Dryland ecosystems diverge in the sensitivity of litter accumulation to drought across an elevational gradient

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Cassandra M.L. Miller, Sevilleta Long Term Ecological Research Program, Albuquerque, NM, Cassandra M.L. Miller, Biology, University of New Mexico, Albuquerque, NM, Jennifer A. Rudgers and Marcy Litvak, Department of Biology, University of New Mexico, Albuquerque, NM

Background/Question/Methods
How much climate change disrupts the terrestrial carbon (C) cycle will determine the strength of C feedbacks to further climate change. Dryland ecosystems are linchpins to this understanding because they comprise ~45% of Earth’s land surface, account for ~33% of the global soil organic C pool, and store ~470 Pg organic C in the top meter of soil. Drylands also contribute more to interannual variation (IAV) in global land C fluxes than any other terrestrial ecosystem. As climate change disrupts historic net primary production (NPP) patterns, it is vital to understand how plant litter accumulation responds because these inputs drive soil C pools, which are 3X larger than aboveground C in Earth’s biota. Using a comparative ecosystem ecology approach, we leveraged long-term monthly litter accumulation data from the New Mexico Elevational Gradient (NMEG) Ameriflux eddy-covariance tower network to test how plant litter accumulation trends have changed over the last decade. Our models examined whether five common dryland ecosystem types differed in the sensitivity of litter inputs to climate variables (e.g., drought index) or variability in climate.
Results/Conclusions
Sites included Chihuahuan Desert grassland, shrubland, juniper savanna, piñon-juniper woodland, and high elevation conifer forests dominated by ponderosa pine, representing >104 million ha of drylands across the southwestern US. Analyses across NMEG demonstrated that ecosystem types significantly diverged in sensitivity of both NEE and litter accumulation to hot/dry climate conditions (negative SPEI drought index) vs. cool/wet climate conditions (positive SPEI drought index). High elevation conifer forests were predicted to store significantly less C under hot/dry conditions (negative SPEI, P <0.05) and produce less litter under climates that are both drier and more variable (concave nonlinearity). In contrast, low elevation sites stored more C when it was hot/dry (P <0.001). Exploring the climate sensitivity functions for litter in each ecosystem highlighted the large variation among dryland ecosystem types in how plant litter accumulation varies with yearly changes in climate. This work provides novel insight into the response of plant litter to increasingly variable climates, adding to the growing body of work to demystify the observed IAV in C fluxes across global drylands.