Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 349

FIGURE 7.7 Hypothetical temperature-predicted treeline limits modified by local environmental con-
ditions and disturbance. Treeline (solid line), timberline and tree species limits (dashed line), and
eco-tone breadth are indicated by horizontal lines. Causes and hypothetical extent of treeline
depression are indicated by letters, with treeline depression indicated by arrows and light gray
shading. Krummholtz are noted by gray triangles. Gray circles represent isolated populations of
erect (non-krummholtz) trees. Dark gray indicates closed forest conditions. White areas repres-
ent the nival zone (areas of permanent snow). (Modified from Körner 2007).
Another unusual form of local timberline is the ribbon forest, glade, or “shimagare”
common in the central Rocky Mountains (Billings 1969, Miles and Singleton 1975) and
other midlatitude mountains (e.g., Iwasa et al. 1991; Figure 7.8). These long, narrow
features form as a consequence of downwind snow accumulation and subsequent peri-
ods of prolonged soil saturation, resulting in a short growing season that inhibits tree
growth and seedling establishment (Billings 1969; Hattenschwiler and Smith 1999). A
similar but more insular form of timberline occurs in regions of heavy snowfall such as
the Pacific Northwest region of North America (Franklin and Dyrness 1988; Fig. 7.8).
Timberline can also be depressed in response to the underlying geology, grazing,
forest clearing, lack of local seed sources, and fire. Such factors can result in the form-
ation of grassy balds found in the southern Appalachians and Oregon Coast Range, and
the parks found in the Rocky Mountains and Great Basin ranges in Nevada (e.g., Billings
and Mark 1957; Gersmehl 1971, 1973; Lindsay and Bratton 1979). Fire plays a similar
role in determining the location of treeline in arid and semiarid regions. In these moun-
tain environments, forest succession is typically slow, because most timberline tree spe-
cies are not fire-adapted. As a result, trees growing near the previous treeline may re-
quire hundreds of years to reestablish (Peet 1981; Shankman 1984; Daly and Shankman
1985). In the absence of conifers, aspen can become both the dominant species and the