Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 346

Characteristics of Timberline
In general, upper timberline elevations increase toward the equator (Fig. 7.6). The
highest timberlines, however, occur in the subtropics between 20° and 30°S latitude in
the Peruvian and Bolivian Andes and on the high plateau of Tibet (Miehe et al. 2007).
Global timberline ranges from sea level in the Arctic to 3,500-4,000 m (11,500-13,200
ft) in the moist tropics, and more than 4,500 m (14,900 ft) at its extreme upper limits
in the dry subtropics. Timberline elevation increases in the middle latitudes at an av-
erage of 110 m (360 ft) per degree of latitude until it reaches the subtropics at about
30°, where it peaks before descending near the equator (Holtmeier 2003; Körner 2007).
This pattern differs on islands and in marine locations, where timberlines are lower re-
lative to continental areas, in response to higher precipitation and cloud cover (e.g.,
Höllermann 1978; Fig. 7.4). In the United States, timberline varies from west to east,
occurring at 1,800 m (6,000 ft) in the marine climate of the Olympic Mountains and
Cascade Range, rising to >3,000 m (10,000 ft) in the dry continental climate of the cent-
ral Rocky Mountains, and descending to 1,500 m (5,000 ft) in the humid continental
climate of Mount Washington in New Hampshire. Similar to islands, timberlines often
reach higher elevations on larger mountains than on smaller mountains because of the
mountain-mass effect noted above. This relationship can be observed in all climatic re-
gions, including the central Alps and the Rocky Mountains, where trees grow at higher
elevations than they do on marginal ranges (Brockmann-Jerosch 1919; Griggs 1938).
An important characteristic of global timberline is the dominance of evergreen tree
species. These tree species are typically long-lived, have long reproductive life spans,
and are capable of regenerating asexually (e.g., Lloyd 1997). Each of these traits is
likely to contribute to the superior adaptation of conifers to the extreme environmental
conditions that occur at upper timberline (Daubenmire 1954). Nonetheless, broadleaf
deciduous species also form timber-line in some midlatitude areas and the tropics, and
can be found along with the needle-leaf conifers in the northern hemisphere.
Timberline Floristics
The vast majority of timberline trees of northern hemisphere mountains are pine
( Pinus ), spruce ( Picea ), and fir ( Abies ) . These genera are represented by similar species
throughout the northern hemisphere and occupy similar ecological niches, as illustrated
by the ecology of the white-bark pine ( P. albicaulis ), a western North America timber-
line species, and its close relatives in Eurasia. Whitebark pine produces large, wingless
seeds that rely on a bird, the Clark's nutcracker ( Nucifraga columbiana ), for seed dis-
persal through the harvesting and caching of seeds as a winter food source (Vander
Wall and Balda 1977). Unused portions of these caches subsequently serve as an im-
portant seed source for seedling establishment. An identical relationship exists between
pines and nutcrackers in Western Europe from the Alps to the Caucasus (e.g., between
P. cembra and Nucifraga caryocatactes ), and in the mountains of eastern Asia, between
P. pumila and N. caryocatactes var. japonicus (Holtmeier 1973).
Other, less common genera of the pine family found at timberline include hemlocks
and larches. The timberline hemlocks are represented by mountain hemlock ( Tsuga
mertensiana ) in the Cascade and Olympic ranges of western North America. The de-