The ecoregion is characterized by dramatic vertical zonation of vegetation and asssociated fauna. This zonation is a consequence of abrupt elevational gradients between flatlands and mountains. Secondary climatic effects of topographic relief (e.g., rain-shadow effects, exposure to or shelter from prevailing winds, and thermal inversions) likewise influence zonation (Peet 1988). The overall elevation of the ecoregion is very high and reaches greater altitudes than any other part of the Rockies. It therefore exhibits the full range of life zones seen in other ecoregions farther north, while having much greater spatial extent of certain zones than other ecoregions. In addition to this distinctive physiography, the ecoregion's latitude and contiguity distinguish it as a clear ecological unit.
The dominant vegetation type in the ecoregion is coniferous forest. Species composition and associations are similar to those of the South-Central Rockies, with a few exceptions. Bristlecone pine (Pinus aristata) replaces whitebark pine (Pinus albicaulis) as the predominant upper treeline/ krummholz species (Peet 1988). Lodgepole pine (Pinus contorta) is relatively rare, while there is a good deal more Ponderosa pine (Pinus ponderosa) woodland in the Colorado Rockies than more northerly sites in the cordillera (Peet 1988). Extensive stands of aspen (Populus tremuloides) are prominent in the Colorado Rockies, apparently out-competing lodgepole pine as a major post-fire seral species (Peet 1988). In addition to expansive conifer forests, the ecoregion contains several other vegetation communities. Mountain meadows, foothill grasslands, riparian woodlands, and upper treeline / alpine tundra communities exist throughout the ecoregion.
Large, important herds of elk (Cervus elaphus) and mule deer (Odocoileus hemionus) inhabit the ecoregion. Mountain lion (Puma concolor) and black bear (Ursus americanus) are also abundant. It is likely that a remnant population of grizzly bears (Ursus arctos) still survives in the San Juan Mountains in SW Colorado and NW New Mexico. The Colorado Rockies may in fact have nearly all the species that were present prior to European settlement. Wolverine (Gulo gulo), Lynx (Lynx canadensis), and American marten (Martes americana) all occur in the ecoregion, reaching perhaps the southern limit of their peninsular distribution here. As such, the ecoregion may hold long-term evolutionary potential for these species.
Fire, snow avalanches, and wind are major disturbance patterns in this ecoregion. Prevailing winds alter distribution and morphology of tree species at higher elevations. Alpine tundra systems are some of the most extensive in the Western Hemisphere.
Habitat Loss and Degradation
Logging, hard-rock mining, oil and gas development, and recreational-residential construction are all major anthropogenic threats to the ecoregion. Domestic livestock grazing and introduction of exotic species are altering species compositions. The Medicine Bow / Snowy Range mountains in southern Wyoming have been subject to particularly intense commercial logging and sheep grazing. Low-elevation areas in the San Juans have been logged extensively, including late-seral and old growth stands of Ponderosa pine.
Burgeoning recreational use of remote areas is also affecting the ecoregion. Expansion of existing downhill ski resorts, and development of new resorts, is a pressing threat to the alpine and subalpine environment. Additionally, human settlement and transportation impacts associated with ski developments at Vail and Aspen are rapidly consuming low-elevation habitat and dramatically increasing vehicle traffic. Environmental pollution from the Denver metropolitan area and urbanizing areas in the mountains is also a concern. Densely-concentrated recreational use near the Front Range metroplex threatens fragile alpine environments; unregulated off-trail travel in easily accessible tundra can cause damage that will remain for decades.
Remaining Blocks of Intact Habitat
Rocky Mountain National Park, Indian Peaks Wilderness, Commanche Peak Wilderness, and Rawah Wilderness form one large, relatively contiguous block of wildlands in the northern part of the ecoregion. The San Juan wilderness complex in the southwest is a massive conglomerate of high, wild mountains. Large wilderness areas around Aspen and Vail, centered around the Maroon Bells and Collegiate Peaks, and the Flat Tops area north of I-70 protect extensive habitat areas in the central part of the ecoregion. The Sangre de Cristo Range, a long, narrow, range extending into New Mexico, occupies the southeastern part of the ecoregion.
Degree of Fragmentation
The ecoregion is fragmented by several major roads. Along these roads, development tends to creep outward in an ever-expanding zone. The Gunnison River corridor and I-70 are particularly problematic. Continued development is disrupting elk and bighorn sheep movements; the bighorns are particularly susceptible to stress. Ski area development and expansion represents another source of fragmentation. Disrupting everything from treeline to valley bottom in many locales, downhill skiing is a particularly damaging form of recreation - perhaps worse than ORV use.
Degree of Protection
There are several major protected areas, corresponding to many of the intact habitat blocks listed above. These include:
•South San Juan and associated roadless areas protect the wild core of the southwestern part of the ecoregion - southern Colorado
•Rocky Mountain National Park and adjacent wilderness areas in the northeast protect a fairly wide elevational range, although river valleys and foothills are greatly under-represented - north-central Colorado
The same could be said of many other wilderness areas in the ecoregion, which do an adequate job of protecting alpine and subalpine zones but do not embrace the full complement of habitat types. Lower-elevation old growth, particularly in the San Juans, is poorly represented in the protected area system.
Types and Severity of Threats
Loss of valley bottoms (and increasingly, hillsides) to homebuilding has major impacts on ungulate movements in many parts of the ecoregion. Downhill ski resort expansion is another source of habitat destruction. Concentrated, heavy recreational use of mountain areas near resort towns and the urban Front Range is another threat. Logging of low-elevation forests remains a threat as well.
Suite of Priority Activities to Enhance Biodiversity Conservation
•Halting any further old growth logging in the ecoregion is a high priority.
•To maintain lynx, wolverine, and marten, it will be necessary to conserve adequate areas of the forest habitats they require, which includes old growth to some extent. Efforts to restore natural predator-prey relations, as well as natural fire processes, are both possible and desireable in an area of such expansive wildlands.
•Management of recreational use is a key conservation action to avoid long-term degradation of sensitive alpine communities. Ending public subsidies for ski resort expansion would likely bring an end to further downhill ski development, as demand has remained flat over the past decade or more.
•Colorado Environmental Coalition
•Southern Rockies Ecosystem Project
•Sierra Club, Southwest Regional Office
•The Wilderness Society
Relationship to other classification schemes
The ecoregion boundary is taken from Omernik (1995). It corresponds closely to Bailey's M331F, G, H, and I, and Küchler's 14, 17, 20, and 45 for the same geographic area.
Prepared by: S. Primm