Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 338

Broadleaf deciduous trees constitute the second common forest type found in northern
hemisphere mountains. These forests occur at low to middle elevations in humid midlat-
itude regions and are best developed in Western Europe, eastern Asia, and the eastern
United States. Although consisting of distinct species, the similar genera, appearance,
and structures of these forests suggest former connections among these widely separ-
ated regions during the Tertiary period (ca. 65 to 1.8 million years before present) (Del-
court and Delcourt 2000). The dominant genera in these forests include oak ( Quercus
spp.), maple ( Acer spp.), beech ( Fagus spp.), elm ( Ulmus spp.), hickory ( Carya spp.),
chestnut ( Castanea spp.), ash ( Fraxinus spp.), hornbeam ( Carpinus spp.), and birch
( Betula spp.).
Broadleaf deciduous forests in general differ from needle-leaf conifer forests by
possessing multilayered canopies and higher species diversity (Delcourt and Delcourt
2000). European beech forests are an exception to this “rule,” differing from their North
American counterpart by virtue of their single-layered canopy and low species richness.
These forests occur as a natural “monoculture,” free of surface mosses resulting from
leaf fall and burial.
Midlatitude deciduous trees are generally smaller than those found in tropical rain-
forests, but share their proclivity to grow larger and in more diverse communities with
increasing moisture. Seasonal leaf loss in broadleaf deciduous forests can result from
several factors, including low air temperatures and frost damage, low light levels and
shorter photoperiods, seasonal moisture deficit, and freezing ground that limits the
availability of water, thus inhibiting transpiration.
The broadleaf deciduous forests generally give way to conifers at higher elevations,
but broadleaf deciduous trees (mainly birches) serve as the upper timberline species in
some areas (Troll 1973; Price 1978; Nagy and Grabherr 2009). Disturbances, including
fire, windstorms, landslides, and avalanches, may also allow broadleaf deciduous trees
to extend into higher elevations normally dominated by conifer forests. Where both con-
ifers and deciduous trees are present, disturbance can result in a mosaic of interming-
ling communities of different ages and compositions.
Tropical and Subtropical Mountain Forests
Tropical mountain forests are characterized by high species diversity and the domin-
ance of broadleaf evergreen trees. The high species diversity of these forests is en-
hanced by several factors, including a multilayered canopy and a nearly continuous
cycle of reproduction under the near-constant thermic conditions. Although distinct
thermic seasons are generally absent in the tropics below 15°-20° latitude, many trop-
ical areas experience seasonal variations in the form of alternating wet and dry periods.
Deciduous trees that shed their leaves during the dry season dominate mountain forests
in areas with pronounced wet/dry seasons.
Similar to other mountain forests, the characteristics of tropical forests change
markedly with elevation. These changes include gradual decreases in tree height, spe-
cies richness, and structural complexity, as well as a shift from a three-storied to a two-,
and then a single-storied forest with increasing elevation. Changes in tree character-
istics with increasing elevation include a decrease in leaf size, loss of leaf drip tips, a
decrease in the frequency of trunk buttressing, and thickening of tree bark. The abund-