Climate variability is one of several important biotic and abiotic factors regulating ANPP in tallgrass prairie. Multiple factors, including fire, nutrients, grazing by large ungulates, and topography, are involved in the regulation of ANPP in tall-grass prairie. For example, a synthesis of a 20-year record of ANPP at Konza Prairie showed that, in general, early growing-season fire and moderate-intensity grazing increased ANPP (Knapp et al. 1998). Herbivores and fire in some ways have similar effects, with both removing the plant canopy and detritus layers, allowing increased penetration of light to the soil surface, which warms the soil and enhances plant growth. Interactions among these factors are pervasive. For example, topographic position influenced ANPP most strongly in annually burned sites, where deep-soil lowlands were more productive than more shallow-soil uplands. In contrast, at long-term unburned sites, there was little topographic effect on ANPP. The simultaneous presence of these multiple interacting controls on ANPP means that there is considerable temporal variation in limitations on ANPP in tallgrass prairie and that ANPP depends strongly on the degree to which the multiple controlling factors reinforce each other or cancel each other out (Knapp et al. 1998). Climate variability can be viewed as the backdrop against which these other productivity-limiting factors operate.
The climate variability influencing ANPP in tallgrass prairie operates in a larger
spatial context. The Central Plains is a vast west-to-east gradient in grassland species composition and ANPP, with a strong increase in ANPP (r2= 0.90) following the eastward increase in annual rainfall (Kuchler 1974; Sala et al. 1988). For individual locations, relationships between grassland ANPP and rainfall quantity are strongest in the drier western portions of the Central Plains (Epstein et al. 1997) and weaker in the more mesic (and more variable) eastern portions of the Great Plains (Knapp et al. 1998).
The basic patterns of plant community structure in modern grasslands provide the foundation for a mechanistic understanding of productivity responses to climate variability in tallgrass prairie. Native plant communities in the Central Plains grasslands are composed of species from several functional groups (Korner 1994). These include warm-season C4 grasses, cool-season C3 graminoids (grasses and sedges), and a diverse array of other C3 herbaceous dicots (hereafter referred to as "forbs"), nitrogen-fixing leguminous species, and woody species. The C4 grasses consist of relatively few species but are abundant, widely distributed, temporally stable, and they account for roughly 80% of the biomass and canopy cover (Briggs and Knapp 1995; Knapp and Medina 1999). Conversely, forbs constitute a small fraction of the biomass but a large fraction of the species, and forb species are temporally dynamic in presence and abundance (Collins and Glenn 1991; Hartnett and Fay 1998).
The impact of climate variability on ecosystem structure and function depends on species and functional group differences in morphology and physiology, and their resultant ability to track and acclimate to changing climate conditions. Forbs and grasses respond in different ways to interannual variation in rainfall (Briggs and Knapp 2001). Most of the interannual variability in ANPP is due to fluctuations in grass productivity, whereas forbs tend to be unresponsive to these climate elements. This stability of forbs in terms of annual productivity stands in contrast to the dynamic species composition of the forb assemblage through time, potentially reflecting compensatory responses among various members of the forb functional group in response to the prevailing conditions of each growing season.
When studying the influence of climate on ANPP, it is important to carefully consider how the term climate is defined and quantified. Most studies of tallgrass response have used indexes derived from basic meteorological data (i.e., temperature, precipitation), but they also take into account climate/vegetation feedbacks. Briggs and Knapp (1995) used total precipitation (1 January-31 December), growing-season precipitation (1 April-30 September), and summer pan evaporation (1 July-30 September) as their climate indexes. These indexes integrate both temperature and precipitation effects, and in the case of the growing-season precipitation and pan evaporation, they concentrate on the period of the year when ANPP is determined. Using these indexes, Briggs and Knapp (1995) found correlations with ANPP ranging from 0.53 to 0.65 for productivity at all site at Konza. When sites were differentiated by treatment type (burned vs. unburned) and topographic position (upland vs. lowland), correlations as high as 0.87 were found between growing-season precipitation and total ANPP in annually burned uplands. Correlation values for other combinations of climate variable and treatment/position ranged from 0.40 to 0.85.
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