Eli R. Perez-Ruiz, Enrique R. Vivoni and Osvaldo E. Sala
Arizona State University
Though dryland ecosystems have a fundamental role influencing the global carbon cycle, the seasonal variability of their water and carbon dynamics is not well understood. In this study, we analyze a long-term micrometeorological flux record (2011-2019) in a mixed shrubland of the Jornada Experimental Range (JER) to determine the relation between water availability and net ecosystem production (NEP). The ecosystem exhibits a bimodal regime in the water balance, with a wet season dominated by a large rainfall (R) leading to high evapotranspiration (ET) and streamflow (Q), and a dry season with low R when ET depends on subsurface water storage. We show that subsurface water is carried over between the wet and dry seasons, thus sustaining carbon fluxes during the dry season when NEP is highest. This surprising result is due to the significant gross primary productivity and low ecosystem respiration during the dry season. The net effect is that the mixed shrubland is a net annual sink of carbon regardless of the amount of rainfall during the wet season. However, the proportional distribution of wet and dry season rainfall plays a key role in defining the seasonal contribution of net ecosystem productivity.