Five-year numerical response of saproxylic beetles following a dead wood pulse left by moth outbreaks in northern Scandinavia

Saproxylic insects are important for the decomposition of dead wood, and therefore crucial for maintaining healthy forest ecosystems. There is, however, little knowledge about their ability to respond to massive resource pulses of dead wood caused by outbreaks of defoliating insects. This study investigates the numerical response of saproxylic beetles to increased availability of dead wood caused by outbreaks of geometrid moths in the mountain birch forest of Finnmark, Northern Norway. Beetles were sampled with window (flight interception) traps in 2011, 2012, 2015 and 2016 along two 20 km long transects with differences in habitats and defoliation histories. Both transects started in forest that was damaged by outbreaks during the period 2001-2009 (high dead wood abundance) and ran towards healthy, undamaged forest (normal dead wood abundance). Galleries and exit-holes made by saproxylic beetles on dead stems were also counted along the same transects in 2016. This were done to assess differences in utilization rates of dead trees by the beetles between damaged and undamaged forest and transects. The abundance of saproxylic beetles in the study system increased from 2011 to 2015. However, this response was seemingly driven by a few dominant species related to early successional stages of wood decay. Habitat variations between transects did in addition appear to heavily affect most of the beetle community, including the dominant species. The beetles appeared to favor areas with more variation and productivity, which corresponds with the utilization rates of dead trees by saproxylic beetles. The rates of galleries and exit-holes were highest in such areas as well as on stems with high diameters. However, the saproxylic beetle community has yet to utilize all the dead wood resources of the area as of 2016, and the wood remains in an early stage of decomposition. Less preferable dead wood resources and the cold climate of the region may partially explain why the beetle community have been unable to mount a sufficient numerical response to handle all the dead wood resources. The dead wood created by the moth outbreak may therefore spend longer time to decompose past the earliest successional stages than anticipated.