Responses of C4 grasses to aridity reflect species-specific strategies in a semiarid savanna

The C₄ Poaceae are a diverse group in terms of both evolutionary lineage and biochemistry. There is a distinct pattern in the distribution of C₄ grass groups with aridity; however, the mechanistic basis for this distribution is not well understood. Additionally, few studies have investigated the functional strategies of co-occurring C₄ grass species for dealing with aridity in their natural environments. We explored the coordination of leaf-level gas exchange, water use, and morphology among five co-occurring semiarid C₄ grasses belonging to divergent clades, biochemical subtypes, and size classes at three sites along a natural aridity gradient. More specifically, we measured predawn and midday water potential, stomatal conductance, water use efficiency, and photosynthesis. Leaf tissue was also collected for the analysis of stable isotopes of carbon and oxygen as well as for measurement of specific leaf area (SLA) and leaf width. Species differences in responsiveness of stomata to changes in vapor pressure deficit (VPD) were also assessed. It was expected that NAD-me species would maintain higher rates of photosynthesis, higher water use efficiency, and have more responsive stomata than other co-occurring species based on observed biogeographic patterns and past greenhouse studies. We found that Aristidoideae and Chloridoideae NAD-me-type grasses had greater stomatal sensitivity to VPD, consistent with a more isohydric strategy. However, midgrasses had both greater apparent water access and water use efficiency, regardless of subtype or lineage. PCK-type species had less responsive stomata and maintained lower levels of photosynthesis with increasing aridity. There were strong interspecific differences in δ¹³C, leaf width, and SLA; however, these were not significantly correlated with water use efficiency. C₄ grasses in our study did not fit discretely into functional groups as defined by lineage, biochemistry, or size class. Interspecific differences, evolutionary legacy, and biochemical pathway are likely to interact to determine water use and photosynthetic strategies of these plants. Control of water loss via highly responsive stomata may form the basis for dominance of certain C₄ grass groups in arid environments. These findings build on our understanding of contrasting strategies of C₄ grasses for dealing with aridity in their natural environments.