Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 337

spp.), and fir ( Abies spp.), which comprise 90 percent of the conifer species present in
both regions (Eyre 1968). The shared adaptive zones (environmental regions where spe-
cies coexist while exploiting the same resources) and current distribution of these gen-
era also implicate the historical role of mountain climates as a critical factor leading to
the southerly migration of these trees into the midlatitudes and subtropics (e.g., Perry
et al. 1998).
Many needle-leaf conifers exhibit broad ecological amplitudes (environmental toler-
ance among species) developed over a long history of natural selection under diverse
environmental conditions. Consequently, conifers dominate such diverse mountain
forests as the cloudy, marine climate of the Pacific Northwest in the United States, the
xeric slopes of the Atlas Mountains in North Africa, and the cold and windy slopes at
the upper timberlines throughout the northern hemisphere. In all these environments,
conifers have proven to be well suited to cold winters and short growing seasons.
Some of the success of conifers can be attributed to their compact and often conical
crowns, which expose a comparatively small surface area. This has the advantage of
shedding snow quickly and being less vulnerable to wind damage. The small, needle-
shaped leaves of conifers provide further advantages, including low transpiration rates,
low susceptibility to mechanical damage and wind abrasion, and high photosynthesis
efficiency under diffuse light conditions. Moisture retention is critical during periods of
high summer temperatures and the onset of freezing conditions in fall and winter, when
the desiccation potential is high. The ability to use diffuse light is advantageous in both
high-latitude regions, where incident radiation occurs at a low (oblique) angle, and in
areas with cloudy, marine climates. In the latter case, the ability of conifers to photo-
synthesize under low light conditions extends their growing season well beyond that of
competing deciduous broadleaf trees (Waring and Franklin 1979).
The evergreen habit of most needle-leaf conifers provides the additional advantage
of early growing season photosynthesis by avoiding the delay caused by new leaf
growth. This adaptation is critical in areas characterized by short growing seasons.
Northern hemisphere conifers can also exhibit high resistance to frost, depending on
their evolutionary history. For example, Pinus cembra, a European treeline species, can
survive temperatures below −75°C, having evolved its frost hardiness in a colder, more
continental climate region such as Siberia (Buchner and Neuner 2011). Paradoxically,
an evergreen habit can also be maladaptive in areas of cold continental climates, such
as portions of northern Siberia and Scandinavia. In these areas winter needle mortal-
ity favors the replacement of evergreen conifers by larch ( Larix spp.), a deciduous con-
ifer, or by broadleaf deciduous trees, such as birch ( Betula spp.) or aspen ( Populus spp.)
(Troll 1973).
The low nutrient requirements of conifers are another important adaptation that
allows them to occupy rocky substrates and unusual edaphic (soil-plant) conditions
(Kruckeberg 2002). Conifers also return fewer nutrients to the soil through leaf fall and
litter accumulation, resulting in relatively acid and infertile soils relative to those that
develop under broadleaf deciduous forests (Perry 1994). These conditions, in conjunc-
tion with canopy shading, result in less diverse understory communities than those typ-
ically found in broadleaf deciduous forests.
BROADLEAF DECIDUOUS FORESTS