The Western San Juan Mountains: Their Geology, Ecology, and Human History

12. Biotic Communities of the Cool Mountains

CHAPTER 12 BIOTIC COMMUNITIES OF THE COOL MOUNTAINS DAVID W. JAMIESON, WILLIAM H. ROMME, AND PRESTON SOMERS Forest vegetation is restricted to areas of relatively high precipita~ tion and moderate temperatures. Most of the forests in the San Juan Mountains are dominated by various species ofevergreen co~ niferous trees. Evergreens are especially well adapted to environ~ ments with short growing seasons (because of long winters, summer droughts, or both) and scarce soil nutrients. Evergreen trees need not expend a large amount of energy and nutrients in growing a full set of new leaves every year. Evergreen forests are common in the harsh climates of the northeastern and Great Lake states, in Canada, Europe, and Asia, on the infertile soils of the coastal plain in the southeastern United States, and at middle to high elevations throughout the Rocky Mountain Cordillera. Middle~altitude slopes of the San Juan Mountains are cov~ cred by mixed conifer forests, whereas higher elevations support spruce~fir forests (DeVelice et al. 1986, Romme et al. 1992). Al~ pine vegetation grows in areas above timberline. Interspersed within the coniferous forests are aspen groves and meadows, which provide a pleasant visual contrast to the dark green conifers and support a distinctive suite of plant and animal species. Mixed conifer and spruce~fir forests line many parts of the Skyway, espc~ cially in the vicinities ofSilverton and Rico. The highway does not quite reach the true alpine zone, though it comes close at Red Mountain Pass and other high points. 159 160 The Western San juan Mountains MIXED CONIFER FORESTS At middle elevations in the San Juan Mountains (7,500-9,500 ft; 2,270-2,880 m), forests are dominated by a variety ofconifer species , including ponderosa pine (Pinus ponderosa), sonthwestern white pine (Pinus strobiformis), Douglas-fir (Pseudotsuga menziesii ), white fir (Abies concolor), corkbark fir (Abies lasiocarpa var. arizonica), blue spruce (Picea pungens), and Engelmann spruce (Picea engelmannii). A complex mix ofenvironmental and historical factors interact to determine the presence and relative abundance of each of these species on any particular site. White fir appears to grow best on relatively fertile soils, blue spruce thrives in cold, wet valley bottoms, and ponderosa pine is restricted to locally drier, warmer conditions. Generally, however, a mixed conifer forest is indicative of relatively cool, moist conditions. The southwestern white pine is particularly interesting because it reveals the floristic affinities between this area and the mountains farther sonth in New Mexico, Arizona, and Mexico. The San Juans mark the northern limit of distribution of sonthwestern white pine. In northern Colorado and on up the Rocky Mountain Cordillera, this species is replaced by the limber pine (Pinus flexilis). Some anthorities lump the southwestern white and limber pines into a single species, but even then the two forms are distinct, and the trees in the San Juans are clearly more similar to populations to the sonth than to populations elsewhere in Colorado. Historically, mixed conifer forests appear to have been subjected to fires less frequently than ponderosa pine forests. Cooler, wetter conditions at the higher elevations where mixed conifer forests grow reduce fire hazards. However, when these forests did burn, the fires were sometimes very hot and killed all ofthe aboveground vegetation. The first trees to reestablish an extensive cover in the burned forests often were aspen. Today on the mountain slopes aspen groves often have fairly abrupt borders with the surrounding mixed conifer forest; some ofthese groves mark the sites of forest fires that raged during the late 1800s. Today young conifers , but very few young aspens, are becoming established in the shade ofthe adult aspens. Eventually these conifers will replace the Biotic Communities ofthe Cool Mountains " 161 aspens, and a mixed conifer forest similar to that which burned more than a century ago will once again be in place. Numerous mammals and birds inhabit the mixed conifer forests. One that is specifically dependent on ponderosa pines is the Abert's squirrel (Sciurus aberti). This large, gray tree squirrel is not often seen but is readily distinguished by the long tufts of hair that protrude upward from its ears. A smaller relative, the red squirrel (Tamiasciurus hudsonicus), lives wherever Douglas-firs or spruces occur and often announces its presence by its long, chattering alarm call. White-breasted nuthatches and pygmy nuthatches are common in conifer forests, where they feed on bark-dwelling insects. SPRUCE-FIR FORESTS As we approach timberline, reaching elevations...