Elevation dependent response of soil organic carbon stocks to forest windthrow br

Storms represent a major disturbance factor in forest ecosystems, but the effects of windthrows on soil organic carbon (SOC) stocks are poorly quantified. Here, we assessed the SOC stocks of windthrown forests at 19 sites across Switzerland spanning an elevation gradient from 420 to 1550 m, encompassing a strong climatic gradient. Results show that the effect size of disturbance on SOC stocks increases with the size of the initial SOC stocks. The largest windthrow-induced SOC losses of up to 29 t C ha-1 occurred in high-elevation forests with a harsh climate developing thick organic layers. In contrast, SOC stocks of low-elevation forests with thin organic layers were hardly affected. A mineralization study further revealed high elevation forests to store higher amounts of easily mineralizable C in thick organic layers that got lost following windthrow. These findings are supported by a meta-analysis of available windthrow studies, showing an increase of storm-induced SOC losses with the size of the initial SOC stocks. Modelling simulations further indicate longer-lasting SOC losses and a slower recovery of SOC stocks after windthrow at high compared to low elevations, due to a slower regeneration of mountain forests and associated lower C inputs into soils in a harsh climate. Upscaling the experimental findings/observed patterns by linking them to a data base of Swiss forest soils shows a total SOC loss of similar to 0.4 Mt. C for the whole forested area of Switzerland after two major storm events, counteracting the forest net carbon sink of decades. Our study provides strong evidence that the vulnerability of SOC stocks to windthrow is particularly high in forests featuring thick and slowly forming organic layers, such as mountain soils. Thus, the risk of losing SOC to more frequent windthrows in mountain forests strongly limits their potential to mitigate climate change.

Automated summary

Storms have a significant impact on forest ecosystems, but the effects of windthrows on soil organic carbon (SOC) stocks have not been well studied. This study assessed SOC stocks in windthrown forests across Switzerland and found that the disturbance effect on SOC stocks increases with initial SOC stocks. High-elevation forests with thick organic layers experienced the largest SOC losses, while low-elevation forests with thin organic layers were less affected. Additionally, modeling simulations showed that SOC losses after windthrow are longer-lasting and slower to recover at high elevations due to slower forest regeneration and lower carbon inputs in harsh climates. These findings highlight the vulnerability of SOC stocks in mountain forests to windthrows and the limitations on their ability to mitigate climate change.