Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 374

Depending on one's scale of interest, the plant geography and ecology of mountain
forests and alpine tundra can be viewed as part of a broad environmental continuum, or
as a complex mosaic of unique vegetation types reflecting highly localized environment-
al conditions. At the larger scales, mountain vegetation manifests itself as elevation-
defined vegetation zones responding most closely to attitudinally controlled changes in
temperature and precipitation. Locally, however, plant species and community struc-
tures vary dramatically in response to topo- and microscale environmental conditions
and disturbance history.
The plant species in any mountain region are also the product of migration, adapt-
ation, and evolution. In some cases, including most midlatitude mountains, subalpine
and alpine plants are recent arrivals following the melting of extensive Pleistocene gla-
ciers. These plants represent pre-adapted species moving among similar (cool) environ-
ments, often recolonizing formerly occupied areas. In more isolated ranges, mountain
plants have evolved from ancient floras and have remained local or regional endemics.
These plants have adapted slowly over the millennia as the mountains themselves were
created and uplifted. In still other cases, the mountain vegetation is composed of sur-
rounding lowland species that have been able to adapt to the conditions at altitude and
have migrated slowly to higher elevations. These too may have become isolated and no
longer interact with their distant relatives in the lowlands, especially in tropical regions.
Relationships between individual species and their environment ( autecology ) are
highly specialized among most alpine tundra species, reflecting their adaptation to ex-
treme environmental conditions. In some ways, they are similar to “weedy” plants com-
mon in midlatitude lowlands. Both types of plants exhibit extreme hardiness, the ability
to live under extreme environmental conditions, and the ability to pioneer new sites.
Alpine plants differ from weedy species by their perennial life cycle and other adapta-
tions that allow them to survive repeated periods of extreme differences in temperature
and moisture. Although some species are highly adapted to local conditions and limited
in their distribution, others have broad ecological amplitudes and wide geographical
ranges, living successfully from the lowlands to the highlands.
The Future of Mountain Vegetation
Humans have played a major role in altering the structure and spatial patterns of moun-
tain vegetation for thousands of years. Since prehistoric times, people have used fire
for hunting and to clear land for agriculture. In the tropics, where these activities have
been practiced longest, it is likely that few, if any, tropical timberlines are truly “natural”
(e.g., Ellenburg 1979; Horn 1998; Young and Keating 2001). This may be illustrated by
the predominance of the heath family, Ericaceae, near tropical timberlines and its ability
to regenerate after fire (Sleumer 1965; Janzen 1973; Wesche et al. 2000). Ericaceae is
also well represented in the subalpine and alpine zones of northern hemisphere moun-
tains, in part because Native Americans regularly set fires in the upper forest to main-
tain and increase their harvests of Vaccinium spp. berries (Boyd 1999).
Humans have also altered the character of forests and timberline through logging,
agricultural clearing, firewood collection, and charcoal production (e.g., Zimmerer and
Young 1998; Bolton 2005). Intensive livestock grazing has further eliminated young
trees and hindered seedling reestablishment through soil compaction (Byers 2005).