Drought-induced piñon mortality alters the seasonal dynamics of microbial activity in piñon–juniper woodland

Piñon (Pinus edulis) – Juniper (Juniperus monosperma) (PJ) woodlands cover 17+ million hectares in the western USA. These woodlands have been particularly sensitive to recent changes in climate, with drought induced die-offs of piñon occurring across 1.5 million ha, since 2000, triggering large changes in the structure and function of these biomes. To assess the effects of this large scale mortality on soil processes, we analyzed rhizosphere extracellular enzyme activity (EEA) and fungal biomass in soil samples collected beneath tree canopies at two piñon–juniper woodland sites: one site where piñons were experimentally killed by girdling, and a paired intact site with no piñon mortality that served as a reference. We also quantified soil water content (SWC) from soil samples, and sap flow density fluxes as an indicator of tree physiological status. Soil EEA patterns varied as functions of the identity of the closest competitor trees, host-tree physiological status, and SWC. At the girdled site, we observed higher plant cell wall related decomposition activities under live juniper canopies, compared to dead piñon trees. In contrast, at the control site, we observed higher plant cell wall decomposition rates under live piñon rather than juniper canopies, particularly with higher soil moisture availability. At the site level, ordination plots for intact PJ woodlands showed a decreasing trend in microbial cell wall decomposition activity as soil water availability, and fungal biomass increased. We observed the opposite trend at the girdled site. Overall, these results suggest that widespread piñon mortality significantly affects the functional behavior of rhizosphere microorganisms at multiple scales, in part by shifting the focal substrates of microbial community decomposition.