Montane forests

Mountains rise like islands from the surrounding landscape, serving as landmarks, recreation areas for humans, and refugia for plants and animals that cannot be found in nearby lowlands. Sometimes mountain ranges serve as migration routes, other times they serve as migration obstacles. Often, the resources they hold are biologically and economically important.

Mountains are valuable because of their uniqueness—they are areas of extensive environmental change over relatively small distances. Despite the distinct nature of individual mountains or mountain ranges, some generalizations may be drawn about the characteristics of the mountain environment. For example, temperatures typically decline with elevation, while precipitation usually increases. Even with the greater precipitation, soil development on mountain slopes may be inhibited because steep slopes inhibit the accumulation of organic matter, and soil moisture may be lacking because steep slopes prevent storage of water. High peaks and ridges may be enshrouded in clouds, yet the intensity of solar radiation increases with elevation as there is less atmosphere with which to filter incoming solar radiation.

Other important environmental variables are not dependent on elevation. Geology may influence plant distributions by influencing the physical and chemical characteristics of the parent material from which the soils that plants grow in are derived. Landform characteristics may influence the pattern and frequency of disturbances such as landslides.

Exposure likewise adds another level of complexity to how plants are distributed on mountains. Different exposures—the direction that a slope faces—are subject to different patterns of diurnal (daily) shading, heating, and cooling, which in turn influence moisture supply. Likewise, sites shaded or sheltered by adjacent slopes or adjacent mountains tend to be wetter. More shade leads to cooler temperatures, and adjacent slopes may also dampen wind velocity—both likewise lead to reduced evapotranspiration, a term combining evaporation from soils and plant surfaces with transpiration, or loss of water from stomates (openings) in leaves. Exposed sites feel the full brunt of sunlight and desiccating winds and thus are usually drier.

One by-product of this seemingly infinite number of combinations of site characteristics is an incredible patchwork of biotic diversity in mountainous areas. Nevertheless, by analyzing the montane landscape in terms of environmental gradients, some reasonable generalizations about how montane forest composition and structure change across mountain landscapes can be drawn.

The most useful environmental gradients to evaluate montane forests are elevation, latitude, and moisture gradients. More properly, these gradients are “complex gradients” as they represent gradients of several environmental factors that change—though not necessarily in the same way—in more or less the same direction. A good example of a complex gradient is the elevation gradient in which both temperature and precipitation change with elevation. On a global scale, changes along one gradient may mimic changes along another. In the case of mountain ecosystems, the elevation gradient mimics the latitude gradient in terms of temperature. According to the bioclimatic law, the temperature effect of a change of 400 feet (122 meters) of elevation is equivalent to a change in 1 degree of latitude.

Elevation

One of the most noticeable characteristics of mountains is how vegetation and other phenomena change with elevation. Temperature inversions, where warmer air overlays cooler air, may occur, but, in general, temperatures decrease with increasing elevation. The change in temperature—a measure of the available solar energy that powers biological systems—with elevation is measured by lapse rates. The rate of temperature change ranges between 5.5 degrees F per 1,000 feet (3 degrees C per 300 meters) in dry air to 3 degrees F per 1,000 feet (1.7 degrees C per 300 meters) in air saturated with water vapor. The decreasing temperature affects other important variables. One is vapor pressure, a measure of the amount of water vapor in the atmosphere. Since temperature is the primary variable influencing the amount of water vapor air can hold, vapor pressure decreases with elevation as well. The decrease is logarithmic, with the rate of change faster at lower elevations than at higher elevations. The drier the air, the greater the evapotranspiration demand. Drier air is likewise more effective at transmitting solar and infrared (heat) radiation, which translates into faster heating and cooling.

As temperature decreases, so does the length of the growing season—at least in temperate locations. The growing season may be year-round in tropical locations. Rather than a hot season and a cold season, tropical mountains have hot days and cold nights. Diurnal (daily) temperature ranges decrease with elevation.

Latitude

One of the main latitudinal relationships is the decrease in temperature and annual incoming solar radiation from the equator to the poles. The equatorial zone, with nearly 12 hours of daylight year-round, gets more than enough energy and, as a result, has on average high average temperatures. Where water is plentiful, lush plant growth—especially forest growth—is possible.

In subpolar and polar latitudes where daylight alternates between nearly 24 hours of daylight in the summer and nearly none in the winter, short growing seasons and limited solar energy inputs result in low temperatures and short growing seasons. The combination drastically limits the type of plant growth possible. Because of the low angle of the sun in the sky, slopes cast longer shadows, thus magnifying topographic effects on temperature and moisture conditions in mountainous environments.

Latitude has a major effect on temperature ranges. In tropical regions, diurnal temperature ranges are greater than annual temperature ranges. Outside the tropics, the reverse is true.

Moisture

For decades, researchers studying vegetation patterns in mountainous environments have used slope position and exposure as a proxy for the moisture gradient. In the Great Smoky Mountains of North Carolina and Tennessee, for example, sheltered (shaded) coves would be at the extreme mesic (moist) end of the moisture gradient. Moving from the mesic to the xeric (dry) end of the gradient, ravines and draws would be next, followed by sheltered slopes, open slopes, and exposed ridges and peaks.

In the Northern Hemisphere, southwest-facing slopes, because they are exposed to direct sunlight during the warmest part of the day, tend to be drier because the higher air temperatures, combined with solar heating, increase loss of water through evapotranspiration. On the other hand, northeast-facing slopes, which are exposed to direct sunlight during the coolest part of the day, tend to be wetter because evapotranspiration demand is lower. Between those extremes, from mesic to xeric, are north-, east-, northwest-, west-, southeast-, and south-facing slopes.

Another possible component of the moisture gradient is slope configuration (shape). At one extreme are convex slopes, which tend to disperse moisture and thus are drier. At the other extreme are concave slopes, which tend to collect water and thus are wetter.

Other Factors

Other factors also influence the characteristics of montane forests. For example, where a long mountain range of sufficient height runs perpendicular to prevailing wind directions, the windward slopes of the range force air upward. As the air rises, it cools, and the water vapor it carries condenses and falls as orographic precipitation. Over the ridge, the air follows the contour of the land, sinking, warming, and, in effect, drying. Such a process produces a lush, wet forest on the windward slopes and a scrub forest or woodland with lowland desert on the leeward slope.

This pattern produces the temperate rainforests characteristic of the Pacific Northwest region of the United States. Mountain ranges, such as the Pacific Coast Ranges, Cascades, and Sierra Nevada, intercept moisture-laden air masses driven off the Pacific Ocean by the mid-latitude westerlies. As the winds descend back down the slopes, they produce dryland environments in areas such as California's Central Valley and Nevada's Great Basin.

Geology and soils can create unique patterns on smaller scales. Shales, for example, give rise to fine-grained soils from which plants have difficulty extracting water. Serpentine soils are toxic to many plants. The type of vegetation that typically occurs on shale or serpentine barrens tends to have a composition and structure more typical of drier environments.

The Ridge and Valley Province of Virginia features a number of shale barrens. Whereas most slopes are covered with thick stands of tall oak (Quercus), hickory (Carya), pine (Pinus), or hemlock (Tsuga), the shale barrens are open, with stunted, scraggly versions of some of the same species. Serpentine barrens in the mid-Atlantic region of the United States often feature post oak (Q. stellata) and blackjack oak (Q. marilandica), two species more common in the Cross Timbers, a savanna formation on the plains of Oklahoma and Texas.

Generic Patterns

In tropical or warm temperate climates with adequate precipitation, the lower flanks of a mountain (lower montane belt) are typically covered with broadleaved species. In the tropics, the dominant tree species are broadleaved evergreen. In temperate regions they are typically broadleaved deciduous species. There may be a mixture of coniferous and deciduous species in the montane belt above, but conifers typically dominate. Conifers, particularly spruces (Picea), firs (Abies), and pines (Pinus) again dominate the subalpine zone in the Northern Hemisphere. Other species, such as southern beech (Nothofagus) and conifers of the genus Podocarpus are the main dominants of the Southern Hemisphere subalpine zone.

Above the subalpine zone is the alpine zone, where tundra—a plant community dominated by herbs, grasses, and low shrubs—replaces forest where climatic conditions prohibit the growth of trees. The forest-tundra ecotone (boundary) is the upper timberline (or tree line). In arid environments, the lowlands are dominated by desert or steppe vegetation because the lack of water likewise inhibits tree growth. Woodlands replace desert or steppe vegetation at higher elevations where precipitation is sufficient for tree growth. The desert/woodland or steppe/woodland ecotone is called the lower treeline.

Tree lines and boundaries between vegetation belts may change depending on local or regional conditions. On any given mountain slope, boundaries between vegetation belts tend to be lower on wetter slopes than on drier slopes. Tree lines on isolated mountains tend to be lower. On a much larger scale, tree lines occur lower the closer one is to the poles.