Changes of forest cover and disturbance regimes in the mountain forests of the Alps

Abstract

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.

Keywords: Disturbance interactions; European Alps; Forest fire; Land-use history; Secondary succession; Snow avalanches; Windthrow.

Fig. 1

Examples of photographic time series visualizing forest cover changes in the Alps since the 19th century: (A and B): Ceüse, southern French Alps, at the end of the 19th century (A) at the beginning of the 21st century after so-called RTM (mountain terrains rehabilitation) works (B); (C and D): Davos (Central Swiss Alps) in 1900 (C) and 2010 (D); (E and F): Vermiglio (Trentino, Italian Alps) in 1915 (E) and 2000 (F). -Source: Trento Autonomous Province archive.

Fig. 2

Average annual burnt area in ha per year between (1985 and 2011) for different regions of the European Alps. Original data source: European Forest Fire Information System (EFFIS), Valese et al. (2014) and Vacik et al. (2011).

Fig. 3

Forest cover changes in different regions of the Alps since the availability of data. Black bars represent for each region the area of forest cover during the last available survey relative to the area of forest cover during the first available forest cover data (grey bars, standardized to same bar size for each region). Note that the time for the first available survey varies considerably between 1827 (Slovenian Alps; Petek, 2005) and 1981 (Piedmont).

Fig. 4

Increase of forest cover percentage since 1880 as a function of distances from potential treeline (calculated based on moving window approach of the highest trees in a 10 km neighborhood). Fig. 4a shows forest cover percentage in 1880, 1940 and 2000 based on digitized historic maps. Fig. 4b shows relative increases between 1880 and 1940 (black bars) and between 1940 and 2000 (grey bars) as a ratio of original forest cover percentage.

Fig. 5

NFI-plots windthrown by the storms Vivian (1990) and Lothar (1999), established prior to 1880 (blue dots, n = 110) and after 1880 (orange dots, n = 24) in Switzerland. Barplots with corresponding colours represent the frequency of observed windthrow areas in pre- and post-1880 forests relative to the expected frequencies according to the area of each forest type.

Fig. 6

Representation of forest fire in NFI-plots which overlap with pre-1880 forests (blue dots, n = 236) and post 1880-forests (orange dots, 193 Fires) in the Canton Graubünden (eastern Switzerland). Barplots with corresponding colours represent the frequency of observed fire areas in pre- and post-1880 forests relative to the expected frequencies according to the area of each forest type.