal phytoliths to establish paleovegetation and make inferences to Holocene climate change.

In order to make regional inferences regarding paleoenvironmental conditions, we must determine:

• The regional extent of paleosols,
• The areal extent and provenance of major alluvial and eolian deposits, and
• The relationship of paleosols to their paleo landforms. An understanding of paleosol occurrence, in the context of the geologic episodes that resulted in their burial, is critical to interpreting the prevailing paleoclimate.

Influence of current environmental perturbations: climate and human landuse: We recently conducted a study to evaluate the influence of interannual variability in temperature and precipitation on aboveground net primary production using a CPR data set from 1938. Our analysis suggested that the response of primary production to increased or decreased precipitation and temperature is small relative to the inherent spatial variation in the system. We hypothesize that alteration of species composition as a result of human use will dramatically influence ecosystem responses to climatic variation.

Large herbivores have been an important component of shortgrass steppe ecosystems since the retreat of the Pleistocene glaciers. Currently, livestock grazing occurs over approximately 44 percent of the shortgrass steppe. Long-term grazing intensity treatments and exclosures were established at the site in 1939.

We have conducted a wide range of studies on these treatments, including plant population dynamics, primary productivity, vegetation structure above- and belowground, other consumer populations, and nutrient dynamics, transport and volatile losses as influenced by cattle. Spatial scales of the investigations range from the individual plant to patch, catena and landscape. Research conducted by the USDA Agricultural Research Service and the LTER project has established that grazing has a very small effect on plant species composition and net primary production. In fact, invasions by exotic and native “weed” species are more likely to occur in ungrazed than in grazed treatments. Recently, we found that soil organic matter pools and nutrient availability are only slightly affected by grazing. Our results have led us to the conclusion that shortgrass ecosystems are unique in their resistance to climatic variation and grazing. These two potential disturbances fall within the evolutionary history of the ecosystem. What attributes determine resistance to certain disturbances? Our hypothesis is twofold. First, all of the organisms of the shortgrass steppe evolved under selection pressure and periodic drought. in the case of plants, this selected for species with morphological and physiological adaptations to drought and grazing. Second, shortgrass steppe systems maintain over 90% of their organic matter in soils, with a relatively slow turnover rate. Disturbances such as cattle grazing that target the aboveground plant component have a very small impact on these aspects of ecosystem structure and function.

Our LTER project has recently focused efforts on understanding the recovery of shortgrass steppe ecosystems following cropland abandonment. We also recently evaluated the recovery of vegetation and soils on old fields that had been abandoned for 53 years, on both the CPR and the Pawnee National Grasslands. We found that variability in recovery patterns of vegetation are not easily explained by simple environmental factors, such as climate. Historical factors, such as reseeding and grazing intensity, may provide a better explanation. Total soil organic matter appears to recover extremely slowly relative to the amounts lost due to cultivation (1% recovery relative to 30% losses). Nitrogen mineralization, however, and other indices of active soil organic matter, appear to recover within about 50 years, probably because the turnover time of those pools is small relative to the long recovery period.

Plant—soil interactions:

Shortgrass ecosystems are characterized by low-growing vegetation with a distinct plant-interspace pattern at a small scale. The resulting “islands of fertility” associated with individual plants are similar to patterns documented in desert systems. The small scale pattern of plants and interspaces provides the framework for much of our vegetation and soil process work. We recently examined the role of individual plants and abiotic factors in the development of soil heterogeneity in two intersite studies, one addressing the gradient from shortgrass steppe to tallgrass prairie and the other on the gradient from shortgrass steppe to desert grassland. The shortgrass to tallgrass gradient encompassed the CPR, a long-term research site at Hays, Kansas, and the Konza Prairie LTER site. We found that small-scale heterogeneity induced by plants was most important at the shortgrass steppe site; the identity of the plant had minimal influence on nutrient accumulation or cycling. Alternatively, in the mid- and tallgrass prairie sites, there was minimal heterogeneity induced by plant presence or absence, but there was a significant effect of plant species or lifeform on nutrients, suggesting that interactions between plant communities and ecosystem function are strongly influenced by climate. The shortgrass to desert grassland study used three LTER sites: Central Plains, Sevilleta and Jornada. We are still in the process of analyzing these results, but here we also found that the two dominant grasses at the three sites, Bonteloua gracilis and B. eriopoda, were associated with significant variation in soil properties.

Opportunities

Located in the middle of the moisture and productivity gradient along which the grassland LTER sites lie, the CPR provides unique opportunities to investigate ecosystems whose components have had long evolutionary histories of intense selection pressure by both herbivory and drought. Additionally, it represents the point along the grassland productivity gradient at which the spatial location of individual plants and their life histories have strong influences on the biogeochemistry of the system. Finally, the CPR is located in the portion of the grassland region in which landuse change is most dynamic and the conversion
of land from rangeland to cropland, or vise versa, is heavily influenced by weather, economics, and government programs. This conversion of land from one use to another raises many interesting and challenging ecological questions with important policy implications. Our LTER research group invites interested collaborators to join us in answering these and other questions on shortgrass ecosystems.

For more information: Deborah Coffin, Colorado State University, Range and Ecosystem Science Department, Fort Collins, CO 80523, 303-491-7662, dCoffin@LTER.net.edu