Semi-arid biomes in many parts of the Southwestern U.S. have recently experienced one of the driest decades in the past 40 years. While annual carbon uptake in semi-arid biomes of the Southwestern US is relatively low, compared to more temperate ecosystems, collectively these biomes store a significant amount of carbon on a regional scale. It is therefore of great interest to understand what impact this drought has on inter and intra annual variability in regional carbon dynamics. We use an 9 year record from 2007-2015 of continuous measurements of net ecosystem exchange of carbon (NEE) and its components (gross primary productivity (GPP) and ecosystem respiration (Re), made across a network of flux towers along an elevation/aridity gradient in New Mexico, the New Mexico Elevation Gradient (NMEG), to quantify biome-specific responses of carbon dynamics to drought over this time period. Biomes include a desert grassland, creosote shrubland, juniper savanna, piñon-juniper woodland, and ponderosa pine and subalpine mixed conifer forests. We report here on the sensitivity of seasonal and annual NEP, GPP and Re across the NMEG to drought using the Standardized Precipitation Evaporation Index (SPEI) as a measure of drought, the degree to which these biomes are exhibiting resiliency to this drought, and estimate the consequences of this drought for regional carbon balance.
Results/Conclusions
In all biomes, except juniper savanna, GPP declined more than Re, as drought intensified. Both GPP and Re decreased with drought in the juniper savanna, but these fluxes are so closely coupled, NEE has remained relatively constant in this biome despite wide fluctuations in precipitation over the past decade. The decrease in C sequestration between non-drought and drought years in all other biomes across the NMEG was significant (100, 250, 100, 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 grassland site experienced a 200% increase in C fluxes out in the most intense drought year (2011). The lower elevation sites appear to be the most resilient to drought. The large implications for the carbon sink strength in this region will be discussed.