Geography Reference
In-Depth Information
FIGURE 7.8 (A) Ribbon forests (photo by L. W. Price) and (B) “insular” treeline pattern in the Olympic
Mountains, Washington (photo by K. S. Hadley).
The idea that more than one climate variable controls treeline has also been shown
using pollen evidence. Markgraf and Scott (1981), for example, suggest that the upper
and lower treelines advanced in opposite directions between 10,000 and 4,000 YBP in
central Colorado in response to warmer temperatures (upper treeline) and increased
precipitation (lower treeline), associated with the development of strong monsoon con-
ditions. Their idea of diverging treelines, and the importance of drought in treeline dy-
namics (Lloyd and Graumlich 1997) illustrate the complex relationship between treeline
and environmental factors.
Causes of Timberline
Several theories have been proposed to explain the cause of upper treeline; all remain
disputed. At any given site, timberline can be related to such factors as low temperat-
ures, UV radiation, carbon dioxide concentrations, aridity, excessive snow, strong winds,
poor or excessive drainage, lack of soil, recent disturbance by fire, disease, avalanches,
volcanic eruptions (e.g., Lawrence 1938; Holtmeier 2003; Bader et al. 2007; Körner
2007; Harsch and Bader 2011), or by stature- or growth-related factors, such as plant
competition and facilitation, deep root systems, or carbon balance (Stevens and Fox
1991; Callaway et al. 2002; Nagy and Grabherr 2009). At the global scale, however,
the relatively consistent pattern of timberlines (Fig. 7.6) suggests that some overrid-
ing environmental or ecological factor transcends local conditions and accounts for the
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