Summer-Time Carbon and Water Fluxes in Sagebrush Ecosystems Spanning Rain- to Snow-Dominated Precipitation Regimes

Abstract

Sagebrush ecosystems consist of different communities of species and subspecies of sagebrush marked by distinct ecotones along elevation gradients, yet few studies have quantified how ecosystem-scale carbon dioxide (net ecosystem exchange, NEE) and water fluxes (evapotranspiration, ET), as well as their environmental drivers, vary among communities dominated by different sub/species of sagebrush at daily and seasonal time scales. To address this knowledge gap, we measured daytime (6 a.m.–6 p.m.) NEE and ET using a tent chamber and associated environmental drivers in three sagebrush communities spanning an elevation gradient of 1 425–2 111 m at the Reynolds Creek Critical Zone Observatory in southwestern Idaho. Daytime NEE and ET were greatest at the highest elevation (snow-dominated) site during the study period except NEE in June. By late summer, NEE declined by > 80% at the lower (rain-dominated) sites but only 50% at the highest site, compared with maximal values in June. In contrast, ET declined ∼95% in late summer compared with June at all three sites. Ecosystem-scale NEE and ET were mainly controlled by soil moisture and vapor pressure deficit at the rain-dominated sites and by deep soil moisture and air temperature at the snow-dominated site. Cumulative (June−August) modeled daytime NEE was greatest at the midelevation site, whereas cumulative daytime ET was greatest at the highest-elevation site. Ecosystem models often assume that sagebrush landscapes are homogeneous and do not differ in fluxes and controls, yet our data demonstrate that there are fundamental differences in CO2 and water fluxes and their controls among different shrub communities that should be accounted for in these models.