Snow is an important environmental factor in alpine ecosystems, which influences plant phenology, growth and species composition in various ways. With current climate warming, the snow-to-rain ratio is decreasing, and the timing of snowmelt advancing. In a 2-year field experiment above treeline in the Swiss Alps, we investigated how a substantial decrease in snow depth and an earlier snowmelt affect plant phenology, growth, and reproduction of the four most abundant dwarf-shrub species in an alpine tundra community. By advancing the timing when plants started their growing season and thus lost their winter frost hardiness, earlier snowmelt also changed the number of low-temperature events they experienced while frost sensitive. This seemed to outweigh the positive effects of a longer growing season and hence, aboveground growth was reduced after advanced snowmelt in three of the four species studied. Only Loiseleuria procumbens, a specialist of wind exposed sites with little snow, benefited from an advanced snowmelt. We conclude that changes in the snow cover can have a wide range of species-specific effects on alpine tundra plants. Thus, changes in winter climate and snow cover characteristics should be taken into account when predicting climate change effects on alpine ecosystems.

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Interactions between natural disturbances are widely recognized as important determinants of vegetation patterns in forested landscapes but have only rarely been investigated quantitatively. In a subalpine forest landscape in northwestern Colorado, we quantified spatial associations of fire and spruce beetle (Dendroctonus rufipennis) outbreaks over more than a century and developed a multivariate logistic model of probability of occurrence of spruce beetle outbreaks. The study area, an ∼2800 km2 landscape in White River National Forest, was affected by severe widespread fires around 1879 and by a spruce beetle outbreak in the 1940s, which affected most of the forests that were dominated by Picea engelmannii (Engelmann spruce) and Abies lasiocarpa (subalpine fir). The boundaries of the fires ca. 1879 and of the 1940s spruce beetle outbreaks were digitized based on an 1898 vegetation map and modern aerial photograph interpretation, and overlaid in a Geographic Information System. The areas disturbed by the ca. 1879 fires and the 1940s beetle outbreak were also overlaid with 303 fires recorded after 1950 as well as with topographic and forest structural variables. Forests that had burned in 1879 were less affected by the 1940s outbreak than older stands. On the other hand, areas affected by the 1940s spruce beetle outbreak showed no higher susceptibility to subsequent fires. A multivariate logistic model indicated that, although fire history had the greatest effect on stand susceptibility to spruce beetle outbreak, dominance of neighboring stands by spruce as well as elevation were also important predictors of outbreaks. Predictive modeling of spatial interactions between fire and spruce beetle disturbances needs to consider the high degree of variability in the nature of these interactions related to contingencies such as time since last major disturbance, topographic position, and weather during windows of potential interaction. Corresponding Editor: F. W. Davis.

Aim There is increasing research attention being given to the role of interactions among natural disturbances in ecosystem processes. We studied the interactions between fire and spruce beetle (Dendroctonus rufipennis Kirkby) disturbances in a Colorado subalpine forest. The central questions of this research were: (1) How does fire history influence stand susceptibility to beetle outbreak? And conversely, (2) How does prior occurrence of a beetle outbreak influence stand susceptibility to subsequent fire? Methods We reconstructed the spatial disturbance history in a c. 4600 ha area by first identifying distinct patches in the landscape on aerial photographs. Then, in the field we determined the disturbance history of each patch by dating stand origin, fire scars, dates of mortality of dead trees, and releases on remnant trees. A geographical information system (GIS) was used to overlay disturbance by fire and spruce beetle. Results and main conclusions  The majority of stands in the study area arose following large, infrequent, severe fires occurring in c. 1700, 1796 and 1880. The study area was also affected by a severe spruce beetle outbreak in the 1940s and a subsequent low‐severity fire. Stands that originated following stand‐replacing fire in the late nineteenth century were less affected by the beetle outbreak than older stands. Following the beetle outbreak, stands less affected by the outbreak were more affected by low‐severity fire than stands more severely affected by the outbreak. The reduced susceptibility to low‐severity fire possibly resulted from increased moisture on the forest floor following beetle outbreak. The landscape mosaic of this subalpine forest was strongly influenced by the interactions between fire and insect disturbances.

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 Europe PMC Funders Group Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts 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.