Mechanistic and predictive understanding of needle litter decay in semi-arid montane ecosystems experiencing unprecedented vegetation mortality 

Jonathan O. (Josh) Sharp – Colorado School of Mines (Principal Investigator)

Eoin L. Brodie and Kenneth H. Williams – Lawrence Berkeley National Lab (Co-Investigators)

Carl Gable, Ethan Coon and Adam Atchley – Los Alamos National Lab (unfunded collaborators)

Mountainous regions across the globe are experiencing major forest disturbances associated with a changing and variable climate. These disturbances alter ecosystems with potential effects and feedbacks on interdependent carbon and nitrogen cycling at the terrestrial interface. Recent large-scale insect infestations are of particular concern as they have the potential to switch the Rocky Mountain region from a carbon sink to a source through the defoliation and degradation of millions of acres of evergreen trees. While it is understood that this will lead to cascading effects in both the hydrosphere and atmosphere, current predictive models are not equipped with the information to incorporate these dynamic and comparatively sudden disturbances relating to carbon decay and release. This one-year project focuses on establishing, instrumenting, and deploying a set of experiments designed to understand needle litter decay rates and underlying mechanisms between evergreen needles released from insect-killed trees to that of naturally senesced litter from healthy trees. The proposed work will be conducted within Rocky Mountain Biological Laboratory’s (RMBL) East River watershed in southwestern Colorado, whose forests are threatened by changing temperature, altered hydrological and snowmelt patterns, and invasive insect infestation. Needle litter from two members of the Pinaceae family experiencing large-scale insect infestations (spruce and lodgepole pine) will be collected and redeployed. We will experimentally test three tractable parameters that are known to impact litter decay and biogeochemical cycling and are relevant to bark beetle-induced tree mortality: (1) moisture content, (2) temperature by using elevation as a surrogate, and (3) changes to litter compositional chemistry that result from beetle infestation. We will establish a baseline for decay processes while monitoring the migration of soluble and gaseous nitrogen and carbon species. In addition, we will evaluate variables that are associated with the rate of microbial litter decomposition such as metal availability, enzyme activity, and the temporal succession of associated microorganisms. The studied processes are expected to improve formulation and parameterization of reactive transport models being developed by DOE National Labs. This approach can help unravel the intertwined response of carbon and nitrogen cycling, microbial processes, and the impacts moisture conditions and temperature have on litter decay feedbacks in a changing climate.