Identifying and quantifying mountain pine beetle (Dendroctonus ponderosae) induced watershed scale changes to metal input, storage, and export, Rocky Mountains, USA

Abstract

Warming climates and extended droughts enabled mountain pine beetle populations to grow and impact larger areas, as observed during the 2008-2013 widespread epidemic in the Rocky Mountain region, USA. After the decline in mountain pine beetle populations, spruce beetle populations were the most widespread and destructive forest pest in Colorado for the fourth successive year (2011-2015). In mountain pine beetle impacted areas, tree death occurs an average of three years after the initial infestation. In this short period of time the trees stop transpiring, defoliate, and die. The rapid drop of pine needles to the forest floor, and subsequent decomposition of needles, increases organic carbon availability and releases metals. While column experiments and tree scale studies have begun to elucidate changes in metal cycling with mountain pine beetle infestation, questions still exist regarding the watershed scale response to these perturbations. This work focuses on the post beetle infestation shifts in metal input, storage, and export at the watershed scale by investigating 1) control of tree type and pine beetle infestation stage on shifts in metals leached from needles, 2) watershed scale shifts in metal storage and export with increasing severity of pine beetle infestation, and 3) control of forest composition and severity of beetle infestation on exported metals through reactive transport modelling.Needle leaching studies were used to determine how changes in forest composition and severity of pine beetle infestation would alter the input of metals and organic carbon into the forest floor. lodgepole pine and engelmann spruce needles were sampled to compare their individual potential organic carbon and metal contributions to soils. Generally, spruce needles released more dissolved organic carbon and metals than pine needles. Additionally, pine needles from infested trees leached more dissolved organic carbon and metals than from healthy pine trees. Greater amounts of metals leached from spruce needles compared to pine needles could mask changes in metal loading as a result of increased mountain pine beetle infestation and pine tree mortality. Additionally, increased spruce tree mortality as a result of increased spruce beetle infestations could lead to more severe changes in water quality. Analysis of watershed metal storage in soils and effluent metal concentrations revealed a lack of correlation between severity of pine beetle infestation and metal storage and export. The lack of correlation was the result of heterogeneity between individual soil sampling sites and that within the studied watersheds pine and spruce were equally dominant resulting in a dampening of the signal from increased pine needle litter by the presence of spruce needle litter. To investigate the control of forest composition (pine versus spruce) and severity of pine beetle infestation on effluent fluxes of Al3+, Cu2+, and Zn2+, we used a two-dimensional reactive transport model with varied infiltration solution composition which represents a range of forest types (pine versus spruce) and degrees of beetle infestation (0-100% areal infestation). Our results demonstrate that changes in effluent metal fluxes are not only controlled by increasing tree mortality but also by the dominant tree type.