Mountain Forests - Mountains and Plains: The Ecology of Wyoming Landscapes - page 174

than might be expected based solely on elevation. Val-
ley bottoms are often cool because of cold-air drainage.
thus, species typically found high on the mountains
may extend to low elevations in ravines or on north
slopes with less direct sunlight, and species requiring a
warmer environment may be found at unusually high
elevations on south slopes (fig. 11.1).
Soils also have a pronounced effect on the landscape
mosaic. For example, meadows occur in the Bighorn
Mountains on comparatively dry, fine-textured soils—
even at high elevations where forests might be expected
(see chapter 12). Forests of Douglas-fir, lodgepole pine,
engelmann spruce and subalpine fir grow on other
kinds of soils (inceptisols and Alfisols), where moisture
stress is less severe in late summer (such as on north
slopes). Lodgepole pine is typically the most common
tree in forests on coarse, less fertile soils derived from
granite or rhyolitic lava. Multiple factors—soils, local
climate, influences of herbivores and pollinators, and
local history of disturbance and species migrations—

all interact to determine the spatial distribution of any
given plant species . 4

Surviving in the Mountains

Perhaps the primary challenge for plant survival in the
Rocky Mountains is the short, cool, and sometimes
dry growing season. Several distinctive adaptations are
involved. Most notable is that photosynthesis occurs
in montane plants at temperatures near freezing, or
sometimes even below, 5 a nd many plants are evergreen
or wintergreen (that is, they have chlorophyll in their
leaves or stems all year). the ability to tolerate low tem-
peratures, combined with the presence of chlorophyll
throughout the year, enables some plants to extend
their growing season into late fall and begin the next
one earlier in the spring. evergreen conifers are excel-
lent examples, but other plants have similar adapta-
tions. For example, such deciduous plants as aspen
and dwarf huckleberry have chlorophyll in their stems,
which permits photosynthesis even when they have no
leaves. the green stems of the huckleberry may account
for a substantial portion of the plant's annual photo-
synthesis . 6 Both aspen and huckleberry are examples of
deciduous evergreen plants.
Some shrubs and herbaceous plants have leaves
that remain green even under the snow—for example,
elk sedge and wintergreen. they are capable of photo-
synthesis and growth as soon as the snow is shallow
enough to permit light penetration, when the tempera-
ture is at or near freezin g. 7 t he capacity for photosyn-
thesis at cold temperatures is an important adaptation,
but simply being able to survive the cold is another. A
comparison of low-temperature tolerances of numerous
tree species found that, unlike the trees from warmer
climates, those of the Rocky Mountains could tolerate
temperatures of -76°F. 8
Another problem for mountain plants is acquiring
nutrients from infertile, coarse-textured soils. All of the
tree species have mycorrhizae (fig. 11.2), which are con-
centrated in the top 12 inches of the soil, where limiting
nutrients are most likely to be available. once incorpo-
rated into plant tissues, the nutrients are retained for
extended periods simply by the longevity of leaves and
twigs. Lodgepole pine needles, for example, persist for
5-18 years, depending on environmental conditions.

Fig. 11.1. Approximate distribution of different forest types in
relation to elevation and moisture availability. note that each
kind of forest typically occurs at the lowest elevation in cool,
moist ravines. Foothill shrublands occur just below wood-
lands and forests at lower treeline.

12000

3500

Alpine

Fellfield

9000

Turf

Wet
meadow

Spruce-fir
Lodgepole pine
Aspen

9000

3000

Willow

Aspen

9000

Lodgepole pine

2500

Willow
Alder
Birch

8000

Douglas-fir
or
Ponderosa pine

Aspen

7000

Cotton-
wood

Foothill
shrublands

2000

6000

Wet
valley
bottom

Cool
ravine

North
slope

E-W
slope

South
slope

Ridge

MOISTURE GRADIENT