Alpine Forest Biogeography

Most of the ecological research focusing on the alpine forest has involved vegetation studies, although many animal species use this zone seasonally, especially later in summer when lower elevations have dried from the longer summer. This area is a prolonged green zone where food for herbivores, especially, is still in abundance compared to lower elevations where most annual plants have completed their life cycle, and the perennial species have undergone a seasonal senescence due to accumulating summer drought. Alpine forest is found on all continents except Antarctica, as well as several oceanic islands. The mountain regions of the Western Hemisphere form large, N—S cordilleras that connect polar regions to the subtropics. For example, the Cascades, Rocky,

# Seedling/sapling -f- Forest tree <y> Flagged tree

15 m

15 m

Timberline

Timberline

Intact forest Snow glade

Abundance and wind direction

Treeline it it

Krummholz mats

Flagged trees Mats with few Mats only Ribbon forest with mats flagged trees

Intact forest Snow glade

Figure 2 Schematic representation of Figure 1 showing the relative size and spacing of individual tree forms and tree clusters making up a typical alpine forest within the treeline ecotone of a dry, continental mountain range. See text for further explanation.

and Sierra Nevada Mountains of the western US extend from the most northern boreal forest to southern Mexico where very high volcanic mountain ranges occur, while the high ranges of the Andes connect the full latitudinal extent of South America along its western seaboard. In contrast, the high ranges of the Alps of Central and Southern Europe, as well as the Himalayas of the Eurasia, are formed along an E— W axis and are much more discontinuous between the boreal and subtropical latitudes. Further south, high mountains of southern and eastern Africa represent much more isolated ranges compared to the more continuous cordilleras of the Western Hemisphere. In the Southern Hemisphere where there is much less land mass, alpine forests are less extensive and found in only a few mountain regions that tend to be close to coastlines and, thus, have a strong maritime influence (e.g., Andes, Australian Alps, New Guinea, and New Zealand).

The question of why treelines across the globe occur at specific altitudinal limits, and no higher, has been a focus of research and discussion for over a century and a half. Although it is well known that the altitude of upper tree-lines have been strongly influenced by anthropogenic causes (e.g., grazing and fires), the primary focus of these studies has been on identifying the abiotic factors that are most limiting to the growth and survival oftrees. However, there is also evidence that certain seed-dispersing bird species (e.g., Clark's nutcracker and the gray jay) may play a crucial role in the distribution of certain species in the high-altitude treeline (e.g., limber and whitebark pine of the western US). The high-altitude environment involves particularly extreme values of cold temperature, high wind, high and low (clouds) sunlight levels, low air humidity, high long-wave energy exchange, and rapid mass diffusion due to low ambient pressure. On wetter tropical mountaintops, forests may be cloud-immersed for much of the year. In general terms, the temperature lapse rate (dry adiabatic) associated with altitude generates a maximum decrease in air temperature of approximately 1°C per 100 m of increasing altitude. Thus, this environmental factor alone is a dominant environmental factor influencing differences in the alpine forest located at dryer continental versus more moist coastal mountain ecosystems. Coastal mountains experience much lesser lapse rates (<0.3 °C per 100 m) because substantial condensation of moisture with greater altitude transfers heat to the thinning atmosphere. In addition to this extreme abiotic environment, the total length of the growth season is severely curtailed (often <90 days), and even growth during summer is severely limited for short, but often frequent, time periods that occur periodically during the entire summer growth period. Because of these factors alone, adaptation and survival of alpine forest species is most often perceived as driven by abiotic pressures.

The distribution and species composition of alpine forests on a global scale vary strongly according to both latitude and longitude. In general, the Western Hemisphere of North and South America has an N—S and S—N cordillera that extends from the boreal forests of northern Canada (Canadian Rocky Mountains) all the way across North and Central America to the southernmost portions of South America (southern Andes). In addition, this long expanse of alpine forest may be strongly influenced by nearby oceanic influences. In contrast, the major mountain ranges of Europe and Asia have a more E—W distribution and in the case of the Asian provinces, are much further away from strong oceanic influences. There are also high mountains with treelines located on volcanic islands in all the major oceans. The mountains of New Guinea, southeastern Australia, Tasmania, and New Zealand are examples of island-like alpine forests with strong oceanic influence and that also extend from the subtropics to the extreme south temperate zone of the Southern Hemisphere. There are only a few Antarctic treelines that occur on small islands relatively close by. The distribution of different treeline tree species on a global scale and according to plant type and latitude is summarized in Table 1 and Figure 3. Treelines of the northernmost temperate zones of the Northern Hemisphere are dominated in the eastern hemisphere by white birch at the highest latitudes of the Scandinavian, Ural, and eastern Siberia ranges, followed by the Scotch pine and European spruce treelines of central Norway and Sweden. In the more interior, continental areas of central Europe, the Swiss mountain pine (Pinus mugo) forms the alpine forest, while European larch (Larix decidua) and stone pine (Pinus cembra) form the treelines of the central Alps. In the maritime mountains of the western and southern Alps, European beech (Fagus silvatica) is dominant. In the Western and Northern Hemisphere, evergreen conifers dominate the alpine forest (e.g., larch, bristlecone pine, subalpine fir, and Engelmann spruce), while deciduous conifers and broad-leaf species are found less frequently, along with the rare occurrence of evergreen broadleaf species in South America (Table 1 and Figure 4). Many of these distribution patterns appear influenced by differences not only in abiotic factors, but also in historical factors related to dispersal mechanisms and historical factors related to continental drift over geological timescales.

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