Photosynthetic traits in C3 and C4 grassland species in mesocosm and field environments

The North American tallgrass prairie is composed of a diverse Mix of C-3 and C-4 plant species that are subject to multiple resource limitations. C-4 grasses dominate this ecosystem, purportedly due to greater photosynthetic capacity and resource-use efficiency associated with C-4 photosynthesis. We tested the hypothesis that intrinsic physiological differences between C-3 and C-4 species are consistent with C-4 grass dominance by comparing leaf gas exchange and chlorophyll fluorescence variables for seven C-4 and C-3 herbaceous species (legumes and non-legumes) in two different settings: experimental mesocosms and natural grassland sites. In the mesocosms, C-4 grasses had higher photosynthetic rates, water potentials and water-use efficiency than the C-3 species. These differences were absent in the field, where photosynthetic rates declined similarly among non-leguminous species. Thus, intrinsic photosynthetic advantages for C-4 species measured in resource-rich mesocosms could not explain the dominance of C-4 species in the field. Instead, C-4 dominance in this ecosystem may depend more on the ability of the grasses to grow rapidly when resources are plentiful and to tolerate multiple limitations when resources are scarce. (C) 2007 Elsevier B.V. All rights reserved.