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In many mountain regions, there have been dramatic changes in agricultural land use in recent decades. In some cases, these are related to changes in technology, such as the increased use of machine harvesting of hay or a switch from one breed of grazing animals to another. In other cases, the trend has been to abandon agriculture on less productive and least accessible land (Lambin et al. 1999). In the European Alps, for example, 16% of all farm holdings were abandoned within ten years (1980–1990). In addition, almost 70% of the farms that are still in operation today are run only as a secondary source of income. With regard to the land use issue, this means that an average of about 20% of the agricultural land of the Alps has been abandoned, and in some areas as much as 70% (Tappeiner et al. in press). In contrast, farming in the better agricultural locations is being intensified. Hence, land-use changes are considered to be a major driving force behind changes in landscape patterns, ecosystem function and dynamics in Europe (MacDonald et al. 2000).

Climate change can alter ecosystems and thereby trigger feedback effects that can either enhance or retard the climate change (Lashof et al. 1997). Such feedbacks are especially likely in montane and high-latitude ecosystems where soils are carbon-rich (Whittaker 1975; Schlesinger 1997), ecotones are prevalent as a result of topographic variability, vegetation is sensitive to climatic variables such as snowmelt date and length of growing season (Körner 1992; Harte and Shaw 1995; Goulden et al. 1998), and climate change is expected to be large due to snow-albedo feedback (Groisman et al. 1994). Predicting the chronology and magnitude of such feedbacks is a major challenge in ecology today, as well as an important issue both for global climate change science and policy and, locally, for people whose livelihood is dependent upon montane climatic and ecological regimes.

The rapid rise in human populations and technological advances since 1850 have caused changes in several global scale biogeochemical cycles, including the global nitrogen cycle. The Haber-Bosch process to convert atmospheric nitrogen gas (N) to ammonia (NH) is now almost universally used to fertilize food crops. The production of nitrogen oxides (NO) from combustion for industrial purposes, energy production, and transportation is the other large source of reactive nitrogen to the atmosphere. Combined, these two human alterations have added approximately 140 Tg N yr to the global reactive N pool, a value that now exceeds natural source contributions of about 100 Tg N yr (Galloway and Cowling 2002).

High elevation lake ecosystems are regarded as potentially sensitive indicators of global change because of their cold and dilute abiotic environment, low biodiversity, poor functional redundancy, and relative lack of local human perturbations (Skjelkvâle and Wright 1998; Sommaruga 2001; Battarbee et al. 2002; Psenner et al. 2002). Mountain lakes located near treeline are expected to be the most responsive to long-term impacts of stratospheric ozone depletion and increased flux of solar ultraviolet-B radiation (UV-B; 290–320 nm), climatic warming, and other stressors because of sharp transitions in control processes (Fig. 1) associated with vegetation development and snowpack albedo (Vinebrooke and Leavitt 1998; 1999a; Fyke and Flato 1999). As detailed below, increased flux of solar UV-B and global warming may be already interacting to restructure food webs and biogeochemical cycles in many mountain lakes (Leavitt et al. 1997; Sommaruga-Wögrath et al. 1997).

The International Geosphere Biosphere Program report on mountain ecosystems stresses the potential role of mountainous regions in the Earth’s geophysical cycles (Becker and Bugmann 2001). However, mountain environments have rarely been addressed specifically in studies of terrestrial carbon dynamics. Although it was first suggested that the US carbon sink was localized in eastern US forests (Fan et al. 1998), more recent studies that partition the US sink into specific regions suggest that a significant fraction is located in the western US (Schimel et al. 2000; Pacala et al. 2001 ; Schimel et al. 2002). As increasing development puts pressure on arable lands in North America and Temperate Asia, forests and other high carbon storage ecosystems are increasingly relegated to mountain landscapes. Inspection of recent land cover databases (e.g. IGBP or DeFries et al. 2000) shows clearly that in Temperate North America, Europe and China, a large fraction of forested landscapes is found in major and minor mountain ranges. Figure 1 shows an index of carbon uptake in forests based on forest cover from satellite observations (Defries et al. 2000) and growing season length (with longer growing seasons indicating a higher carbon uptake potential). Growing season lengths are scaled to eddy covariance estimates of carbon uptake per growing season day (Falge et al. 2002). Since the majority of current terrestrial sinks are found in the Northern Hemisphere mid-latitudes, montane forests have the potential to contribute significantly to current carbon sinks.

A striking change associated with modern human society has been the increase in atmospheric CO due to the increased burning of fossil fuels (coal, petroleum, natural gas) since the industrial revolution (Sarmiento and Siegenthaler 1992; Sarmiento and Bender 1994). One potential consequence of this atmospheric change is the so-called “greenhouse effect”, a global climatic warming induced by elevated atmospheric CO modifying the atmosphere’s opacity to infra-red radiation. Because CO is an essential component of plant photosynthesis, an increase in ambient CO levels immediately leads to the question as to whether these changes might be altering plant function globally and might also be changing vegetation patterns.

In recent years, predictive modelling of plant species’ distribution has been shown to be a powerful method for obtaining preliminary assessments of potential ecological impact of rapid climatic change (e.g. Brzeziecki et al. 1995; Kienast et al. 1996; Saetersdal and Birks 1997; Iverson and Prasad 1998; Lischke et al. 1998; Gottfried et al. 1999; Guisan and Theurillat 2000; 2001; Bakkenes et al. 2002). Such models give static results: they reveal where suitable species’ habitats might be located in a climatically changed future, but they do not explicitly consider all the processes leading to the predicted changes. A basic assumption behind their application is thus to consider present and future distributions of species to be in equilibrium, or at least in pseudo-equilibrium, with their environment (Guisan and Theurillat 2000). Although this assumption obviously does not hold in all ecological situations, scenarios obtained from these models nevertheless constitute an interesting spatially-explicit and quantitative basis for discussing how climate change might impact plant distribution. Examples of such discussions are provided in the next section.

Mountain forests fulfil a multitude of functions, including the provision of timber, fuelwood, edible and medicinal plants, the storage of carbon, the purification of air and water, the regulation and reduction of peak streamflow, the protection from natural hazards, and the contribution to the aesthetic beauty of the landscape. The importance of these functions varies greatly from one mountain region to the other, but in some way, forested landscapes and their fate under a changing climate are important for the capability of mountain regions to provide many of the goods and services that humanity depends on.

Mountain systems are characterized by strong environmental gradients, rugged topography and extreme spatial heterogeneity in ecosystem structure and composition. Consequently, most mountainous areas have relatively high rates of endemism and biodiversity, and function as species refugia in many areas of the world. Mountains have long been recognized as critical entities in regional climatic and hydrological dynamics but their importance as terrestrial carbon stores has only been recently underscored (Schimel et al. 2002; this volume). Mountain ecosystems, therefore, are globally important as well as unusually complex. These ecosystems challenge our ability to understand their dynamics and predict their response to climatic variability and global-scale environmental change.

Mountain ecosystems provide unique opportunities to detect and understand global change impacts due to their strong altitudinal gradients coupled with the presence of parks and biosphere reserves in many mountain areas where direct human impacts are minimal (Graumlich 2000; Becker and Bugmann 2001). Alpine treeline, the distinctive boundary between forest and tundra on high mountains, has been a particularly important focus of this research. While alpine treeline can appear to be a simple ecotone and thus a ready indicator of changing temperatures, a rich history of research has revealed that the dynamics of treeline are complex. Issues of lags and inertia, as well as multiple drivers across diverse scales abound. In this essay, we outline key issues for understanding alpine treeline in the context of global climate change. While our views strongly reflect research that has been done in the temperate zone, particularly in North America, the questions underlying the interpretation of treeline are applicable to many biotic indicators of climate change.