The Southern Rocky Mountains Physiographic Area (62) covers much of the central region of Colorado, with small extensions into southern Wyoming and northern New Mexico. This physiographic area encompasses the majority of the forested lands in Colorado, except for pinyon-juniper forests, most of which occur in the Colorado Plateau Physiographic Area (87). The topography is rugged, with over 50 mountain peaks rising above 4300 m (14,000 ft) elevation. Very large, flat, open, mountain basins (called "parks") also occur.

Annual precipitation varies from 25-100 cm (10-40 in), much of it occurring as snowfall during the winter months. Some permanent snowfields and remnant glaciers are found at higher elevations. Local precipitation is heavily influenced by elevation. Elevation and exposure, and their effects on soil moisture, also strongly influence plant communities. Understory vegetation is sparse in most forest types except for aspen.

Forests in the Southern Rocky Mountains may be naturally "patchier" than most other forest types in North America, due to the severity of the weather and topography and the effects of other forces such as avalanches, snow accumulation, fire, insects, and disease. The resulting landscape pattern is a complex mosaic of open meadows and forest stands of varying age and species composition.

The primary large-scale disturbance agents are fire and insect outbreaks. Other agents may act on a smaller scale. In some areas, dwarf mistletoe is common. This is a parasitic growth that affects the growth form of some conifers. Long considered a destructive pest by timber managers, its presence has been correlated with higher bird abundance as the witch's brooms (clusters of small branches) that result provide nesting platforms for some species, and the weakening of trees by mistletoe allows for attacks by insects, which can provide prey for birds (Bennetts et al. 1996).

A breakdown of Colorado timber types and ownership (as of 1983) demonstrates that much of the forested land in this physiographic area is publicly-owned (Benson and Green 1987).

Significant federal holdings (and approximate areas) include the Arapaho (415,000 ha; 1,024,000 ac), Grand Mesa (140,000 ha; 346,000 ac), Gunnison (674,000 ha; 1,665,000 ac), Pike (449,000 ha; 1,110,000 ac), Rio Grande (752,000 ha; 1,859,000 ac), Roosevelt (327,000 ha; 808,000 ac), Routt (455,000 ha; 1,126,000 ac), San Isabel (452,000 ha; 1,117,000 ac), San Juan (760,000 ha; 1,878,000 ac), Uncompahgre (382,000 ha; 945,000 ac), and White River (794,000 ha; 1,962,000 ac) National Forests, and Rocky Mountain National Park (108,000 ha; 266,00 ac). Lands managed by the BLM comprise 810,000 ha (2 million ac).

Important bird habitats in Physiographic Area 62 include ponderosa pine, mixed-conifer, aspen, and spruce-fir forests, mountain shrubland, cliff/rock, sagebrush shrubland, lowland riparian, high-elevation riparian, wetlands, and alpine tundra.

Conservation Issues

Hejl (1994) identified six ways that western coniferous forests have been altered by humans over the past 100 years: fire exclusion, timber harvesting, grazing, residential development, chemical applications, and introduction of exotic diseases, plants, and animals. The effects on bird populations of these actions are understudied and poorly understood, although more is known about the first two than the others.

Fire exclusion has resulted in stands overstocked with small trees or heavy fuel loads of dead and down trees, especially in ponderosa pine habitats, where the forest evolved with frequent (low-intensity) fires. Overstocked stands and heavy fuel loads have altered fire intensity, leading to larger and hotter catastrophic fires. Increases in road density have resulted in fragmented habitats, with unknown impacts on bird populations. Even-aged timber harvest techniques move the forest toward homogeneity, rather than the more natural landscape mosaic of uneven-aged stands.

Timber harvesting may have reduced the density of snags, especially large snags, to the detriment of cavity-nesting species. For many years, harvesting was driven by economics and expedience, with little thought given to ecological consequences. Southern Rocky Mountain forests were harvested with even-age management techniques such as clearcuts, which simplify harvesting but result in unnatural, homogeneous forests. Compared to some other physiographic areas that are strongholds for industrial forestry, this area is drier and the forests experience shorter growing seasons due to the elevation. As a result, Southern Rocky Mountain forests do not regenerate quickly after harvesting and rotations must be longer.

Brown-headed Cowbird populations in Physiographic Area 62 increased at a 6.70% annual rate (P<0.01) over the 1969-96 period. If this trend continues it could seriously impact populations of those species that are susceptible to nest parasitism.

Conservation Opportunities

Many bird species in this physiographic area are not adequately monitored by the BBS. Its limitations include its restriction to roads which, in mountainous areas, often parallel streams or rivers. This emphasizes riparian birds and edge species, and under samples forest interior species. Also, routes that run along streams climb (or descend) in elevation, so the route may sample different forest habitat types. CBO's M2001 monitoring program avoids these problems by utilizing habitat-based surveys, with point count transects oriented randomly through those habitats. Once this program is in place for several years, it should provide valuable data on trends in bird populations.

The U.S. Forest Service is shifting its emphasis away from timber production to genuine multiple use of forests (including wildlife habitat), and adopting a more holistic management strategy (for example, use of prescribed burns) - this is partly a result of growing public pressure to change forest management practices, and USFS and other agencies have responded by adjusting their policies on clearcuts, forest fragmentation, and other practices.

New tools and techniques are available for measuring and analyzing landscape patterns and bird distribution (Geographic Information Systems, Global Positioning Systems, remote sensing, computer models in landscape ecology, etc.), and their application can do much for forest bird conservation. With these advances, forest managers have the ability to manage forests to mimic natural landscape patterns resulting from disturbance regimes. While forest management practices do not exactly replicate natural disturbance agents, the possibilities are promising and, from a conservation standpoint, more desirable than working without any sense of natural patterns.

Avifaunal Analysis

Of the 123 species that regularly breed in this physiographic area, populations of 3 species are strongly represented in alpine tundra, 10 in aspen, 13 in cliff/rock, 13 in high elevation riparian, 11 in mixed-conifer, 9 in mountain shrubland, 11 in ponderosa pine, 4 in sagebrush shrubland, 17 in spruce-fir, and 32 in wetlands. However, few species are confined to a single habitat type--an outcome of adaptation to a complex forest mosaic. Taking into consideration all of the shifting among habitats and use of several habitats by each species, aspen forests support the most bird species and the most individual birds of any of the forest types in the Southern Rocky Mountains.

Species for which this area has high responsibility (20% of the global population within the physiographic area) include the Broad-tailed Hummingbird, Brown-capped Rosy-Finch, and Dark-eyed (Gray-headed) Junco. Other species that have a significant presence in the Southern Rocky Mountains (10% of the global population) include the Williamson's Sapsucker, Virginia's Warbler, Green-tailed Towhee, and Cordilleran Flycatcher.