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The influence of wildfire on the protective capacity of beech forests against rockfalls

 

Mountain beech forests often protect our communities and roads against rockfalls. But is a beech tree still able to dissipate the energy of falling rocks after a forest fire?

In a research project funded by the National Environmental Agency (BAFU), employees of the WSL in Cadenazzo and the SLF in Davos joined forces with fellow scientists from Turin University, the School of Agricultural, Forest and Food Sciences (HAFL) and Stuttgart University to investigate the protective capacity of beech forests after a wildfire. They selected more than 30 forest stands in Ticino and Piedmont (Italy) that had burnt once within the last 40 years. Considering the intensity of the fire and the elapsed time since the fire, they analysed how the vegetation has developed in the individual stands. They categorised and measured trees having a diameter of at least 8 cm and a height of at least 2 m. Additionally, lying and standing deadwood was inventoried. All parameters were implemented in the rockfall model Rockfor.net– a tool that helps to calculate the protective capacity of a forest stand against rockfalls. Based on Rockfor.net, the researchers were able to calculate how effectively each of the investigated stands fulfilled that role in the years following the fire.

The results show that beech forests are able to regenerate quickly and produce stable forest after single wildfires. However, the post-fire recovery period as well as the regeneration capacity and the development of a structured forest stand, fundamentally depends on the severity of the forest fire. Wildfires of low severity appear to have only a marginal impact on the protective function of beech forests, and the forest structure remains comparable to that of an unburnt stand. As a result, no post-fire technical or sivicultural measures are required in order to ensure protection against rockfalls.

Severe fires reduce protection by up to 50 percent

After medium to high-intensity fires, in contrast, the forest structure changes radically. Small and medium-sized beech trees in particular fall victim to moderately severe fires, whereas high-severity fires kill trees regardless of their size. However, both the fallen deadwood and the surviving beech trees are still capable of dissipating the energy released by falling rocks. In addition, the seeds of the surviving trees contribute to the forest's regeneration capacity. But protective capacity declines owing to the combination of dying fire-injured beech trees and decaying deadwood on the one hand, and the low protective capacity of the emerging regeneration on the other. Therefore, the researchers observed a reduction in the protective capacity of burnt beech forests of up to 50% within a period of ten to thirty years after the fire event. The extent of the reduction depends on the intensity of the fire, structure of the affected forest, size of the rocks, and length and gradient of the forested slope. In such cases they recommend installing rockfall nets, the small-scale felling of dying trees and the placing of stems at an oblique angle to the slope. These measures need to be inspected periodically in order to verify their effectiveness (particularly in view of the accumulation of loose stones or rocks).

One key result of the project was the development of a methodical approach for practitioners. This approach enables foresters to estimate the protect capacity of burnt beech stands against rockfalls within the first three years after the fire event – giving consideration to the fire severity, the expected rock size and topographical characteristics (forest slope length, slope inclination). Such forecasts allow foresters to initiate appropriate protective measures in good time, if needed.