Wind disturbance in mountain forests: Simulating the impact of management strategies, seed supply, and ungulate browsing on forest succession

Fifteen years after the heavy storm "Vivian", it is still not clear how succession in subalpine forests that were affected by the storm will continue and when regrowing forests will provide effective protection from natural hazards such as avalanches. We used a simulation model to evaluate forest succession, forest structure and the protective effect in subalpine blowdown areas after 50 simulation years under different scenarios. The scenarios included the effects of different management strategies such as clearing the fallen logs or leaving the sites untouched ("uncleared"), variations in seed supply, and ungulate browsing. The simulation results indicated that forest structure was heterogeneous after 50 years, with a high amount of trees between 11 and 100 cm height, and a low amount of trees taller than 1 m. The number of trees >5 m, which is important for the protective effect of a site, was lower at uncleared areas if the area was covered with high amounts of fallen logs, but diversity of microsites was higher than at cleared areas. We found that it is particularly important that abundant seed supply occurs within the first few years after the blowdown at cleared sites, because in later stages there was high competition by tall herbs, which prevented the establishment of tree regeneration. Larger time lags between seed years in the simulations led to retarded tree regeneration. Particularly at cleared sites, ungulate browsing retarded tree regeneration. In contrast, uncleared sites had a higher potential to recover from high browsing pressure due to a high amount of favourable microsites that are provided by decaying logs. These results of our model simulations may help understanding the dynamics of forest regeneration and providing perspectives for management after blowdown events.