Legacy of bark beetles (Dendroctonus spp.) on soil carbon and nitrogen cycling seven years after forest infestation

Abstract

Between years 2007 and 2013, an outbreak of pine- and spruce-bark beetles (Dendroctonus spp.) has decimated coniferous forests of the northwestern United States. Beetles introduced blue stain fungus to the tree xylem that caused blockage and discontinuation of plant water conductance. One of the immediate disease symptoms was premature and immediate drop-off of all needles rich in nitrogen (N). Soil surface-accumulated needle litter in combination with elevated soil water levels, have led to accelerated N and carbon (C) mineralization. Understanding of the soil C and N cycling during forest transition from “standing dead trees” to “snagfall and forest regrowth” is still unclear. Specifically, the potential lasting legacy of beetle-caused surface-deposited tree litter is of importance in order to assess long-term forest health. The main objective of this study was to assess whether soil C and N mineralization continues to be affected by litter chemistry in this transitional period of post beetle disturbance ecosystem recovery. Mineral soil (0–10 cm) and corresponding surface tree litter were obtained from beneath dead and live tree clusters in three forest types in Medicine Bow-Routt National Forest in Wyoming, USA. All forests were located in the same watershed, grew on similar soils, experienced up to 35% of forest mortality and consisted of: Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa) mixed with lodgepole pine (Pinus contorta), and lodgepole pine alone. Results suggest that tree litter uniformly affected mineral soils across all forest types regardless of beetle infestation. In laboratory incubations, the presence of litter doubled mineral soil heterotrophic respiration. While litter had the most uniform effect on soils, beetle infestation and forest type resulted in a number of differences. Soils beneath dead tree clusters continued to have elevated inorganic N concentrations when normalized by total N, which was twice as high compared to soils beneath live tree clusters across all forest types. The lowest NH4+ concentrations were observed in soils beneath lodgepole pine trees, which was accompanied by the highest C to N ratio of microbially available substrates. In conclusion, the legacy of forest die-offs caused by bark beetle infestation was still observable seven years later during the early transitional period. This confirms ecosystem modeling simulations that, while entering decadal stage of forest recovery, forest structure may differ depending on the forest type, and that the forest types that contain lodgepole pines may encounter an earlier onset of potential N limitation.