Department of Biological Sciences, Fort Hays State University Instructor: Mark Eberle                  Course Homepage

Rocky Mountains Snowy Range Scenic Byway Medicine Bow Mountains Wyoming

As an air mass from the Pacific Ocean travels eastward across the western United States, it rises over mountain ranges, which cool the air and reduce its ability to hold moisture. As a result, precipitation falls at the higher elevations on the western slope of the mountains. As the air crosses the mountains and drops down the eastern slope, it is holding less moisture because of the losses to precipitation as it rose up the western slope, and because the air becomes warmer, giving it a greater capacity to hold moisture, further reducing its relative humidity. Thus, the eastern slope generally receives less precipitation than the western slope of the mountains. Differences also exist on north-facing and south-facing slopes. The tilt of the Earth on its axis means that northern exposures receive less solar radiation than southern exposures in the Northern Hemisphere, and this reduces the amount of evapotranspiration that occurs on the north-facing slopes. This leads to generally wetter conditions on the north-facing slopes than on the sunnier south-facing slopes under otherwise similar conditions. In addition, temperature gradients from the base of the mountains to their peaks mean that the length of the growing season decreases as elevation increases.

These environmental factors and others (e.g., soil type) create generally recognizable communities on the different slopes and elevations of a mountain range, as we will see in the mountain ranges we visit. However, disturbance (e.g., fire, windthrow, insect damage, fungal damage) is a regular component of some communities. These disturbances and the transitional zones in local environmental conditions produce a mosaic of forest communities that can make generalizations about comunity boundaries difficult.

The lower forests of the Southern Rockies in Colorado and New Mexico typically are comprised of ponderosa pine (Pinus ponderosa), often bordered at lower elevations by woodlands of juniper and piñon (photographs above). These pine forests typically have an open understory dominated by grasses and forbs (photographs above and below). Because of the higher latitude and elevation in Wyoming, piñon and ponderosa pines are absent from our route through the Snowy Range. As we leave the high-elevation grasslands around Centennial, we drive through stands of lodgepole pine (Pinus contorta) and the white-barked quaking aspen (Populus tremuloides) (photographs below), 2 species that often dominate the middle montane forests and occupy areas that have been "recently" burned. Recent is a relative term, though; lodgepole pine forests can perpetuate themselves for centuries.

The natural frequency of fires in the southern Rocky Mountains is higher than in the coastal forests we visit in the Pacific Northwest. Some species native to the interior forests (e.g., Rocky Mountains, Cascade Mountains, and Sierra Nevada) are adapted to low-intensity fires in the understory of the forest. For example, lodgepole pines of the Rocky Mountains (Pinus contorta latifolia) can produce closed (serotinous) cones sealed with a resin that has a melting point of 63°C (145°F). Heat supplied by fires can open the cones to release the seeds onto a mineral-rich forest floor cleared of its understory. Lodgepole pines in the Sierra Nevada and Cascade Mountains (Pinus contorta murrayana) produce open (nonserotinous) cones that release their seeds at maturity without waiting for a fire (summaries by Lanner, 1999:80-87, and Benedict, 1991:428-429). [We will see a third subspecies, Pinus contorta contorta, referred to as shore pine, on the coast.] Forest pines, including lodgepole and ponderosa pines, tend to loose their lower branches (photograph above), which helps larger trees escape the formerly more frequent ground fires. With fire suppression efforts, however, more plant material builds up in these forests, which is one factor that now makes crown fires more likely (photograph below).

Continuing our climb over the Snowy Range, we soon reach the subalpine meadows and groves of Engelmann spruce (Picea engelmannii), the tree that dominates these higher elevations (photograph below). In other areas of the Southern Rockies, Rocky Mountain subalpine fir (Abies lasiocarpa) joins Engelmann spruce at tree-line. Thin bark, resinous sap, and retention of lower branches make these subalpine trees susceptible to damage or death from fires; however, natural fires at this elevation were less frequent than in the coniferous forests at the warmer and drier lower elevations. The pyramidal shape of the spruce and fir trees helps them to shed heavy snows and gives them a lower wind resistance than broad-crowned conifers, such as the ponderosa pine. These spruce and fir trees also can carry out optimal rates of photosynthesis at lower temperatures than other conifers, allowing them to "extend" the brief growing season at this high elevation (summary by Benedict, 1991:469-470).

Eventually, we cross Snowy Range Pass (3,300 m or 10,800 feet). At this elevation, the cold prevailing winds during the winter damage the branches (buds) on the windward side of the Engelmann spruce. With branches growing only on the lee side of the tree, they look like tall pennants, which gives us the common name for this process: flagging (photograph below left). It is a common misconception that the wind simply bends the branches to one side of the tree, where they remain like flags on a pole. [Isolated trees in western Kansas also can exhibit flagging resulting from hot, dry, southerly winds.] Subalpine trees also grow in "tree islands" (photograph below right), where trees on the windward side offer protection to the other trees in the stand. In some areas, cold winds prevent subalpine trees from growing taller than shrubs, a growth form known as krummholz.

Literature

• Benedict, A. D. 1991. Sierra Club Naturalist's Guide: The Southern Rockies. Sierra Club Books, San Francisco, California.
• Lanner, R. M. 1999. Conifers of California. Cachuma Press, Los Olivos, California.

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