The understory and overstory partitioning of energy and water fluxes in an open canopy, semiarid woodland

Eddy flux studies have traditionally focused on total ecosystem exchanges of energy and water by making measurements in the well-mixed surface layer, but this approach does not provide information about the partitioning of the total ecosystem fluxes between overstory and understory sources and sinks. In more open canopy environments, information about partitioning of fluxes is often required in order to understand the relative importance and functioning of key ecosystem components and their response to climate forcing. In this paper, we present results from a series of experiments carried out in a riparian mesquite (Prosopis velutina) woodland. Three eddy covariance systems were deployed before, during, and after the onset of the summer rainy season to measure energy and water fluxes. One eddy covariance system was installed on a tower to measure whole ecosystem fluxes. The other two were installed at a height of 2 m, one in a relatively closed understory patch and the other in a more open understory patch. Our results indicate that the understory and overstory moisture sources were mostly decoupled. The trees apparently had access to deep moisture sources, and thus, their water use was relatively insensitive to local precipitation. In contrast, the contribution of the understory to the total ecosystem fluxes was highly variable due to the presence or absence of near-surface soil moisture.