Large-scale importance of bark beetle outbreaks for standing deadwood and woodpeckers

Resource pulses associated with forest disturbances can significantly influence biological communities, with interacting and sometimes lagged effects on species numbers. Among the most common disturbances in temperate forests are bark beetle outbreaks, which represent a resource pulse and initiate additional resource pulses of other insects and of deadwood. Moreover, bark beetle outbreaks are often followed by changes in habitat conditions due to canopy cover loss or salvage logging that can have contrasting effects on co-occurring forest species. However, the effects of this multi-resource pulse on forest species have rarely been quantified simultaneously. Here, we investigated if and to what extent changes in woodpecker population density were related to resource pulses, specifically spruce bark beetle and standing dead wood of Norway spruce. We used data from long-term monitoring projects collected across Switzerland from 1984 to 2020 to build a network model that estimated relationships among species and resources. We built a semi-mechanistic model that included structured equations of parameters of interest. The equations modelled Norway spruce standing deadwood volume, bark beetle density and density of three woodpecker species as a function of their interactions, as well as climatic, environmental and management variables. The model allowed both same-year and 1-year lagged effects. We found weak relationships between bark beetle outbreaks and woodpecker densities, and strong positive relationships between deadwood amount and woodpecker densities. In turn, standing deadwood was strongly linked to bark beetle outbreaks, highlighting the indirect role of bark beetles in promoting woodpecker populations by means of providing deadwood for foraging and nesting. Salvage logging, that is, the removal of trees infested by bark beetles, had only minor effects on bark beetles. Bark beetle outbreaks contributed to the generation of deadwood that ensured the continuity of its associated ecosystem functions, which benefited woodpeckers and, potentially, forest biodiversity as a whole. Large-scale consequences of these multi-resource pulses were reflected by species populations over longer time periods than the pulses.