The San Juan National Forerst (SJNF) occupies 847,638 ha in the approximate geographic center of the South Central Highlands Section of the southern Rocky Mountains Province in southwestern Colorado (Romme et al. 2003)(Figure-study area map). Elevation ranges from 2,400 m in the valley bottoms to over 4,200 m mountain peaks along the continental divide. The study area includes three major watersheds, including the Dolores River, Animas River, and San Juan River, all major tributaries of the Colorado River. The geology of the area is quite complex. Parent materials date from ancient Precambrian rocks to recent alluvial deposits (Ellingson 1996a; Campbell and Brew 1996). Current landforms were created by a variety of geomorphological processes, including plate tectonics, volcanism, glaciation, and erosion (Brew 1996; Ellingson 1996b; Blair 1996). The climate varies significantly in relation to pronounced elevation and topographic gradients. Temperatures range from an average high of 23 degrees (c) in July to an average low of -18 degrees (c) in January. Precipitation ranges from a mean of more than 152 cm on the highest peaks to less than 51 cm in the lower reaches of the study area, and predominately falls in late summer (July and August) and winter (January through March), although there may be significant local variation (Keen 1996). Overall, the study area is typical of the South Central Highlands Section with respect to physiographic variation and landscape structure.
Seven major vegetation types of ecological and economic significance occur within the study area. Each of these types has a unique ecological setting and history (Romme et al. 1992; Floyd-Hanna et al. 1996; Jamieson et al. 1996; Somers and Floyd-Hanna 1996; Spencer and Romme 1996), as well as distinctive human impacts and changes since EuroAmerican settlement. At the lowest elevations, the vegetation is dominated by semi-desert grasslands and savannahs and pinon-juniper woodlands (primarily Pinus edulis and Juniperus osteosperma). At the foothills and on tops of broad plateaus and mesas, the vegetation ranges into Ponderosa Pine (Pinus ponderosa) forest interspersed with shrub-dominated stands (Petran chaparral dominated by Quercus gambelii). The middle slopes are covered by a mosaic of mixed conifers (Pinus ponderosa, Psuedostuga menziesii, Abies concolor, Picea pungens) and quaking aspen stands (Populus tremuloides), broken by occasional meadows and grasslands. The highest elevations contain extensive spruce-fir forests (primarily Picea engelmannii and Abies lasiocarpa), subalpine meadows, and treeless alpine communities on the highest peaks. Running through all these types are riparian woodlands and meadows along the borders of perennial rivers and streams.
Prior to European settlement, landscape dynamics were driven primarily by the patterns of wildfire, which varied dramatically with vegetation type. The median fire interval was only 10-20 years in the lower elevation ponderosa pine type; 20-30 years in the dry mixed-conifer type; 50-100 years in the aspen type; and >100 years in the spruce-fir type (Romme et al. 1998). Many individual stands escaped fire for far longer than the median return interval and some burned at shorter intervals, creating a complex vegetation mosaic at the landscape scale. Under this “natural” disturbance regime, stand replacement fires initiated stand development and maintained a coarse-grain mosaic of successional stages and cover types across the landscape, although other disturbance processes, such as landslides, floods, windthrow, insects and disease also played a role on a finer scale. In particular, the disturbance regime of individual stands in the later stages of development is dominated by chronic, fine-scale processes that kill individual trees or small groups of trees (Veblen et al. 1989; Lertzman and Krebs 1991; Veblen et al. 1991a, b; Roovers and Rebertus 1993).
Although limited logging by Euro-American settlers began as early as 1875, the scale and impact of logging increased dramatically in the late 1800's with the advent of railroad logging (Pearson 1950). Most of the activity was confined to the pine forest at lower elevations, such that by 1950, essentially all of the old-growth ponderosa pine forests of this region had been exploited and profoundly altered (San Juan National Forest 1962; Anonymous 1971). In contrast, extensive logging at the higher elevations generally did not begin until much later in the twentieth century. The first large-scale spruce logging operation began in 1946 (US Forest Service, unpublished data). Logging in spruce-fir and mixed-conifer forests was accelerated dramatically in the 1950s and reached a peak in the 1960s and 1970s. Logging was carried out using clearcutting and a variety of partial cutting methods. Clearcutting was discontinued by 1980 in all but aspen forests because of problems in regenerating clearcut stands.