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

Abstract

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 (Psipd), greater leaf photosynthetic rates (Anet) and stomatal conductance (gs), 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.

Partitioning of evapotranspiration and its relation to carbon dioxide exchange in a Chihuahuan Desert shrubland

“…Likewise, Fierer and Schimel (2002) found that frequent exposure to drying-rewetting events decreased the amount of CO 2 released upon rewetting. Respiration occurred throughout this study, even when the soil was very dry, most likely due to the continued activity of perennial plants (Huxman et al, 2004b). Reflecting the plant activity alone, T and GEP had similar responses in time, though peaks in GEP appeared to precede T by a day or two.…”

“…Consequently, we caution that the computed R and GEP presented below likely contain considerable error. This uncertainty is not unexpected owing to the myriad of processes, not accounted for in the standard respiration models, that may influence total CO 2 efflux in semiarid areas and which include (1) the physical displacement of CO 2 following rainfall event, (2) the biological activity of both aboveground and belowground autotrophic biomass, (3) the microbial activity in close association with roots and the microbial (Huxman et al, 2004b;Fisher and Whitford, 1995;Emmerich, 2003).…”

“…This likely derives from changes in the diurnal pattern of instantaneous photosynthetic gas exchange relative to maximum achievable photosynthetic rates of individual leaves within a canopy that influence different patterns of leaf water loss. Likewise, E and R both spiked in response to moisture pulses with a pattern of R peaking slightly after E. The slight differences in these processes in time could arguably be due to either increased resistance for CO 2 diffusion from the deeper soil layers as water infiltrates the surface soil immediately following rainfall (Conant et al, 2004) and/or the added contribution of plant respiration to the total CO 2 efflux, which would show a peak in activity several days after the event, which is likely to be in some fairly constant proportion to the GEP response (Huxman et al, 2004b). Overall, GEP was well correlated with T, but R and E were not (Figure 8).…”

Emerging Issues in Rangeland Ecohydrology: Vegetation Change and the Water Cycle

“…In addition, recent work has examined the relationship between exotic grasses and rainfall pulses (Huxman et al 2004;Clarke et al 2005). But very little work has directly examined how invasive species might be influencing the water cycle in these environments.…”

The impact of invasion and subsequent removal of an exotic thistle, Cynara cardunculus, on CO2 and H2O vapor exchange in a coastal California grassland

“…Similarly, evapotranspiration (ET) represents the sum of stomata-mediated transpiration and evaporation from soil and canopy surfaces and is an important link in the movement of water between ecosystems and the atmosphere. Chamber-based measurements of NEE and ET have been used to demonstrate the impacts of invasive plants on above-and below-ground carbon cycling and ecosystem water balance in semi-arid grasslands of southern Arizona (Huxman et al 2004;Potts et al 2006a) and in the Great Basin (Prater et al 2006).…”