Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 348

Timberline patterns vary considerably from the tropics to the high latitudes, reflecting
the many different species and life forms, different environmental regimes, and vari-
ations in local conditions (e.g., Gieger and Leuschner 2004; Harsch and Bader 2011;
Fischer et al. 2013). Nevertheless, all latitudes share the general timberline pattern of
decreasing forest density and height and the development of a forest-alpine ecotone.
THE POSITIONING OF TREELINE
The location of upper treeline is sensitive to several environmental factors including lat-
itude, continentality, and topography. This is illustrated by the topographic position of
timberline in the tropics, where trees grow to their highest elevations in the valleys but
reach their maximum elevations on ridges at higher latitudes (Troll 1968; Körner 2007;
Fig. 7.7). These differences can be partly explained by the increasing depth and dura-
tion of snowcover with increasing latitude and its influence on the length of the growing
season and regeneration success (e.g., Hättenschwiler and Smith 1999; Cuevas 2000;
Wipf et al. 2009). Where snow-packs are deep, trees in northern hemisphere mountains
can be found above the valley floors on the ridges or valley slopes. Aspect also influ-
ences seedling establishment patterns, inhibiting regeneration success on warmer and
drier slopes exposed to higher levels of solar radiation (Elliott and Kipfmueller 2010).
Snow is comparatively unimportant in the tropics, making valleys more favorable
habitat than exposed ridges. Under these diurnal climatic regimes, valleys experience
smaller temperature ranges and a lower probability of frost than the ridges. Valleys also
have deeper soils and are less exposed to direct sunlight, thereby increasing local mois-
ture availability (Troll 1968). Mountain valley bottoms surrounded by high valley walls
can provide an exception to this rule, supporting alpine vegetation rather than trees.
The result is an inverted timberline caused by poorly drained soils, fire, cold air drain-
age (Barry 2008), and frost pockets (cf. Goldblum and Riggs 2002). Mountain meadows
occurring below timberline in midlatitude mountains experience similar conditions. In
these areas, however, greater snow accumulation and poorly drained soils are probably
more important than cold air drainage (Wardle 1974; Miller and Halpren 1998).