Past and current research provides an important base and source of information for accomplishing our LTER goals. Our core research emphasizes: the relations between the hydrologic cycle and primary production evaluation of key microbial responses, studies of plant succession, plant and animal population dynamics, processes associated with the aggradation or degradation of soil organic matter, the response of plant community and soil processes to long-term cattle grazing, the nature of the erosion cycle and its influence on redistribution of matter, nutrients, and pedogenic processes, the influence of atmospheric gases, aerosols, and particulates on primary production and nutrient cycles.

Our conceptual framework asserts that one must consider the interplay of several forces, which occur at a variety of spatial and temporal scales, in order to understand the structure and function of SGS ecosystems. There are five components that we have identified as particularly important in shaping the SGS: climate, natural disturbance, physiography, human use, and biotic interactions (Fig. 2.2 ). Below, we provide an overview of the SGS in order to frame the unique interactions of these components,and then elaborate on each in turn.

1. Overview

The 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 (Mack and Thompson 1982).

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 low- growing plants that are either tolerant or resistant to grazing and drought. The most common plants are two species of C₄ grasses, a cactus, and several species of dwarf-shrubs, all of which have a large fraction of their biomass belowground. The vegetation is strongly dominated by Bouteloua gracilis (blue grama), the grass species with the greatest tolerance to grazing and drought (Weaver and Albertson 1944). This species contributes 60-80% of the plant cover, biomass, and net primary productivity of shortgrass ecosystems. 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).

This distribution of biotic components, with a large bias towards the belowground portion of the system, leads to important predictions about the response of SGS ecosystems to disturbances: those events that largely affect aboveground plant biomass, such as fire or grazing, are not likely to cause changes in total organic matter, or large alterations in the biotic community ( Fig. 2.5 ). Recovery from such disturbances may occur in as few as 2-3 years due to the tolerance of the biota and the high rate of turnover of aboveground plant biomass. In other types of grasslands, there may be shifts in plant species composition in response to grazing or drought, leading to alterations in biota, and ecosystem structure and function. However, the evolutionary adaptations of organisms in the SGS provide additional resistance to grazing.

2. Conceptual Framework: Five important components

Climate Atmospheric processes, including radiation inputs, determine the water and energy balance of the system. Precipitation is the key driving variable for the temporal variability of biota in SGS ecosystems (Lauenroth and Sala 1992). Interannual variation in precipitation is high, but despite this, the seasonal pattern is quite reliable, with a peak in May and June and a dry period in December, January, and February (Lauenroth and Burke 1995). Spatially, atmospheric inputs are variable on the scale of kilometers.

Our research indicates that the biota have a large response to interannual and seasonal variations in precipitation (Lauenroth and Sala 1992). Perennial C₄ bunchgrasses maintain their presence and cover in all years, changing slowly with dry and wet periods. By contrast, C₃ grasses and annual forbs respond significantly to individual wet or dry years. Abundance and species composition of small mammals reflect precipitation-induced changes in vegetation structure (Grant et al. 1977) and resulting fluctuations in production of seeds and arthropods (Crawford 1991). Ecosystem functions such as net primary productivity (Lauenroth and Sala 1992), net N mineralization (Hook and Burke in prep), and trace gas flux (Mosier et al. 1991) are very responsive to interannual and seasonal variation in precipitation.

In distinguishing our 5 components, we exclude both drought and grazing from our list of natural disturbances due to the long evolutionary history of these selection pressures in the SGS. The climate of the SGS has fluctuated among subhumid, semiarid, and arid over the past 10,000 years (Table 2.1). At the time of settlement, the SGS was the home of native Americans and large numbers of bison, prairie dogs, elk, pronghorn, bighorn sheep, and deer. These herbivores had been present since the retreat of the Pleistocene glaciers, 10,000 years ago. Conceptually, we list herbivory except by cattle under biotic interactions, and cattle grazing under human use; drought is dealt with under atmospheric processes.

Physiographic features provide the parent material for soils and determine the rates of development and the mineralogical composition of the resulting soils. Temporal variability in physiography is very slow, operating on the scale of decades to centuries. Spatially, physiography is variable on the scale of 0.1-100 kilometers. Three aspects of physiography are most important in influencing ecosystem structure and function: landscape position, soil depth, and soil texture. In early years of the SGS LTER, we conceptualized the influence of physiography on ecosystems as a catenary landscape sequence, along which soils became finer and deeper downslope (Schimel et al. 1985). Recently we have found that the geomorphic and pedologic history of the region has resulted in a complex array of physiographic units across landscapes (Yonker et al. 1988).

Soil texture and depth play an important role in controlling water availability, a key constraint for the biota of SGS ecosystems. In sandy soils, water moves to greater depths and surface evaporation is lower than in fine-textured soils, leading to lower bare-soil evaporation rates and different vegetation structure (Noy-Meir 1973, Lane 1995). Deep sandy soils often support large shrubs which, in turn, strongly influence the presence and abundance of small mammals (Stapp in prep). Soil texture also exerts control over decomposition and N mineralization ( Fig. 1.4 ). Areas of fine texture are characterized by higher SOM accumulation, due to the lower mass-specific decomposition rates, and leading to higher N mineralization (Hook and Burke in prep) and N₂O flux (Mosier et al.submitted).

Human use has been and continues to be a major force influencing the structure and function of SGS ecosystems. Grazing by cattle is a major influence; virtually every hectare of native SGS is currently being grazed. Our LTER site has the oldest, largest, and most numerous livestock grazing exclosures in the entire shortgrass region. Our past LTER research indicates that shortgrass ecosystems are relatively resistant to negative effects of grazing (Milchunas et al. 1989). Long-term (50+ years) heavy grazing during the growing season has small effects on plant species composition and diversity and results in increased importance of blue grama( Fig. 2.6 )(Milchunas et al. 1989). Furthermore, heavy grazing appears to reduce the vulnerability of the plant community to invasion by exotics ( Fig. 1.18 , Milchunas et al. 1992).

The SGS has been subjected to several periods of conversion to croplands and subsequent periods of cropland abandonment. Sixty percent of the SGS is currently being cultivated (Lauenroth et al. 1994) and up to 30% more was cultivated at some time in the past 100 years. As a result, a component of our studies deals with the recovery of abandoned croplands, although our research focuses on native steppe. Cropping drastically alters biological diversity (e.g. Holland and Coleman 1984), reduces SOM (Haas et al. 1957, Burke et al. 1989), and alters the temporal and spatial distribution of plant biomass. Our work on abandoned fields indicates that while vegetation, nematodes, active SOM, CH₄ consumption and N₂O emissions may recover within 50 years, total SOM does not.

Beyond the obvious role of organisms as elements of the structure and function of shortgrass ecosystems, interactions among and between organisms and the other 4 forces (climate, natural disturbance, physiograpy, and human use) are the critical determinants of both the current state and the future vulnerability of SGS ecosystems. Biotic interactions provide the mechanisms whereby ecosystem structure and function are affected by and, in turn, affect climate, disturbance, physiography, and human use. Biotic interactions are the key focus of our LTER research ( Fig. 2.3).