RESPONSES OF A C-4 GRASS AND 3 C-3 FORBS TO VARIATION IN NITROGEN ANDLIGHT IN TALLGRASS PRAIRIE

In tallgrass prairie, high plant species diversity results not from a large number of grass species, but from a large number of forb (nongra ss, herbaceous) species. Forbs exhibit morphological, life history, an d ecophysiological characteristics that contrast sharply with those of the dominant C-4 grasses. Success of the subdominant forbs varies str ongly with topographic position and burning regime, and landscape scal e patterns of abundance are well documented. But comparatively little is known about the mechanisms determining these patterns in persistent tallgrass prairie forbs. To elucidate these mechanisms, (1) leaf-leve l physiological characteristics of the dominant C-4 grass, Andropogon gerardii, and four co-occurring C-3 forbs were measured in response to natural and experimentally manipulated gradients of N availability, a nd (2) seasonal light environments of forbs in contrasting topographic positions and burning regimes and their morphological and physiologic al responses in these environments were compared to determine whether resource availability and utilization patterns contributed to patterns of forb distribution and abundance. The effects of burning regime and topographic position on maximum rate of photosynthesis (A) and stomat al conductance to water vapor (g) measured at the leaf level were not consistent with patterns of forb abundance. Nitrogen did not appear to limit forb physiological processes, even though increased N availabil ity resulted in higher tissue N concentrations and greater biomass. Th ere was no consistent increase in A or decrease in g in response to fe rtilization. However, A at low light levels was as much as 67% higher in fertilized Vernonia baldwinii and A. gerardii compared to unfertili zed plants. Greater light availability to forbs in the canopy was asso ciated with lower grass biomass production in uplands compared to lowl ands and in unburned compared to burned sites. Forbs did not appear to adjust morphologically (leaf area and plant height) to different ligh t environments at different sites. As a result, as much as 90% of forb leaf area in the burned lowland was displayed in low light, whereas a s little as 30% of forb leaf area was in low light in the uplands at m idseason. Estimates of potential whole-plant carbon uptake, based on l eaf area distribution relative to available light and A as a function of light availability, agreed well with patterns of forb abundance and production. Differences in light availability may account for much of the variability in forb abundance related to burning regime and topog raphic position by limiting carbon gain in forbs more in burned lowlan ds than in other sites.