Indirect biogeomorphic and soil evolutionary effects of spruce bark beetle

Abstract

Outbreaks of bark beetles, for example Ips typographus L. in Eurasia or Dendroctonus ponderosae Hopkins in North America, have serious impacts on forest resources, biodiversity, and ecological dynamics, with economical and social ramifications. Moreover, many models predict increasing frequency and severity of such biotic disturbances due to ongoing climate change, and land use driven changes in forest structure and composition. Bark beetles are recognized as keystone species due to their strong and complex effects on ecosystem dynamics. However, due to the increasingly widely recognized biogeomorphic impacts of trees, bark beetles may have significant indirect biogeomorphic and pedogenetic impacts through their effects at scales ranging from individual trees to forest landscapes. These include: (1) Reduced uprooting, with associated impacts on topography, mass movements, regolith and soil formation, and slope hydrology; (2) Reductions in bioprotection via trapping of downslope sediment movement; (3) Hydrological impacts, including increased total runoff and increased proportion of subsurface flow; (4) Decreased microtopographic irregularity (and associated hydrological and pedological impacts); and (5) Changes in biochemical and biomechanical effects on soils, regolith, and hillslope morphology. Five separator factors (discriminators between different developmental trajectories) were revealed for the case of the central European region. These factors may determine the occurrence and severity of biogeomorphic impacts: First is whether the site is prone to potential uprooting or whether an spruce bark beetle (SBB) outbreak is initiated by a blowdown/uprooting event. Second is whether the site is dominated by mineral soils or Histosols. A third discriminating factor is whether the forest is managed or unmanaged, which determines the pre-attack tree species composition and coarse woody debris and disturbance regimes; and a fourth is the post-outbreak management. Finally, the fifth separator factor relates to slope thresholds that determine the significance of impacts on mass movements and erosion. These findings support the need, and provide guidelines, for research on geomorphic impacts of bark beetle infestations. Though we mainly restrict our consideration to bark beetles in Europe, both our approach and findings are likely to have broader relevance for biogeomorphic impacts of extensive tree mortality. • Spruce bark beetles (SBB) influence various geomorphic and pedogenetic processes. • SBB influence soils and hillslope dynamics from local scale of tree to landscape scale. • Five separator factors may determine the occurrence and severity of SBB impacts. • Findings have broader relevance for geomorphic impacts of other biotic disturbances.