Gas exchange strategy differs among co-occurring C4 grass species and is highly sensitive to water availability

Background/Question/Methods

Spatial and temporal variation in water availability is known to affect semi-arid and arid grassland productivity. Previous studies have shown that C4 grasses co-occurring in a community may experience differences in water access and adopt various gas exchange strategies, water use strategies, and associated functional traits. For example, grasses adapted to low mean annual precipitation environments tend to have smaller, denser leaves and exhibit isohydric stomatal responses, with stomata closing in response to low water availability in order to maintain leaf water potential. Evolutionary lineage and C4 decarboxylating subtype may also play roles in determining traits associated with water use and physiology but have rarely been explored in field settings. Additionally, vapor pressure deficit (VPD) has often been overlooked in C4 grass drought response studies, even though atmospheric dryness is known to affect stomatal behavior and photosynthetic performance for many species. We explored gas exchange and water relations of five species of semi-arid C4 grasses differing in photosynthetic lineage, biochemical subtype, and height across a natural aridity gradient over three years. We also accessed whether patterns in bulk leaf δ13C and specific leaf area (SLA) tracked with water environment or VPD experienced by these species.

Results/Conclusions

C4 photosynthetic responses of grasses were highly coupled to temporal and geographic differences in water availability (p< 0.001). Photosynthesis (Anet) and stomatal conductance (gsw) were much lower at the driest site in 2019 (p< 0.001). Stomatal conductance (gsw) was unresponsive to VPDleaf regardless of species. Stomatal response of E. sericea, the only Panicoideae species, was decoupled from declines in midday water potential (Ѱleaf), with Anet and gsw unchanging across the gradient. All other species exhibited higher Anet and gsw when water was more available, and were responsive to Ѱleaf. Eriochloa sericea had the least negative midday and pre-dawn Ѱleaf, while the shortgrasses E. pilosum and Hilaria belangeri experienced lower water availability across the gradient (p< 0.001). Patterns in bulk leaf δ13C differed by species (p< 0.001) and generally reflected patterns in midday Ѱleaf by year and location (p< 0.001). Specific leaf area (SLA) differed by species (p< 0.001) but did not track with water availability. Grasses at our sites were partitioning into two water-response strategies: 1) drought escape via deep water access and maintenance of low gas exchange functioning across the gradient (E. sericea) and 2) declines in gas exchange coupled to declines in water availability (all others).