Moth outbreaks reduce decomposition in subarctic forest soils

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>Tree mortality from insect infestations can significantly reduce carbon storage in forest soils. In subarctic birch forests (<em>Betula pubescens</em>), ecosystem C cycling is largely affected by recurrent outbreaks of defoliating geometrid moths (<em>Epirrita autumnata, Operophtera brumata</em>). Here, we show that soil C stocks in birch forests across Fennoscandia did not change up to 8 years after moth outbreaks. We found that a decrease in woody fine roots was accompanied by a lower soil CO₂ efflux rate and a higher soil N availability following moth outbreaks. We suggest that a high N availability and less ectomycorrhiza likely contributed to lowered heterotrophic respiration and soil enzymatic activity. Based on proxies for decomposition (heterotrophic respiration, phenol oxidase potential activity), we conclude that a decrease in decomposition is a prime cause why soil C stocks of mountain birch forest ecosystems have not changed after moth outbreaks. Compared to disturbed temperate and boreal forests, a CO₂-related positive feedback of forest disturbance on climate change might therefore be smaller in subarctic regions.</p></div>