Elevational belts and socioeconomic aspects

A characteristic feature of high mountains is their vertical zonation into climate-driven elevational belts: 1) the snow and ice covered nival belt, 2) the alpine belt (treeless, above treeline), 3) the mainly forested montane belt (below tree-line), and 4) the valley floors and forelands. Figure 1 shows a typical mountain region with clearly identifiable belts.

People live and use resources in these altitudinal belts (Allan 1986). However, at higher elevations, conditions become increasingly difficult, creating societal interdependencies with lower belts and the lowlands. Traditionally, there are 3 ways to secure a livelihood at high elevations despite the harsh environmental conditions:

FIGURE 1

View of a typical high mountain region: the Saas Valley/Simplon region, Switzerland, as derived from a combination of information from Indian Remote Sensing satellite (IRS) and Landsat satellite images. The image shows the glacier belt (white and gray, about 3000 m; red lines define the extent of the glacier in 1850 and blue the position in 1973), the periglacial belt (blue and pink; where blue represents permafrost and pink the transition from warm/thin permafrost to seasonal soil freezing), the alpine vegetation belt (light green), and the montane forest belt (dark green) down to the valley bottoms (about 1500 m). Potential pathways of debris flows, rock avalanches and lake outbursts (GLOFs), estimated by integrative modeling, are indicated in light yellow and green. (Source: Huggel et al 2004 and 2005)

Mountain Biosphere Reserves (MBRs): an essential component of GLOCHAMORE

MBRs often cover a large fraction of this geographical teleconnection. Hence their management affects societal benefits outside MBR boundaries; these should be quantified in economic terms. Furthermore, a re-visitation of the potential for traditional food production in light of the growing demand for organic products is needed. These traditional land use forms preserve options for future generations which otherwise might be lost through land abandonment (Körner and Ohsawa 2006).

Mountain regions and MBRs usually have greater geological, topographical, and climatic diversity and hence greater biodiversity across altitudinal zones than the surrounding lowlands. Land use interacts with these natural drivers, creating the well-known mosaics of land cover types. The management of MBRs influences these patterns, which in turn affect biodiversity and ecosystem processes, including the delivery of goods and services to society such as freshwater, food, and slope stability. The mitigation of risks associated with slope forces (avalanches, debris flows, floods) and the provision of clean water are crucial tasks for MBR management. Knowledge-based decision-making is thus imperative.

Traditional experimental knowledge and skills, including observation and monitoring practices, are abundant in MBRs. However, owing to global environmental change, population growth, and technological pressures, MBR management requires a broader knowledge base than previously. The EU-funded GLOCHAMORE project (Global Change and Mountain Regions) addresses recent and future challenges to the world's mountain regions and aims to develop a research strategy to meet them. The strategy provides guidance to scientists, MBR managers, decision-makers, and policymakers (Björnsen Gurung 2005). A selection of key themes was made (Becker and Bugmann 2001) and dealt with in a 2-year series of 4 workshops: 1) long-term monitoring, 2) the impacts of global change, 3) tools and methodologies to be applied. The fourth and final workshop led to the current report and associated publication emphasizing altitudinal gradients.

Transects crossing altitudinal belts cover various environmental gradients over relatively short distances (Figure 1). At various scales, such gradients illustrate “experiments in nature.” For example, thermal gradients over several hundred meters present us with the potential steady-state responses of biota to global warming (space for time approach).

Conditions and drivers across altitudinal gradients

It is common knowledge that atmospheric pressure and temperature decrease with elevation. However, several other phenomena often considered “common trends” are not so common in the wide spectrum of mountain regions across the globe (Körner 2003). Common to all mountains is declining atmospheric pressure with elevation and the associated partial pressure of oxygen (affecting humans) and carbon dioxide (potentially affecting plants). Humans, wild animals, and plants have adapted to these differences through different evolutionary mechanisms. For instance, humans adapt to oxygen shortage at high altitude with physiological “tricks,” which appear to differ between Andean and Central Asian populations (Beall 2002). Another example are physical “tricks” such as changes in the porosity of egg shells in birds (Rahn et al 1977). Adapted high-elevation populations commonly have superior fitness under highland conditions compared to populations at lower elevations.

Related to declining pressure is reduction in temperature (around a mean of 0.55 K/100 m), causing the above-mentioned altitudinal belts. This gradual temperature reduction influences virtually all environmental conditions, including reduction of evaporative forcing at high elevation under isothermal conditions. Finally, clear sky turbidity decreases with elevation, while clear sky solar radiation increases simultaneously.

However, contrary to widespread belief, all other climatic variables do not vary with altitude in a common manner. Actual solar radiation is influenced by cloudiness and thus often declines with elevation. Precipitation patterns vary greatly, and much confusion had been introduced by associating local peculiarities of rainfall patterns with general altitudinal phenomena. Precipitation may increase (eg European Alps, Rocky Mountains), or first increase and then decrease with elevation (eg many tropical and subtropical mountains). Except for summits or exposed ridges, mountains are also commonly less windy than the plains.

Often the same environmental changes exert different effects across altitudinal gradients. For instance, climatic warming reduces snow pack duration at lower elevations, but—due to associated greater moisture loading in the atmosphere—may actually enhance snow pack at higher elevations. By contrast, warming may cause drought at low elevations and improve the conditions of life at high elevations. Accounting for this spectrum of altitudinal gradients and the linkages between highland and lowland conditions and processes is a key to knowledge-based MBR management and generalization of findings along altitudinal transects.

In the same vein, human interactions with mountain ecosystems do not vary in a systematic way with elevation. Quite often, activities peak in the foothills with a mid-montane minimum (mainly due to the steep slopes) and an upper montane maximum of activities (eg in the Andean Altiplano or in tourist areas in the European Alps). But there are also regions with a mid-montane maximum of land use pressure (eg New Guinea highlands).

Special hydrological aspects

The main components of the water balance—precipitation, evapotranspiration, and runoff along altitudinal transects (slopes)—are not easy to assess. Even precipitation, usually measured with rain gauges (or pluviometers), requires a number of conditions on slopes (horizontal area, no shadow effect of surrounding subjects) for sufficient accuracy of measurement (Sevruk 2004).

Evapotranspiration is assessed in various ways. One is to calculate it via energy balance-related approximations using existing formulas by Penman or Turc, accepting all uncertainties caused by disregarding mountain-specific factors such as exposure, aspect, slope angle, or incoming radiation (in addition to soil and vegetation characteristics). Direct measurement using lysimeters (Körner et al 1989) is laborious and subject to spatial constraints. The best way is to solve the water balance equation, ie subtract runoff from catchment precipitation over longer periods, ideally in “microcatchments” with no uncontrolled subsurface water exchange with neighbor catchments.

At least 3 components of runoff (or stream flow) should be considered (Becker 2005):

Seasonality of runoff and its change play a crucial role in mountain regions, particularly at higher elevations. The dramatic retreat of high mountain glaciers, of reduced snowfall at lower elevations, and of higher winter and early spring rainfall and snowmelt lead to increasing flows at the beginning of the snowmelt period, followed by flow reductions as the snow masses disappear. In the long run this will result in a reduction of river flow in dry summer periods and an increase in drought. Even if total precipitation were to increase, the resulting combination would probably mean lower lake levels, dryer wetlands, and greater shortages of available water in late summer and early autumn.

Temperature is expected to increase by about 3.5 K in the coming 100 years (Houghton et al 2001). The situation is less clear for precipitation, particularly its elevational trends. Shrestha (2005), for example, found a clear decrease in annual precipitation of 35 mm per year in the middle transect in Alaknanda (India)—at the steepest of the world's altitudinal transects, the southern slope of the Himalayas, ranging from 60 m in the Nepalese Terai to 8848 m at Mount Everest. By contrast, he found an increase of 2–31 mm per year in the Kalagandaki (Nepal). Moreover, he noted an increasing trend to greater extremes (monsoonal floods and droughts), as well as increasing precipitation in areas where precipitation is already high and reduced precipitation in areas where it is currently low (in agreement with the last IPCC assessment report, Houghton et al 2001).

Altitudinal gradients as a source of natural hazards

Slopes are the basic condition for the occurrence of hazards in terms of landslides, debris flows, rockfalls, avalanches, etc. The probability of such hazards increases remarkably when slope gradient reaches or exceeds 25° to 30°. Modern technology offers sophisticated tools to describe and model landscapes and their evolution in mountain regions. Figure 1 offers an example of the assessment of landscape elements in a mountain area of the Alps, the Simplon region (Saas Valley) in Switzerland. Using satellite images from Indian Remote Sensing (IRS) and Landsat (Huggel et al 2004), the main altitudinal belts can be identified (between about 1500 and 3000 m).

Decades of worldwide research have helped to create a comprehensive basis of scientific knowledge about landscape evolution in high mountains and their forelands, as influenced by glaciers and frozen ground. Recent and ongoing atmospheric warming, however, has confronted us with new challenges of historic dimensions (Watson and Haeberli 2005). Under the influence of accelerating climate change, the disappearance of glaciers and the degradation of permafrost in the highest parts of the mountains, the conditions of life will change in most mountains and present risks different from those in the recent past (Haeberli and Burn 2002).

Slope stability is very low on recently deglaciated terrain steeper than about 25° to 30°. Thawing permafrost on steep rock walls and non-consolidated sediments induce fragile conditions. Debris flows and rock avalanches may develop under such conditions, especially during snowmelt and heavy precipitation events. Marked changes in glacier extent are commonly associated with the formation of ice- and moraine-dammed lakes, the outburst and flood potential of which constitute extensive hazards for downstream areas. Steep hanging glaciers that are partly or completely frozen to their beds could become unstable in the event of warming. There is an urgent need to extend observation-derived knowledge about these glacial and periglacial processes through improved understanding of processes, especially for runoff formation and slope stability. Robust computer models are needed to simulate such changes in space and time and to assist in the planning of hazard mitigation measures at high altitudes.

High-resolution satellite imagery combined with geoinformatics (as in Figure 1) is now available even for very remote mountain areas (Kääb et al 2003; Huggel et al 2005). Such data are being increasingly used for assessment of high-mountain environmental conditions, including permafrost distribution and formation of dangerous ice- and moraine-dammed lakes with the risk of outbursts. Calibrated numerical spatial gray-box models help to develop scenarios and identify risks. Integrated observation and information systems, including information from modeling and visualization, are needed for planning, mitigation, and adaptation.

Conclusions and recommendations

Elevational gradients and specific transects should be a primary subject of mountain research for the following reasons:

Monitoring activities have been emphasized here, as monitoring data are a prerequisite for understanding processes, which in turn is essential for modeling and, thus, the validation of current concepts of natural and socioeconomic developments. Both monitoring and modeling and cross-disciplinary syntheses will assist knowledge-based land use planning and management of MBRs.