Soil carbon storage in response to forest disturbance 

Forest soils have significant potential to mitigate climate change through their ability to store large amounts of organic carbon. However, forests are increasingly subject to natural disturbances such as windthrow, wildfire or disease outbreaks, which threaten the permanence of this large carbon stock. In response to increasing disturbances and ongoing climate change, forests are expected to lose their ability to return to pre-disturbance conditions involving a reorganization of tree species composition and stand structure. If tipping points are crossed, even a complete vegetation shift and conversion to non-forest ecosystems is possible. Here we aimed to assess the sensitivity of forest soil carbon to disturbance and its recovery with contrasting successional trajectories by combining two field studies on soil carbon stocks in windthrown forest stands and a global meta-analysis on the effects of different disturbance agents. Our results along an altitudinal gradient in Switzerland show that mountain forests with high carbon stocks in thick organic layers were particularly sensitive to disturbance by windthrow, losing up to 90% of their carbon stored belowground. In contrast, low-elevation forest soils with thin organic layers and smaller carbon stocks were barely affected. These results are consistent with our meta-analysis, which shows that disturbance-induced carbon losses increase with the size of initial carbon stocks. Boreal and high-elevation forests with large soil carbon stocks are highly sensitive to severe and long-lasting carbon losses due to damage from storms, wildfire, insects, and harvesting, while in most temperate and tropical forests soil carbon stocks recover more rapidly and losses are smaller. Results from a disturbance chronosequence in Austria also suggest that vegetation shifts following forest damage can strongly influence the recovery of soil carbon stocks after disturbance. Disturbed sites that remained in a non-forest, grass-dominated state for three decades accumulated about a third more soil carbon than sites that regenerated with trees. In addition to high litter inputs from herbaceous fine roots at grass-dominated sites, we relate this difference to changes in microbial community structure and function. In conclusion, our results underline that the magnitude and duration of soil carbon losses after disturbance depend on the forest type and site specific soil properties. Moreover, vegetation shifts during succession significantly modify the re-accumulation of soil carbon after disturbance.