Biogeochemical and ecological impacts resulting from beetle-induced forest mortality

Abstract

Over the last two decades, increasing temperatures and drought conditions have caused unprecedented bark beetle infestation across western North America. As forest die-off ensues, several aspects of the hydrologic cycle are altered due to canopy loss, altered evapotranspiration, and decreased water uptake by infested trees. Additionally, the cessation of root exudates along with the decay of enhanced litterfall following tree death alters biogeochemical cycling. These shifts may alter water quality or produce nutrient feedbacks into the atmosphere and hydrosphere. While modeling and field investigations have begun to elucidate these responses, questions still exist regarding whether a certain level of infestation is required to produce these secondary effects along with perceptions of increased fire risk that drive forest management policies. This work focuses on the ecological and biogeochemical shifts that occur following beetle infestation by investigating 1) shifts in aromatic carbon loading and subsequent disinfection byproduct (DBP) formation potential in beetle-impacted watersheds, 2) alterations in fire severity following mountain pine beetle infestation, and 3) how geochemical responses within soil horizons are influenced as a function of localized tree mortality severity. Analysis of quarterly municipal monitoring data from 2004-2014 from six municipalities in the Rocky Mountain region of Colorado containing varying levels of beetle infestation were analyzed. Watersheds containing >50% areal infestation were found to have significantly increasing total organic carbon and DBP concentrations, with increases continuing nearly one decade after initial infestation. Alarmingly, DBP concentration trends at high-impact sites were found to exceed regulatory maximum contaminant levels during the final two years of analysis (2013-2014). Focused surface water sampling at each municipality further revealed elevated carbon loading and aromaticity in beetle-impacted sites, increasing DBP formation potential particularly during precipitation events. This additional sampling supports the hypothesis that degrading tree material along with elevated groundwater tables may be impairing surface waters used for human consumption following extensive beetle infestation. The relationship between mountain pine beetle infestation and fire severity was analyzed from 2000-2014 across the Western United States. Maps delineating zones of beetle-impacted lodgepole and ponderosa pine forest were overlain with remote sensing fire severity data products providing validation across a range of ecoregions and fire conditions. Results demonstrate a decline in fire risk following beetle infestation with beetle-impacted forests burning 65-77% and 90-94% as severely compared to unimpacted controls; respectively, for lodgepole and ponderosa pine. Forest management decisions should be wary in removing beetle-impacted forests to mitigate fire threats as these actions may not reduce fire risk and can have unintended impacts to water quality. Finally, as the severity of beetle infestation within a forest may impact biogeochemical response, near surface horizons (litter, organic, and mineral soil) were sampled below live and beetle-killed lodgepole pine trees surrounded by varying extents of tree mortality to investigate potential compensatory effects from surviving trees. While some edaphic parameters were significantly different between green and grey phase trees (water content, pH, soil respiration) many biogeochemical signatures tracked with the extent of surrounding tree mortality (carbon aromaticity, C:N ratio, ammonium). Furthermore, the proportion of ammonium in the total nitrogen pool primarily increased once surrounding tree mortality exceeded 40%, demonstrating compensatory effects from surrounding live trees when infestation levels are minimal. Declining C:N ratios and an elevated proportion of ammonium indicate an enrichment of nitrogen in this N-limited ecosystem which may promote regrowth after forest disturbance.