Response of net ecosystem gas exchange to a simulated precipitation pulse in a semi-arid grassland: The role of native versus non-native grasses and soil texture

Physiological activity and structural dynamics in arid and semi-arid ecosystems are driven by discrete inputs or "pulses" of growing season precipitation. Here we describe the short-term dynamics of ecosystem physiology in experimental stands of native (Heteropogon contortus) and invasive (Eragrostis lehmanniana) grasses to an irrigation pulse across two geomorphic surfaces with distinctly different soils: a Pleistocene-aged surface with high clay content in a strongly horizonated soil, and a Holocene-aged surface with low clay content in homogenously structured soils. We evaluated whole-ecosystem and leaf-level CO₂ and H₂O exchange, soil CO₂ efflux, along with plant and soil water status to understand potential constraints on whole-ecosystem carbon exchange during the initiation of the summer monsoon season. Prior to the irrigation pulse, both invasive and native grasses had less negative pre-dawn water potentials (Ψ_pd), greater leaf photosynthetic rates (A_net) and stomatal conductance (g_s), and greater rates of net ecosystem carbon exchange (NEE) on the Pleistocene surface than on the Holocene. Twenty-four hours following the experimental application of a 39 mm irrigation pulse, soil CO₂ efflux increased leading to all plots losing CO₂ to the atmosphere over the course of a day. Invasive species stands had greater evapotranspiration rates (ET) immediately following the precipitation pulse than did native stands, while maximum instantaneous NEE increased for both species and surfaces at roughly the same rate. The differential ET patterns through time were correlated with an earlier decline in NEE in the invasive species as compared to the native species plots. Plots with invasive species accumulated between 5% and 33% of the carbon that plots with the native species accumulated over the 15-day pulse period. Taken together, these results indicate that system CO₂ efflux (both the physical displacement of soil CO₂ by water along with plant and microbial respiration) strongly controls whole-ecosystem carbon exchange during precipitation pulses. Since CO₂ and H₂O loss to the atmosphere was partially driven by species effects on soil microclimate, understanding the mechanistic relationships between the soil characteristics, plant ecophysiological responses, and canopy structural dynamics will be important for understanding the effects of shifting precipitation and vegetation patterns in semi-arid environments.