As the location of northern hemisphere altitudinal and polar treelines are caused mainly by heat deficiency, global warming is expected to cause treelines to advance to higher elevations and more northerly latitudes (e.g. Kauppi & Posch, 1988; Westman et al., 1990; Ozenda & Borel, 1991; Gates, 1993; Neilson & Chaney, 1997; Batchelet & Neilson, 2000; Rupp et al., 2000; Grace et al., 2002). The future position of the upper and northern treelines, which is considered to be an indicator of the effect of changing environment, is increasingly discussed at an international level (e.g. Programme Advisory Committee, 1999; Callaghan et al., 2002). However, climate change is only one aspect of environmental change that may affect the location of treelines.
In this present paper, the term ‘treeline’ is applied to the transition zone extending from closed subalpine or northern forests to the uppermost or northernmost usually scattered and stunted individuals of the forest-forming tree species — regardless of their height (Holtmeier, 1981, 2003). The upper or northern limit of the treeline ecotone is called the tree-limit.
The sensitivity of treelines to environmental change implies a certain state of readiness of the trees to respond to changing conditions. This can take place by changes in growth, growth forms, regeneration and treeline structures (spatial structures, mosaics), and can also be related to changes in alpine or tundra vegetation, age classes and distribution pattern of plant communities. Sensitivity is great where even the slightest change in a limiting factor is followed by prompt response in tree growth and treeline patchiness.
The treeline is a space- and time-related phenomenon. When assessing treeline sensitivity and its potential response to changing environmental conditions, spatial scale plays an important role (Fig. 1). At the global scale, even the relatively broad forest–tundra boundary is reduced to a ‘line’ snaking for about 13,000 km along the northern rim of Eurasia and North America. This ‘line’, defined by a certain coverage (e.g. 30% or 40%), can be mapped from satellite images or other remote sensing techniques (Rees et al., 2002) and correlated to isolines of temperature (e.g. mean temperature of the warmest month or of three or four warmest months, growing degree-days, etc.; see critical review in Tuhkanen, 1980). At this scale, a northern treeline advancing parallel to the northward shift of a ‘treeline-controlling’ isotherm would reflect a high treeline sensitivity. However, this theoretical treeline should not be considered like an organism responding to its changing environment. It has become evident from regional treeline studies that position and structure rarely change in parallel to the shift of any isotherm. Nor do treelines shift synchronously with climatic change (Holtmeier, 1995, 2003; Holtmeier et al., 2003). Moreover, the factors controlling treeline structures are strongly scale-dependent (e.g. Kupfer & Cairns, 1996).
At the landscape scale, treeline varies widely because of the given regional and local geomorphologies, and the fragmented nature and ecotonal character of the treeline become obvious. Even comparatively narrow altitudinal treelines are usually characterized by mosaics of habitats. The ecological conditions within this transition zone are totally different from the mountain or from boreal forest and from the alpine zone or northern tundra (Fig. 2).
Treeline heterogeneity increases from the global to the regional, landscape and local scales (Fig. 1). Thus, assessment of treeline response to changing environment (climate) at the regional and smaller scales requires a much more complex approach than at the global scale (e.g. Körner & Paulsen, 2004). From the landscape-ecological point of view, there is no scientific justification for relating treeline position at this scale with only one factor such as mean air or soil temperature (e.g. Körner, 1998a,b, 2003). Instead, many factors must be considered. These factors control the present position and spatial structure of the treeline ecotone, which also reflect the influence of climate history, vegetation history and historical human impact (Fig. 3). Thus, present conditions reflect treeline history and influence future changes (Holtmeier, 2000, 2003; Payette et al., 2001). They may even override the effects of a general warming on treeline sensitivity and response.
In addition to spatial scales, time-scale plays an important role in assessing treeline sensitivity to changing environments. Short-term response, defined as a year or less, is reflected in individual trees (Table 1). Medium-term response (some years to a few decades) is mirrored in changing tree physiognomy (phenotypical response), tree-ring width and density, survival rate in seedlings and young trees, successional stage of the plant cover, etc. Tree-ring patterns also help to improve our understanding of site history, particularly if supported by radiocarbon dating of wood remains. Tree rings, however, do not allow prediction of future development. Medium-term and long-term responses (several decades to one hundred or more years) may be simulated by scenarios based on the projection of the present empirical relationships between treeline and environmental factors into a warmer future environment. However, we do not really know whether the present more or less well-documented interrelationships of the many tree-affecting factors and their relative intensity will be the same in a warmer climate (see also Giorgi & Hewitson, 2001).
|Short-term response of individual trees (≤ 1 year)||Medium-term response of tree stands/forests (some years to several decades)||Long-term response of tree stands/forest (several decades to hundreds of years)|
|Photosynthesis||Changing tree physiognomy||Forest advance (or retreat)|
|Carbon allocation||Production of viable seeds||Tree limit advance (or retreat)|
|Period of shoot extension||Successful establishment of trees||Feedbacks of increasing forest cover on the regional and local climates (snow cover, albedo)|
|Date of needle flush Annual height and radial growth Maturation of newly formed tissue Ripening of seeds||Effects of increasing tree coverage on microclimates and vegetation in the understory (competition, succession)|