Local to regional scale energy balance consequences of widespread mortality in piñon-juniper woodlands

Abstract:

The southwestern U.S. experienced an extended drought from 1999-2002 which led to widespread coniferous tree mortality throughout New Mexico, Arizona, Utah and Colorado. Piñon-juniper (PJ) woodlands were extremely vulnerable to this drought, experiencing 40 to 95% mortality of piñon pine (Pinus edulis) and 2-25% mortality of juniper (Juniperus monosperma) in less than 3 years. Understanding the response trajectories of these woodlands is crucial given that climate projections for the region suggest that episodic droughts, and associated conifer mortality, are likely to increase in frequency and severity in the coming century. We used a combination of eddy covariance, high-resolution remotely sensed datasets, and sap flow made at an undisturbed PJ woodland (control) in central New Mexico and at a manipulation site within 2 miles of the control where all piñon trees greater than 7 cm dbh were girdled (decreasing LAI by ~ 1/3) to quantify the response of ecosystem carbon, water and energy fluxes in PJ woodlands to piñon mortality.

The girdled site has gradually become both warmer and drier following piñon mortality (annual average temperatures have been 0.6 – 1.2 C warmer than the control site over past 5 years). Our analyses suggest the mortality-triggered decrease in aerodynamic conductance is largely responsible for the increase in surface temperature. In addition, both carbon and water cycling in the girdled site have been more sensitive than the control site to the extreme drought experienced from 2011-2013. We compare these results from our manipulation experiment to: 1) observations in PJ control site and surrounding area following 2013 die-off triggered by bark beetles, 2) responses of MODIS land surface temperature and leaf area index in NM PJ woodlands to climatic variables before and after mortality, and 3) output from CLM4 run in point mode for PJ woodlands where we modified percent vegetation/bare ground cover and quantified the model sensitivity of land surface temperature and energy balance to this induced mortality. We discuss the results in terms of feedbacks triggered by these significant mortality events on the climate system.