Inter- and under-canopy soil water, leaf-level and whole-plant gas exchange dynamics of a semi-arid perennial C4 grass

It is not clear if tree canopies in savanna ecosystems exert positive or negative effects on soil moisture, and how these might affect understory plant carbon balance. To address this, we quantified rooting-zone volumetric soil moisture (θ25 cm), plant size, leaf-level and whole-plant gas exchange of the bunchgrass, bush muhly (Muhlenbergia porteri), growing under and between mesquite (Prosopis velutina) in a southwestern US savanna. Across two contrasting monsoon seasons, bare soil θ25 cm was 1.0–2.5% lower in understory than in the intercanopy, and was consistently higher than in soils under grasses, where θ25 cm was similar between locations. Understory plants had smaller canopy areas and volumes with larger basal diameters than intercanopy plants. During an above-average monsoon, intercanopy and understory plants had similar seasonal light-saturated leaf-level photosynthesis (Anet-sat), stomatal conductance (gs-sat), and whole-plant aboveground respiration (Rauto), but with higher whole-plant photosynthesis (GEPplant) and transpiration (Tplant) in intercanopy plants. During a below-average monsoon, intercanopy plants had higher diurnally integrated GEPplant, Rauto, and Tplant. These findings showed little evidence of strong, direct positive canopy effects to soil moisture and attendant plant performance. Rather, it seems understory conditions foster competitive dominance by drought-tolerant species, and that positive and negative canopy effects on soil moisture and community and ecosystem processes depends on a suite of interacting biotic and abiotic factors.