|Working Group II: Impacts, Adaptation and Vulnerability|
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220.127.116.11. Mountains and Subarctic Environments
Mountain regions are characterized by sensitive ecosystems, enhanced occurrences of extreme weather events, and natural catastrophes. Often regarded as hostile and economically nonviable regions, mountains have attracted major economic investments for tourism, hydropower, and communication routes. The projected amplitude and rate of climatic change in coming decades is likely to lead to significant perturbations of natural systems as well as the social and economic structure of mountain societies, particularly where these are marginal. Because in many instances mountains and uplands are regions of conflicting interests between economic development and environmental conservation, shifts in climatic patterns probably will exarcerbate the potential for conflict (Beniston, 2000).
Nested GCM-regional climate model (RCM) techniques for a 2xCO2 scenario have shown that the European Alps are likely to experience slightly milder winters, with more precipitation, than recently. Summer climate, however, may be much warmer and drier than today, as a result of northward shift of the Mediterranean climatic zones (Beniston et al., 1995). These conditions are likely to have adverse effects on the alpine cryosphere and ecosystems.
Impacts of climatic change on physical systems will affect water, snow, and ice, and shifts in extremes will lead to changes in the frequency and intensity of natural hazards. Water availability in some regions may decline because of a reduction in precipitation amounts and because of reduced snow-pack and shorter snow season. Changes in snow amount will lead to significant shifts in the timing and amount of runoff in European river basins, most of which originate in mountains and uplands; this will have numerous consequences for the more populated lowland regions. Floods and droughts are likely to become more frequent. Indirect impacts include changes in erosion and sedimentation patterns, perhaps disrupting hydropower plants (Beniston et al., 1996).
In most temperate mountain regions, the snowpack is close to its melting point, so it is very sensitive to changes in temperature. As warming progresses in the future, current regions of snow precipitation increasingly will experience precipitation in the form of rain. For every 1°C increase in temperature, the snowline rises by about 150 m; as a result, less snow will accumulate at low elevations than today, although there could be greater snow accumulation above the freezing level because of increased precipitation in some regions. A warmer climate will lead to the upward shift of mountain glaciers, which will undergo further reductions. It is likely that in the 21st century, 30–50% of alpine glaciers will disappear (Haeberli, 1995). Permafrost also could be significantly perturbed by warming, leading to a reduction of slope stability and a consequent increase in the frequency and severity of rock and mudslides. These in turn would have adverse economic consequences for mountain communities.
According to Sætersdal and Birks (1997), mountain plants with narrow July and January temperature tolerances (typically centric species) are most vulnerable to climate warming. These species are characterized by small ranges and population sizes. Holten (1998) documents that these less common centric species have narrow altitudinal ranges—most between 400–600 m in southern Scandes (the Fennoscandian mountain range)—compared with widely distributed mountain plants that have vertical ranges of 800–1,800 m.
Temperature enhancement experiments in the northern Scandes (Henry and Molau, 1997; Molau and Alatalo, 1998) have shown disintegration of present plant communities; “arctic specialists,” such as Cassiope tetragona and Diapensia lapponica, are least responsive to warming, but soon will suffer competitive exclusion from more competitive evergreen and deciduous dwarfshrubs such as Empetrum hermaphroditum, Vaccinium vitis-idaea, Betula nana, and Salix spp. The latter species are most favored by climatic amelioration (see also Jonasson et al., 1996; Graglia et al., 1997). The following vegetation types are regarded as most sensitive to climate change in the Scandes:
As a result of a longer growing season and higher temperatures, European alpine areas will shrink because of upward migration of tree species. After several centuries of invasion of forest into the alpine Scandes, the current alpine area might be reduced by as much as 40–60% (Holten, 1990; Holten and Carey, 1992). The speed and extent of upward migration will depend on species as well as physiographic conditions and climatic regimes. Fairly quick response is expected from pioneer species such as mountain birch (Betula pubescens ssp. tortuosa). However, there seem to be competing explanations for the response time of migration of the timberline in European mountain ranges and how far timberlines will shift under specific climate scenarios (Woodward, 1992). Under optimal topographic/edaphic conditions, the mountain birch, Scots pine, and Norway spruce treelines might be elevated by as much as 300 m in the continental Scandes and less on the coastal slopes. This will probably take several hundreds of years, at least for the more slowly responding Scots pine and Norway spruce (Aas and Faarlund, 1995; Kullman, 1995). The interaction between climate change, acidification, and nitrogen deposition in alpine ecosystems in the Scandes certainly is very important; to date, however, it has been more or less overlooked (Keller et al., 2000).
The predicted future climate suggests that the ranges of many species will extend to higher altitudes. In the topmost zone of the Middle Mountains and the lower external Alps, competition from closed-canopy forest of Norway spruce might restrict the area of small islands of arctic-alpine tundra, destroy patterned grounds and subarctic mires, and kill relict and endemic populations of arctic-alpine organisms (Jenik, 1997). Mountain ecosystems that are particularly vulnerable to climate change in Italy include, for example, shrub vegetation with Pinus mugo of the Apennines (Vaccinio-Piceetalia). This grows 1,500–2,300 m above sea level on glacial residue and is closely dependent on cold continental climate with long-lasting snow cover (Second National Communication on Climate, 1997).
Evidence from past climate changes indicates that species respond by migrating rather than by adapting genetically (Huntley, 1991). According to Scharfetter (1938), the warmest interglacial periods enabled forests to climb higher toward the summits of low mountains (1,800–2,300 m), thereby reducing high-elevation orophyte populations. This is relevant for many isolated endemics and orophytes that presently are living in refugia, such as tops of low mountains in the Alps. In such habitats, they will have no possibility to migrate upward, either because they cannot move rapidly enough or because the nival zone already is absent (Gottfried et al., 1994; Grabherr et al., 1994, 1995).
There is broad agreement that past climatic changes have had a strong impact on the distribution ranges of species, and the same can be expected in the future (Peters and Darling, 1985; Ozenda and Borel, 1991, 1995). However, some of these biotic changes are subject to considerable inertia, especially with long-lived plants such as trees. For treelines to expand upslope, a significantly warmer climate is required for at least 100 years (Holtmeier, 1994). Based principally on palynological and macro-fossil investigations, the forest limit did not extend upward more than 100–300 m during the warmest periods of the Boreal and Atlantic periods in the Holocene (e.g., Bortenschlager, 1993; Lang, 1993; Wick and Tinner, 1997). An increase in mean annual temperature of 1–2°C may not shift the present forest limit upward by much more than 100–200 m in the Alps.
An increase of 1–2°C is still likely to be in the range of tolerance of most alpine and nival species (Körner, 1995; Theurillat, 1995), whereas a greater increase (3–4°C) may not be (Theurillat, 1995; Lischke et al., 1998; Theurillat et al., 1998). This is particularly relevant for endemics and orophytes with widespread distributions throughout the Alps. Where ranges of species already are fragmented they may become even more fragmented, with regional disappearances if they cannot persist, adapt, or migrate. Some categories of vulnerable plants—for instance, isolated arctic, stenoicous, relict species that are pioneers in wet habitats—may disappear. Specialists in distinct relief situations can suffer from habitat loss through lack of suitable escape routes, as observed from modeling studies (Pauli et al., 1999; Gottfried et al., 2000). Such effects can be pronounced—for instance, in the northeastern Alps, where high numbers of endemics occur in narrow altitudinal ranges (Grabherr et al., 1995).
Biogeographically, the Pyrenees are on the edge between the alpine, central European, and Mediterranean regions. In the eastern Pyrenees around Puigmal (2,910 m), some oro-Mediterranean communities occur, as well as some pasture types with the rude Festuca supina (Baudiere and Serve, 1974). On windier and drier areas, an ecological substitution is taking place: Alpine dense pastures are turning to discontinuous oro-Mediterranean communities. There is ongoing degradation of formerly stable soils and communities as a result of periglacial phenomena, including increased cryoturbation (Baudiere and Gauquelin, 1998). With regard to timberlines in the Pyrenees, Montserrat (1992) demonstrates that they have been moving upward in the postglacial period up to the present. Over 1940–1985, an increase in mean annual temperatures is suggested by changes in animal behavior. In Estangento Lake (eastern Pyrenees, 2,035 m), this happens especially in winter months, when the minimum temperatures reach 3°C. This phenomenon affects the hibernation time of a common bat (Miniopterus shreibersi), so that populations enter the caves 1.5 months later than they did 20 years ago. Caves are regarded as stable environments that reflect only general climatic trends (White and Martinez Rica, 1996).
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