Distribution of C4 grass species has previously been linked to biochemical pathway (subtype) and phylogenic lineage, with certain C4 groups more common in drier regions. Semi-arid savannas in North America support a diverse assemblage of C4 grasses and few studies have characterized differences in ecophysiological performance by unique subtype or lineage, instead treating C4 species as one distinct functional group. To explore relationships between physiology, anatomy, and grass subtype in response to aridity, we measured a suite of photosynthetic gas exchange parameters, water status (leaf water potential), morphology (specific leaf area, leaf width), leaf δ13C and δ18O in a variety of C4 grasses. Five species representing different combinations of biochemical subtype, phylogenetic lineage, and physiognomy (Aristida wrightii [NADP-ME, Aristoideae, midgrass], Bouteloua curtipendula [NAD-ME/PCK, Chloridoideae, midgrass], Erioneuron pilosum [NAD-ME, Chloridoideae, shortgrass], Eriochloa sericea [PCK, Panicoideae, midgrass], and Hilaria belangeri [PCK, Chloridoideae, shortgrass]) were studied across three sites representing a natural precipitation gradient on the Edwards Plateau, Texas. We predicted that species belonging to the NAD-ME pathway or the Chloridoideae family would exhibit traits related to drought tolerance and have a photosynthetic advantage over species belonging to other biochemical subtypes.
Results/Conclusions
At the driest site, midday water potential, stomatal conductance, and net photosynthesis were lowest while δ18O was enriched (p<.001). Shortgrasses (H. belangeri and E. pilosum) had more negative water potentials (p=.001) and Anet, perhaps related to their shallower rooting depth. In contrast to previous studies, instantaneous water use efficiency (WUEi) declined for all species along the natural aridity gradient (p<.001) and was most negative for E. pilosum (p=.003). This species also exhibited higher stomatal conductances (p<.001). This was an advantage at the wetter sites but not at the driest site, where Anet was low. Aristida wrightii maintained high WUEi and Anet at the driest site. We attributed this to unique features of morphology associated with subfamily, including narrower leaves, lower SLA (p<.001), and more positive leaf δ13C, characteristics often associated with high water use efficiency under drought. PCK subtype species (H. belangeri and E. sericea) exhibited more stomatal control and lower photosynthetic rates across the three sites. Although C4 grasses are assumed to be highly productive in more arid environments, our data suggest that increased drought on the Edwards Plateau might compromise photosynthetic assimilation. This research also highlights the importance of considering differences in form, biochemical subtype, and lineage as they relate to ecophysiological strategies of C4 grasses in the field.