Linking Climate Change and Mortality in Piñon-Juniper Woodlands, from Leaf to Ecosystem

As global climate changes, the Southwestern US is predicted to experience more frequent and intense drought events. Extreme droughts can drive decreases in both physiological and ecosystem function, and can result in widespread tree mortality. Piñon-juniper (PJ) woodlands are a prevalent ecosystem in the region, co-dominated by two tree species, piñon (Pinus edulis) and juniper (Juniperus monosperma). Drought-induced piñon mortality has occurred over the past few decades, coinciding with outbreaks of a piñon-specific bark beetle. Piñon and juniper have different hydraulic strategies (isohydry and anisohydry, respectively) that should affect the way each species responds to drought. In this dissertation, I used PJ woodlands to quantify both physiological and ecosystem effects of drought and mortality, and the ways in which they interact. First, I focused on a PJ woodland where all of the large piñon had been girdled to simulate drought-related mortality. I looked for evidence of competitive release in both species by measuring photosynthetic and hydraulic parameters in the girdled plot vs. an intact control plot. I did not find evidence of competitive release in response to piñon mortality, likely due to the multi-year drought that followed the girdling event. I next examined whether hydraulic strategy affected piñon and juniper responses to two components of drought, atmospheric and soil moisture drought. I used sap flow measurements at an intact control site to quantify tree responses to soil water potential and vapor pressure deficit (VPD). Over seven years, both species were more sensitive to drying soil than high VPD, and both species were similarly affected by concurrent dry soil and high VPD. Finally, I quantified tree and ecosystem responses to a three year drought and the natural piñon mortality event that followed, combining sap flow and eddy covariance data. Tree and ecosystem function both decreased during drought, and ecosystem net carbon uptake decreased after mortality, although wet conditions following mortality offset this decrease. Taken together, these findings suggest that increasing drought and associated mortality in the region will decrease productivity, but climate conditions following drought will ultimately determine whether the ecosystem recovers or shifts to an alternate state