Quantifying the resilience of carbon dynamics in semi-arid biomes in the Southwestern U.S. to drought

Abstract:

Semi-arid biomes in many parts of the Southwestern U.S. have experienced a range of precipitation over the last decade, ranging from wetter than average years 2006-2010 (relative to the 40-year PRISM mean), extreme drought years (2010-2011) and slightly dry-average precipitation years (2013-2015). 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 range in precipitation variability 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 climate variability 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 compared daily, seasonal and annual NEP, GPP and Re means between pre-drought (2007-2010), drought (2011-2012), and post-drought years (2013-2015). All biomes sequestered less carbon in the drought years, compared to the pre-drought years (~30-40, 270 and 60 g C/m2 less in low and middle elevation biomes, ponderosa pine, and mixed conifer forest, respectively), as GPP in all biomes was more sensitive to the drought than Re. In the post-drought years, GPP was still only 80-90% what it was in the pre-drought years. Re, however, in all biomes except for the creosote shrubland, was 5-15% higher in the post-drought years compared to pre-drought. As a result, carbon sequestration in these biomes was 20-75% lower in the post-drought years compared to what we observed before the drought, suggesting, even 3 years post-drought, none of these biomes has recovered from the extreme drought. The large implications of this lack of resilience for the carbon sink strength in this region are discussed.