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Mountain Complex

The mountain complex biome refers to the totality of vegetation types and associated animals which occupy the mountains and high plateaus of western North America. Elevational transects extending over several thousand feet may exhibit vegetation ranging from desert to alpine tundra. The type of plant community which will dominate a given altitude is dependent on three interrelated factors of the physical environment:
  1. steepness and orientation of slope with respect to incoming solar radiation;
  2. composition of the soil; and
  3. amount of available moisture.

On a more regional basis, precipitation and temperature vary considerably, depending on latitude and the location and orientation of mountain masses relative to the prevailing westerlies and relative to other mountain masses. Thus, we see that timberline decreases in altitude with increasing latitude in both the Rockies and Pacific Coast mountains, but that at a given latitude timberline is lower in the coastal mountains than in the Rockies. The effects of orographic precipitation are seen in the dramatic shifts in vegetation from the windward to leeward sides of mountain masses.

Southern Rockies (Arizona)

The mountain complex of the southern Rockies is exemplified in the vegetation of the San Francisco Mountains of northern Arizona, which reach altitudes in excess of 12,000 feet. The zonation of flora ranges from desert species at the lowest elevations (where rainfall is low and evapotranspiration rates are high), through Douglas-fir and spruce-fir forests at the middle elevations (where moisture from orographic precipitation is high), to alpine vegetation at the highest points (where low temperatures becomes a limiting factor).

The creosote bush-greasewood association (Larcea divaricata and Sarcobatus vermiculatus, respectively) occupies the hot, dry lower slopes below 6000 feet. Once desert grassland, this area was severely overgrazed by sheep and cattle.

The range between 5000 and 7000 feet is occupied by the pinyon-juniper association. The area is characterized by an environment of alkaline soils, low rainfall and humidity, high wind movement, intense sunlight and high evapotranspiration rates. The stands of slow growing, sparsely spaced trees range from pure pinyon (Pinus cembroides) to pure juniper (Juniperus spp.), probably depending upon available seed source as much as on geographical location and local environmental conditions. 

The ponderosa pine (Pinus ponderosa)

zone is found from 6000-8000 feet. Here it forms a well-adapted fire-related
association with the easily combustible grasses found in the understory and having
general park-like appearance. 










The Douglas-fir zone is situated in the 7500-9000 feet elevational range. The area of overlap of the ponderosa pine and Douglas-fir zones is commonly referred to as the lower montane forest. Compared to lower zones, daily temperature fluctuations in the Douglas-fir zone are smaller, the average temperature is somewhat lower, and humidity is significantly higher. The Rocky Mountain variety of Douglas-fir (Pseudotsuga menziesii var. glauca) encountered here is much smaller (and more tolerant in the understory) than its relative in the Pacific Northwest (P. m. var. menziesii) and usually occurs in mixed stands rather than in pure stands. Common associates include white fir (Abies concolor) and blue spruce (Picea pungens) on moist sites, and ponderosa pine and lodgepole pine (Pinus contorta) on drier sites. The dry exposed ridges of this zone are commonly occupied by limber pine (Pinus flexilis) and bristlecone pine (P. aristata). 


In the cold, humid environment above the Douglas-fir zone, between 8,500 and 11,500 feet, is the spruce-fir zone, dominated by Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa). In the lower and middle parts of the range, subalpine fir occupies sites too wet, too dry or too low in nutrients for Engelmann spruce to grow. At higher elevations it is not uncommon to find pure stands of Engelmann spruce. As in the lower montane forest, limber and bristlecone pines occupy the dry, exposed, south-facing ridges and slopes. Many of the bristlecone pines are over 4000 years old. 


Krummholz is the name given to the dwarfed and stunted trees that occupy the transition zone between the spruce-fire zone and alpine tundra. The environment is characterized by intense solar radiation, high winds and large diurnal temperature fluctuations. 


The alpine tundra zone occurs above timberline where climate is even more severe than that in the Krummholz. Short grasses (Poa spp., Festuca spp.), sedges (Carex spp.), tiny alpine flowers, and rocky outcrops dominate the landscape.


Above the tundra lie the mountain tops, a land of perpetual ice and snow. The complete lack of soil and the severe environmental conditions combine to make colonization by even the hardiest invaders an impossibility.

Alpine meadows occur in depressions sheltered by hills or open areas surrounded by trees. Large amounts of snow accumulate and with the mositure provided by the "spring" melt (late June-early July) showy wildflowers come to dominate the landscape.

Moist Temperate Coniferous Forest Biome page    1    2    3    4