Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 358

Forest to Tundra Transition-Northern Hemisphere
Trees above forestline frequently grow in small clumps or “islands” with the tallest trees
in the center, decreasing in height outward to the margins (Fig. 7.9). These “islands”
typically begin as a single tree growing in a favorable microsite (see above). Once es-
tablished, these trees subsequently modify the surrounding environment and enhance
the probability of seedling establishment and the growth of the island. Environmental
modification by early colonizing trees begins through their absorption of incoming solar
radiation and the creation of small heat islands. These higher temperatures accelerate
the rate of spring snowmelt and consequently extend the local growing season, which
promotes seedling regeneration, growth, and needle maturation. The tree canopies also
trap outgoing long-wave radiation during the summer and increase snow accumulation
during the winter, helping to moderate seasonal temperature extremes. Nurse trees
create wind shadows and contribute organic matter to the developing soils. Many tim-
berline tree species also reproduce by layering—the establishment of new stems from
lateral-growing branches that take root when they come into contact with the soil. This
process is common in areas of high snow accumulation, where it aids the vegetative ex-
pansion and merger of tree clumps.
Krummholz forms initially develop a vertical habit, becoming more prostrate with in-
creasing elevation. These small tree clumps typically have one or two primary stems
surrounded by a dense skirt of limbs and shoots extending horizontally around the base.
The height of this skirt reflects the depth of winter snowpack, as buds or shoots pro-
truding above the snow are killed by wind abrasion and dehydration (Hadley and Smith
1983, 1986; Fig. 7.9). The aerodynamic structure of krummholz mats (Daly 1984), along
with their high needle density, promote krummholz survival by increasing snow accu-
mulation, minimizing snow abrasion, and increasing needle temperatures (Hadley and
Smith 1987). While environmental factors are clearly important to the formation of
krummholz forms, the relative contribution of genetics (ecotypes) remains poorly un-
derstood (e.g., Wardle 1974; Grant and Milton 1977).
Several other environmental and ecological factors have been implicated in the de-
velopment and persistence of alpine krummholz. These include snow-related factors,
such as frost damage, snow pressure, pathogens, and snow water content (Daly 1984),
as well as variations in solar radiation, soil moisture, soil properties, and the autecology
of the local timberline species (Weisberg and Baker 1995; Seastedt and Adams 2001).
Hessl and Baker (1997) further found that the combination of warm temperatures and
heavy snowpack occurring over several years are required for tree establishment in the
Colorado Front Range. These factors suggest that the processes influencing tree estab-
lishment and the patterns of upper timberline are highly complex and dynamic. The dy-
namic nature of timberline is further evidenced by the downwind migration of krumm-
holz (Ives 1973; Marr 1977; Benedict 1984).
Forest to Tundra Transitions-Tropics and Southern Hemisphere
Krummholz is generally not well developed in the tropics or some southern hemisphere
mountains. In New Zealand, for example, the Nothofagus forestline occurs as an abrupt
transition from erect trees to a zone of low-lying shrubs (Wardle 1998). These shrubs