Drought is a recurring phenomenon in the Southwestern US, and is expected to increase in both frequency and severity in the twenty-first century. Collectively these biomes store a significant amount of carbon on a regional scale, ranging from a slight source of carbon in low elevation grasslands (~50 g C m-2 y-1) to a significant carbon sink in high elevation coniferous forests (~-350 g C m-2 y-1. It is therefore of great interest to quantify how resilient ecosystem function across the range of biomes represented in the region is to these severe droughts, both in terms of response and recovery. We examine this using an 11-year record from 2007-2017 of continuous measurements of carbon, water and energy fluxes made across a network of flux towers along an elevation/aridity gradient in New Mexico, the New Mexico Elevation Gradient (NMEG), in biomes that include desert grassland, creosote shrubland, juniper savanna, piñon-juniper woodland, and ponderosa pine and subalpine mixed conifer forests. The biomes across this gradient experienced a severe drought from 2011-2013, with climate more similar to the 40 year averages both before and after these years.
Results/Conclusions
The decrease in C sequestration between non-drought and drought years in all other biomes across the NMEG was significant (100, 250, 120, 40 and 60 g C/m2 less sequestered in mixed conifer, ponderosa pine, PJ, creosote and desert grassland, respectively). This represents a 30, 45, 50, and 80 % decrease in carbon sink strength in mixed conifer, ponderosa pine, PJ and shrub sites, respectively. The desert grassland site is the only site that has recovered and exceeded the carbon fluxes measured at this site prior to the drought. We use the responses of net ecosystem exchange of carbon (NEE) and its components (gross primary productivity (GPP) and ecosystem respiration (Re), and canopy conductance to drought, to define both the thresholds of ecological drought (using the Environmental Stress Index defined as (1-ET/PET)), and quantify the observed patterns of recovery across these diverse biomes. We will also discuss the importance of the seasonality of drought across these biomes, how disturbance alters the sensitivity to drought, and use our observations to predict the impact of hotter and potentially more severe droughts in the region.