Spruce-Fir Forest

Abstract

Spruce-Fir Forest occurs in the subalpine zone, at the highest elevation of any major forest in the American Southwest. It is dominated by Engelmann spruce, either subalpine or corkbark fir, and quaking aspen. Most research on Spruce-Fir Forest dynamics is from the central and northern Rocky Mountains, and evidence from the Southwest indicates both similarities and differences. Important drivers of vegetation dynamics are a diverse disturbance regime that is dominated by fire, wind, insects, climate variation, and anthropogenic disturbances that include livestock grazing, fire management, and nearby land use. Historical fire regimes were crown-fire in upper elevations and mixed-severity in at least some lower-elevation sites. Key processes of vegetation dynamics are succession and gap dynamics. Historical descriptions indicate Spruce-Fir Forest was dense before Euro-American influence. Fire exclusion began in the late nineteenth century. Its impacts are unknown for upper-elevation stands, but likely initially involved increases in tree density and basal area in lower-elevation stands. At least some lower-elevation stands decreased in density and basal area during the twentieth century, apparently as a result of density-dependent factors and exogenous factors such as climate change. Vegetation dynamics are illustrated in a nested, three-tiered set of conceptual models. Key conclusions and challenges for researchers and land managers are summarized.