From tree to tap: the impacts of climate change on biogeochemical processes during conifer needle decomposition and broader implications for water quality in Colorado

Abstract

Recent climate change has contributed to large-scale tree mortality across forested regions in Colorado. As forest health declines, concern for associated terrestrial biogeochemical and hydrologic shifts is mounting. These shifts are related to reduced tree canopy cover, cessation of belowground rhizospheric processes, and increased organic inputs to the system. This can in turn affect terrestrial carbon and nitrogen cycling at the tree scale and downstream water quality. These processes within the forest are complex, however, and the observed changes in soil and stream chemistry can vary across watersheds depending on the land cover, local climate, and location. This dissertation explores biogeochemical processes associated with conifer litter decomposition, a source of terrestrial organic matter contributions, and more broadly the associations between forest health and water quality in Colorado. This work includes field-based experiments beginning at the tree-scale and concludes more broadly with watershed-scale evaluations of water quality. Results revealed the inherent chemistry associated with tree species has a significant influence on soil biogeochemistry during isolated needle decomposition. Further, biogeochemical shifts observed with bark beetle impact are likely driven by other changes (e.g., the cessation of rhizospheric processes and tree canopy loss). Comparisons of the roles of elevation, soil type, seasonal shifts in soil moisture, and snowmelt timing on litter decomposition processes revealed needle presence and seasonal variability of soil moisture are influential in soil carbon release and magnitude. In addition, soil carbon fluxes returned the greatest release associated with more bioavailable litter. Microbial community structure was a key variable that demonstrated sensitivity but also resilience to climate shifts. Extended work beyond the field using historical drinking water reports revealed increased organic disinfection byproducts (DBPs) across certain municipalities of Colorado. The trends correlated with climate variables of reduced total annual days of frost and ice conditions. Closer inspection of these trends in a town of interest revealed long-term decadal and seasonal trends of DBPs associated with spring snowmelt events. This work reveals that as terrestrial-sourced organic matter increases under a changing climate, drinking water facilities should monitor the watershed landscape and source water chemistry to proactively respond to heightened DBP production.