Introduction

The forest of the alpine zone occurs near mountain tops and forms a transition zone between the subalpine forest below and the alpine zone above (Figure 1). Whether this zone of overlap represents a definable, stable community with its own inherent structure and stability is open for debate. Observations of the spatial patterns of the tree species do insinuate some successional character, although the long-term encroachment of the subalpine forest into the alpine zone, or vice versa, is a slow process that is detectable only after centuries of change, at least. Although the alpine forest has a well-defined, characteristic vegetation pattern that contrasts with the subalpine forest and alpine zones, animal species are often viewed as community members of either or both. This boundary ecotone between two contiguous communities is often referred to as the upper (or cold) treeline (or timberline) ecotone where the treeline limit is reached. This limit is defined as the highest occurrence of a tree species in any form, or for a tree species that has a certain minimum tree stature (e.g., greater than 2 m vertical height). The latter definition is necessary because this upper limit of

Figure 1 Alpine forest landscape (~3200m altitude) in the treeline ecotone of the Snowy Range, Medicine Bow Mountains, southeastern Wyoming (USA). Alternating snow glades (long-lasting snow pack) and ribbon forest are characteristic of this alpine forest, along with the potentially extreme distortion of individual tree structure and form (see Figure 2). Prevailing winds are from the right in this photo.

Figure 1 Alpine forest landscape (~3200m altitude) in the treeline ecotone of the Snowy Range, Medicine Bow Mountains, southeastern Wyoming (USA). Alternating snow glades (long-lasting snow pack) and ribbon forest are characteristic of this alpine forest, along with the potentially extreme distortion of individual tree structure and form (see Figure 2). Prevailing winds are from the right in this photo.

tree occurrence is often composed of disfigured (flagged branching) and stunted (krummholz mat) tree forms that are more shrub-like than tree-like in appearance (Figure 2). This upper (cold) treeline ecotone can vary in altitude and width according to latitude and proximity to maritime influences, as well as the degree of slope and azimuth at a given location. In addition, plant demographics such as tree size, age, spacing, and clustering among individual trees, plus the structural distortion and disfigurement of individual trees further from the timberline, can vary dramatically.

Regardless of the latitude or altitude of mountain areas, excessive steepness of the slope and, thus, poorly developed soils, will prevent tree establishment and result in sharp boundaries between the timberline and alpine community. Above the timberline, individual trees or patches occur sporadically associated with less wind-exposed microsites where aeolian soil and snow accumulate. These characteristics of the alpine forest landscape can also vary according to the proximity to oceans or other large bodies of water (e.g., 'lake effect' weather patterns). In general, greater latitudes result in a decrease in the altitude at which alpine forest is found, as does a closer proximity to oceans or other large bodies of water. In contrast, the dryer continental mountains tend to have timberlines and treelines at the highest altitude for a given latitude.

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