COMPARATIVE GAS-EXCHANGE AND NITROGEN RESPONSES OF THE DOMINANT C-4 GRASS ANDROPOGON-GERARDII AND 5 C-3 FORBS TO FIRE AND TOPOGRAPHIC POSITION IN TALLGRASS PRAIRIE DURING A WET YEAR

High plant species diversity in tallgrass prairie results not from a l arge number of grass species but, rather, from a large number of forb (nongrass, herbaceous) species. Although morphological characteristics of tallgrass prairie forbs differ strongly from those of the grasses and their patterns of abundance vary substantially with burning regime and topographic position, comparatively little is known about the eco logy of persistent tallgrass prairie forbs. We measured leaf-level phy siological characteristics of the dominant C-4 grass, Andropogon gerar dii, and five co-occurring C-3 forb species in response to burning reg ime and topographic position to determine if there were either absolut e or seasonal differences in resource utilization patterns that could contribute to the patterns of forb distribution and abundance in tallg rass prairie. In a wet year, neither fire nor topographic position aff ected leaf-level physiological characteristics in the grass and forbs studied, and thus responses were not consistent with extant patterns o f forb abundance (greater forb abundance in unburned and upland sites) . Water use efficiency in A. gerardii was 40%-170% greater and foliar N concentration was consistently lower in the grass than in the forbs, but there were no distinct or consistent differences between the gras s and forbs in maximum rates of photosynthesis and stomatal conductanc e or xylem pressure potential. Moreover, seasonal patterns of gas exch ange in forbs and the grass were not different, as might be expected w hen comparing C-3 and C-4 species. These results indicate that, when a ssessed at the leaf level, responses of these prairie plants to fire o r topographic position during a wet year are not indicative of their s uccess in these sites. However, seasonal water stress is a typical fea ture of the tallgrass prairie environment and water use patterns indic ate that the prevalence of soil moisture limitations in most years may be a key factor influencing plant distribution and success in this ec osystem.