Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 363

vergent distributions between two ecologically similar, circumpolar arctic-alpine plant
species resulting from postglacial diffusion, genetic drift, and long-distance dispersal.
FIGURE 7.10 Environmental conditions distributed along a microtopographic gradient. (Billings
1973.)
Although most researched in Europe, alpine plant phylogeography has also been
studied in Africa (e.g., Assefa et al. 2007; Kebede et al. 2007), Asia (e.g., Ikeda and Se-
toguchi 2007; Wang et al. 2008; Li et al. 2010), and South America (Hughes and East-
wood 2006), providing the potential to better understand how alpine plant distributions
are historically linked at a hemispheric to global scale (e.g., Ehrich et al. 2007).
Environmental Gradients in Alpine Tundra
Close examination of alpine tundra reveals a variety of plant communities responding
to different landscape features (e.g., Billings 1974b; Malanson et al. 2008). Many alpine
plants are broadly distributed, yet vary considerably in their sensitivity to environment-
al conditions. Consequently, alpine plant communities often exhibit distinctive spatial
patterns in response to steep environmental gradients over short distances.
The harshest environments in the alpine tundra result from the interaction of wind
and snow (Billings and Mooney 1968). Exposed ridges, for example, are usually blown
free of snow and offer little moisture or protection from wind (Fig. 7.10). Only lichens,
mosses, and a few prostrate cushion plants or dwarf shrubs are able to grow in such
sites. Toe-slope positions and depressions can also present unfavorable growing condi-
tions, if snow accumulation persists late into the season (Fig. 7.10). Midslope positions
are often the most favorable and productive, affording the vegetation both winter pro-
tection and summer moisture. Thus, the pattern of snow accumulation and snowmelt is
a primary factor in the establishment of alpine vegetative patterns (Billings and Bliss
1959; Billings 1973; Canaday and Fonda 1974; Webber et al. 1976; Boyce et al. 2005;
Fig. 7.11a).