Abstract
Tree mortality from insect infestations can significantly reduce carbon storage in forest soils. In subarctic birch forests (Betula pubescens), ecosystem C cycling is largely affected by recurrent outbreaks of defoliating geometrid moths (Epirrita autumnata, Operophtera brumata). 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.
Highlights
Northern forest soils store large amounts of carbon (C) and act thereby as a globally important sink for atmospheric CO2 (Pan and others 2011; Bradshaw and Warkentin 2015)
Based on proxies for decomposition, 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
The three disturbance levels caused by absence/severity of moth outbreaks featured significantly different understory compositions; dwarf shrubs prevailed at the control plots (65% surface cover), whereas the ground vegetation of dead plots was grass dominated (55% surface cover; Table 1)
Summary
Northern forest soils store large amounts of carbon (C) and act thereby as a globally important sink for atmospheric CO2 (Pan and others 2011; Bradshaw and Warkentin 2015). The size of soil C stocks depends largely on the balance between organic matter (OM) input to the soil (C gain) and its decomposition by heterotrophs (C loss). Across disturbed temperate and boreal forests, soil C loss through decomposition seems to predominate—resulting in reduced soil C stocks following disturbance events (Thom and Seidl 2015; Zhang and others 2015). If photosynthetic C fixation cannot compensate for post-disturbance C losses, this may cause a positive feedback on rising atmospheric CO2 concentrations and on climate change (Bonan 2008; Kurz and others 2008). In contrast to temperate and boreal forests (Thom and Seidl 2015; Zhang and others 2015), disturbance effects on soil C stocks and on mechanisms underlying decomposition processes remain uncertain for subarctic forests. Because high-latitude regions are vulnerable to climate warming (IPCC 2014), this information is key to assess additional disturbance-mediated feedback loops to the climate system (Thom and others 2017)
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