Productivity patterns of C3 and C4 functional types in the U.S. Great Plains

Ecology, April, 1997 by H.E. Epstein, W.K. Lauenroth, I.C. Burke, D.P. Coffin

INTRODUCTION

Vegetation patterns are expected to be altered by global climate change (Melillo et al. 1991, Field et al. 1992, Post 1993). One approach for predicting the response of vegetation is to apply a climate change scenario to existing vegetation-environment relationships (Melillo et al. 1991). Large-scale relationships between vegetation and environmental variables have traditionally used biomes or vegetation types as units of aggregation (Holdridge 1947, Whittaker 1975, Prentice 1990). However, there may be considerable variation in the structure of vegetation within a given biome or type (Smith et al. 1993). Examining more resolute units of vegetation (i.e., less aggregated than biome or vegetation type) at large spatial scales will increase our understanding of the environmental controls over plant community structure and may improve our capacity to predict vegetation responses.

The use of species to describe regional vegetation patterns is constrained by availability of data and the limited ranges of species (Smith et al. 1993). Large-scale questions are best addressed by grouping species with similar characteristics into plant functional types (Box 1981, Prentice et al. 1992, Smith et al. 1993, Lauenroth et al. 1996). Aggregating species into functional types creates a component of vegetation that potentially ranges over regions and continents (Woodward 1987) yet preserves the physiological characteristics of the constituent species (Chapin 1993). The plant functional type is gaining widespread acceptance among ecologists interested in the response of ecosystems to global change (Woodward 1987, Schimel 1993).

A major distinction among species in temperate grassland communities of North America is photosynthetic pathway (Teeri and Stowe 1976, Brown 1993). Differential temperature responses result in a geographic separation of species having the [C.sub.3] or [C.sub.4] photosynthetic pathway, with [C.sub.4] species generally occupying warmer locations. However, the extent to which environmental factors relate to the production of [C.sub.3] and [C.sub.4] functional types at regional scales has yet to be determined.

Several studies have attempted to relate the abundances of [C.sub.3] and [C.sub.4] functional types to abiotic variables within central North America (Teeri and Stowe 1976, Sims et al. 1978, Boutton et al. 1980, Barnes et al. 1983, Fan 1993). The spatial extent of these studies varied from portions of states (Boutton et al. 1980, Barnes et al. 1983, Fan 1993) to the central and western United States (Sims et al. 1978) to most of North America (Teeri and Stowe 1976). Data sources have also varied in these correlational analyses from regional flora (Teeri and Stowe 1976) to biomass samples (Sims et al. 1978, Boutton et al. 1980, Barnes et al. 1983) to rangeland surveys (Fan 1993). Most studies on the abundance and distribution of [C.sub.3] and [C.sub.4] plants for grasslands throughout the globe have relied on floristic data (Rundel 1980, Werger and Ellis 1981, Hattersley 1983, Cavagnaro 1988). An exception being Tieszen et al. (1979) who used isotopic data as an indicator of proportional production along an altitudinal gradient in Kenya. No study, however, has assessed the productivities of [C.sub.3] and [C.sub.4] plants throughout the Great Plains of the United States or any similar large temperate grassland region using an extensive production data set. Developing quantitative relationships between the productivities of these functional types and environmental variables is crucial to understanding the dynamics of plant communities in the Great Plains and other grassland regions.

The main goal of our analysis is to quantify the production of [C.sub.3] and [C.sub.4] functional types in terms of the major environmental factors that govern their distribution and abundance in the Great Plains. Our specific objectives are to (1) quantify [C.sub.3] and [C.sub.4] production in terms of climate and soil variables; (2) isolate effects of climate on [C.sub.3] and [C.sub.4] production by maintaining a constant soil texture; and (3) isolate effects of soil texture on [C.sub.3] and [C.sub.4] production by examining specific locations in the Great Plains with different average climatic conditions.

METHODS

We constructed a spatial database of plant species production and environmental variables for the Great Plains of the United States. Plant species production data were collected from USDA Natural Resource Conservation Service (NRCS) range site descriptions. NRCS range sites represent the potential native plant community of well-managed grazing lands in the absence of abnormal disturbances and other management regimes (USDA 1967). Range sites are unique in the combination of total annual yield and plant community composition. Range site descriptions include the relative production (percent) attributed to each species in the plant community, as well as total community production in favorable, normal, and unfavorable years. These data are based on total growth during a single growing season and determined by harvesting plant material at various growth stages (USDA 1967). The NRCS uses range sites to inventory rangelands and to assist users in their management and conservation. We collected and automated [approximately equal to]1700 range site descriptions for our database.