Mechanisms of Treeline Formation

Investigators have been interested for over a century in the question of why trees do not occur above certain altitudes. Ecological studies have shown that the occurrence of both timberlines and treelines decline steeply and almost linearly in altitude as latitude increases between about 30° N and S latitude and over 60° N and S latitude. This linear relationship results in an estimated change in timberline altitudes of ^100 m per degree of latitude. However, between about 30° on each side of the equator, there is a relatively constant, maximum altitude of occurrence that is near 3.5-4.0 km. Little information exists concerning the difference in altitude between the timberline and treeline, or the width of the treeline ecotone (alpine forest) as related to geography or any specific environmental factor. Although most of these studies have associated this altitude of occurrence to the colder temperature regimes at higher latitudes, the actual ecophysiological mechanisms are still being debated, and may involve a large number of abiotic and biotic factors. In addition, major changes in tree habit occur within this life zone, including dramatic alterations in plant height and crown features such as branching pattern. This change in growth form becomes more dramatic as distance from the forest edge (timber-line) increases toward the ultimate treeline limit (Figures 1 and 2). Across this ecotone, the full-tree stature of a typical forest tree becomes twisted and distorted, forming ultimately a small, shrub-like habit commonly referred to as the 'krummholz' mat at treeline. During this transition, trees also become more and more flagged in appearance, with stems occurring only on the downwind side oftrunks and main stems (Figure 3).

Because temperature data have been mostly available for the longest period of time and for most locations worldwide, a host of studies have attempted to correlate measured temperature regimes with the highest altitude of tree occurrence. Within these myriad studies, the occurrence of minimum temperatures and the amount and physical nature of the prevailing snowfall has been a central focus. For example, more continental (noncoastal) mountain ranges of both hemispheres have dryer, colder climates characterized by 'powder snow' conditions. This type of snow is strongly influenced by wind-driven snow that can generate strong abrasive forces due the sharp-edged, crystalline nature of these snow particles. These systems also have distinct snow accumulation patterns across the landscape that are the result of the strong turbulent and eddy flow characteristics. Moreover, snow burial and avoidance of excessive exposure to wind and colder temperatures may be critical for the winter survival of both plants and animals in this alpine-forest belt. In contrast, coastal ranges with lower altitudes of treeline also have higher air humidity levels, snowfall of high water content, and low abrasive power of softer ice crystals that are relatively uncoupled from the influence of wind patterns. This wetter, heavier snow can accumulate on exposed branches, creating severe mechanical forces that can bend, break, and distort stems due to snow and ice loading above the snowpack surface and freeze-thaw compression forces beneath the snow surface. Snow accumulation in the dryer continental alpine forest is much more dependent on drift mechanics and eddy flow dynamics (e.g., burial of krummholz mats), while the wetter snows of more coastal systems generate a more uniform depth and homogeneous distribution pattern across the treeline ecotone. For the dryer powder snow of the continental mountain tops, severe abrasive properties can lead to abrasion of leaf cuticles, removal of paint from highway traffic placards, and the common windburn suffered by skiers on windy days and powder-like snow. Thus, these differences in the physics of snow particles and spatial distribution dynamics also play a major role in the distortion and disfiguration effects on individual trees of the alpine forest (stunting, flagging, and krummholz tree forms), as well as the spatial patterns of tree spacing. These differences in the basic physical make-up of snow have not been considered systematically in terms of their influence on the vegetation patterns and distortions in growth form observed for individual trees within different alpine forests (appearance of krummholz and flagged growth forms). These effects for more maritime versus continental mountain ecosystems need further elucidation, in particular, the impact on the altitude at which trees can no longer regenerate.

The ecophysiological mechanisms regulating the upper elevational limits of treelines across the globe have been contemplated by plant ecologists, biogeographers, and bio-meteorologists for over a century. A recent review concluded that the elevation limits of the upper treelines on a global scale is the result of (1) the inability of alpine plants to metabolically process the carbon gained from daytime photosynthesis because of cold-temperature limitations (e.g., respiratory limitations), and (2) the large size of conifer trees which prevents adequate warming of the soil due to soil surface shading by the closed, overstory canopy. Thus, low soil temperatures due to self-shading was proposed as a major abiotic determinant of the elevational limits of upper forest treelines. However, other studies have provided evidence of strong limitations to resource acquisition at high altitudes, specifically the photosynthetic uptake of CO2 by alpine forest trees. Many other investigators have also questioned conclusions (1) and (2) above.

Despite the longstanding interest in the environmental and physiological mechanisms generating observed altitu-dinal patterns in the formation of alpine forests and their respective treelines, virtually all of this research has focused on the ecophysiological effects measured for adult trees, even though they may show distortions in form and greatly diminished stature, for example, krummholz mats and stunted, flagged trees. Very little research has focused on the establishment of new seedlings away from the forest edge into the treeline ecotone. Yet, it is this life stage within the treeline ecotone that appears critical for migration to a higher altitude and formation of new subalpine forest. The formation of new subalpine forest at higher elevation is dependent on seedling regeneration into the ecotone, whereas the migration of the forest timberline to a lower altitude would require both the mortality of older trees and the successful seedling regeneration at the new altitude of occurrence. However, any mortality of the overstory trees could also introduce an important impact - a decrease in the ecological facilitation of seedling establishment. Likewise, a lack of establishing seedlings in the forest understory at the forest edge, in combination with the death of the overstory trees, would result in a lowering of the timberline and, most likely, the treeline as well. An important component of this process is the ecological facilitation of new seedling survival and growth that results from a more mature forest structure (Figure 2). In other words, the development of trees with forest-like stature (no flagging or krummholz distortion) requires the formation of an intact forest and the resulting amelioration of a host of extreme abiotic factors outside the forest. Thus, the altitu-dinal movement of timberline and treeline boundaries begins with new seedling establishment, either below of above the existing timberline that will act, ultimately, to facilitate further seedling establishment and the gradual development of new subalpine forest either above or below the altitude of the existing timberline. For example, the mechanisms involved in the migration of a timberline/ treeline to a higher altitude must initially depend upon new seedling establishment above the existing timberline, into the treeline ecotone. Moreover, greater seedling/sapling abundance must follow to provide the ultimate facilitation required for continued growth to full forest-tree stature and, thus, the formation of new subalpine forest at higher altitudes. At high elevation, this migration of timberline is possible only with the protective, mutual facilitation provided by neighboring trees and surroundings, similar to that found within intact subalpine forest. Thus, growth to forest-tree stature without structural distortion may require, to some degree, 'the forest before the tree'. In the Rocky Mountains of southeastern Wyoming (USA), the establishment of new tree seedlings into a treeline ecotone appears also to involve considerable microsite facilitation (Figure 5)

Table 2 Factors identified as important for explaining the altitudinal occurrence of alpine forest and its maximum altitude of occurrence as an alpine treeline ecotone

1. Seedling/sapling establishment - seed germination, growth, and survival

2. Mechanical damage - wind abrasion of needle cuticles, apical bud damage, snow loading, and frost heaving cause tissue and whole-tree mortality

3. Physiological tissue damage - low temperature and desiccation limits growth and survival

4. Annual carbon balance - photosynthetic carbon gain minus respiratory demands is less than that needed for successful growth and reproduction

5. Biosynthesis and growth limitationa - greater cold temperature limitation to growth processes than to photosynthetic carbon gain aCold soil temperature due to the large size of conifer trees and consequential soil shading have been hypothesized as a primary environmental factor limiting the processing of assimilated carbon and, thus, maximum altitude of alpine treelines.

by either inanimate objects (e.g., rocks, fallen logs, micro-topography due to freezing and thawing of the soil surface), or by intra- and interspecific spatial associations generating ecological facilitation of microsites. Structural self-facilitation (e.g., cotyledon orientation and primary needle clustering, krummholz mats) may also act to enhance the growth and survival at all structural scales from the seedling to mature trees (Table 2). Increased seedling establishment and abundance is followed subsequently by even greater facilitation, which leads to even greater seedling establishment and sapling growth, and so on (Table 3). Thus, increased seedling/sapling abundance will lead to the same 'sheltering effect that is necessary for the formation of the forest 'outposts', or islands, known to be important shelters for improved seedling establishment. In addition, the ultimate development into a forest tree (nondistorted growth form) is analogous functionally to the biophysical 'escape' of vertical stems from the surface boundary layer of a krummholz mat (Figure 3). Subsequently, continued facilitation of the sapling stage, approaching a similar level as found within the intact subalpine forest at lower elevation, is required before an establishing sapling can reach the stature of a subalpine forest tree.

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