Figure 1. Factors influencing spatial pattern and physiognomy of timberline and ecological conditions in the timberline ecotone.

become lower, but also species composition and structure of the mountain forests changed considerably (Section Although timberline is widely located below its climatic limit the advance of trees to higher formerly forested elevation is strongly hampered by adverse site conditions, even under a warmer climate (Holtmeier, 1965, 1974; Muterthies, 2002). However, it is difficult and frequently impossible to identify and assess these historical influences (occasionally visible in growth rings for example) on the present timberline physiognomy and ecology. In summary, the present timberline is away from being caused only by the present climate (e.g., mean air or soil temperature). The after-effects of the current situation will influence future changes of timberline position and spatial patterns (Figure 90).

Thus, a complex view is needed to understand the spatially varying heterogeneity and dynamics of timberline. A complex view does not only mean consideration of the functional interactions between the many timberline-relevant factors (Figure 1) in different environments but also switching between global and finer (regional, local) scales of consideration (Figure 2) to match the

Figure 2. Timberline-controlling factors at different scales. Modified from Holtmeier and Broll (2005).

particular underlying factors and processes (see also Meentemeyer and Box, 1987). Operating at different scales (e.g., Curran et al., 1997) has been and probably still is the main problem for timberline researchers from different disciplines to better understand the complex nature of their common research object and also each others. Timberline heterogeneity increases from the global to the regional, landscape and local scales. Factors and processes at one scale may not be as important at another scale (Turner, 1989). Lack of soil moisture or waterlogging, for example, may control tree growth and timberline pattern at landscape and finer scales. In a global or zonal view, however, the effects of climatic variables such as temperature and precipitation may be more important. Conversely, coincidences of mean air or soil temperatures and the position of the altitudinal or northern timberline at a global or zonal scale reflect their general dependence from thermal-deficiency but do not provide any information on the possible role of the many other factors.

Deeper insight into the spatial and temporal timberline dynamics and a better understanding of the functional relationships between the timberline-relevant factors and trees can be expected only when considering timberline at the regional and landscape/local scale within different climatic regions. It has been argued, however, that a 'narrow regional perspective' has obscured or will obscure the world-wide dominant role of heat deficiency and its direct and indirect influences on tree physiology and morphology (K├Ârner, 2003b, 2007b) what can hardly be substantiated, however, by the scientific timberline literature. Anyway, 'better' correlations between mean air temperatures or mean soil temperatures and the position of altitudinal and northern climatic timberlines in a worldwide view would just confirm the general rule that treeline, at least outside the tropics, is related to thermal-deficiency in one or other way (Section 4.3.1). This, however, will hardly contribute to a better functional explanation of timberline.

In the present author's opinion, it is the great regional physiognomic, biological and ecological diversity of the upper and northern treeline, which should be considered as the essential feature in the global timberline pattern (e.g., Troll, 1973; Wardle, 1974; Arno, 1984; Holtmeier, 1989; Callaghan et al., 2002a, Broll and Keplin, 2005; Callaghan et al., 2002b). Thus, it is the objective of this book to highlight the physiognomic and ecological variety of timber-lines as well as their spatially varying heterogeneity and temporal dynamics. Without this differentiation, speculation on the more or less great sensitivity and possible response of timberline to the changing environment might result in confusion and too broad implications.

Was this article helpful?

0 0

Post a comment