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Natural disturbances, such as avalanches, snow breakage, insect outbreaks, windthrow or fires shape mountain forests globally. However, in many regions over the past centuries human activities have strongly influenced forest dynamics, especially following natural disturbances, thus limiting our understanding of natural ecological processes, particularly in densely-settled regions. In this contribution we briefly review the current understanding of changes in forest cover, forest structure, and disturbance regimes in the mountain forests across the European Alps over the past millennia. We also quantify changes in forest cover across the entire Alps based on inventory data over the past century. Finally, using the Swiss Alps as an example, we analyze in-depth changes in forest cover and forest structure and their effect on patterns of fire and wind disturbances, based on digital historic maps from 1880, modern forest cover maps, inventory data on current forest structure, topographical data, and spatially explicit data on disturbances. This multifaceted approach presents a long-term and detailed picture of the dynamics of mountain forest ecosystems in the Alps. During pre-industrial times, natural disturbances were reduced by fire suppression and land-use, which included extraction of large amounts of biomass that decreased total forest cover. More recently, forest cover has increased again across the entire Alps (on average +4% per decade over the past 25–115 years). Live tree volume (+10% per decade) and dead tree volume (mean +59% per decade) have increased over the last 15–40 years in all regions for which data were available. In the Swiss Alps secondary forests that established after 1880 constitute approximately 43% of the forest cover. Compared to forests established previously, post-1880 forests are situated primarily on steep slopes (>30°), have lower biomass, a more aggregated forest structure (primarily stem-exclusion stage), and have been more strongly affected by fires, but less affected by wind disturbance in the 20th century. More broadly, an increase in growing stock and expanding forest areas since the mid-19th century have - along with climatic changes - contributed to an increasing frequency and size of disturbances in the Alps. Although many areas remain intensively managed, the extent, structure, and dynamics of the forests of the Alps reflect natural drivers more strongly today than at any time in the past millennium. [ABSTRACT FROM AUTHOR]

When harvesting activities stop, forest stands become steadily richer in very large trees and deadwood, maturity attributes that are crucial for forest dwelling species. On the other hand, the maturation process, associated with large trees and large gaps between trees, has traditionally been thought to be detrimental to the protective function of the forest against gravitational hazards such as rockfall. However, the findings of recent studies have contested this belief, first because they showed that natural dynamics in aging stands is rather gradual in space and time, and second because they highlighted that deadwood may play an important role in protection forests. In this study, we assessed the protection efficiency of the forest along a maturity gradient in uneven aged stands, using a network of permanent sample plots in the French Alps. Plots were selected according to management plans to represent four successive stages along the maturity gradient: young stands, adult stands just after logging, post-adult stands that escaped one to two rotations, and mature stands. We checked that stands gradually matured using the total number of very large trees (dbh > 77.5 cm), the total volume of deadwood, and the total volume of degraded large logs. We then developed a specific module to integrate deadwood into the Rockyfor3D rockfall simulation model and assessed the rockfall protection of the plots. Mature stands, although not having reached the protection efficiency of young and dense stands, can provide adequate protection against rockfall. First, because mature stands are rather dense (more than 500 stems ha −1 ). Second, because large logs increase the surface roughness of the forest floor and act as additional obstacles to the propagation of rocks. Consequently, mature stands originating from aging irregular stands play a fitting role in protection forests, thereby reconciling biodiversity conservation objectives with protective functions. When mature stands originate from aging regular stands dominated by a cohort of very large trees, a cautious approach to management will better initiate a progressive irregularization process before promoting patches of mature stands. [ABSTRACT FROM AUTHOR]