2002), and research that reconstructed past forest structure and composition based on tree rings of current living and dead trees (e.g., Fulé et al.
Individual stands typically had three to five of these species in the canopy, although less diverse stands were present, especially of Douglas-fir.
2003a) and at higher elevations it may have been a crown fire regime characterized by high-severity, stand-replacing fires at intervals of one or more centuries. The characteristic oaks are canyon live oak, interior live oak, California black oak, and Oregon white oak. With increasing elevation, they include three coniferous forests: ponderosa pine forest, mixed conifer forest, and spruce-fir forest. At higher elevations, these stands were more restricted to dry sites, such as ridgetops and south and west slope exposures. One of the typical life zones on mountains is the montane forest: at moderate elevations, the rainfall and temperate climate encourages dense forests to grow. A dense, even-aged canopy formed in a few decades.
Montane forests of the American Southwest are generally divided into four forest types based on the dominant tree species. These fires occasionally crowned, especially in dense stands in dry years (cf. Montane hardwood forests typically lack blue oaks and valley oaks. The fire regime is poorly known for the Southwest, but at relatively low elevations it was likely a mixed-severity regime similar to mixed conifer forest (Fulé et al. The mix of canopy species included ponderosa pine, Douglas-fir, quaking aspen, white fir, limber pine, blue spruce, subalpine fir, corkbark fir, and Engelmann spruce. The elevation at which one habitat changes to another varies across the globe, particularly by latitude. Structure likely depended on successional age and therefore may be similar to today’s structure (above). All areas are relatively small, widely separated and surrounded by much larger areas of ponderosa pine forest. Extensive areas of quaking aspen forest occur on the San Francisco Peaks in northern Arizona, the White Mountains in east-central Arizona, and the southern Rocky Mountains and Jemez Mountains of northern New Mexico.Stand structure prior to settlement by non-indigenous peoples is poorly understood. Stand structure and species composition are determined primarily by moisture and disturbance, which in turn are influenced by elevation, topographic position, and slope exposure.Common canopy tree species are (in approximate order of increasing elevation) ponderosa pine, Douglas-fir, southwestern white pine (Understory tree species include junipers (e.g., Utah juniper: Photograph courtesy of Arizona Historical Society, Flagstaff, Ray Housley CollectionPonderosa pine forest makes up 80% of the montane forests in the Southwest. The fire regime of quaking aspen forest generally mimicked that of adjacent stands of coniferous forest; however, aspen stands are less flammable and therefore had lower fire frequency and severity.In terms of species composition, aspen dominated the canopy of all stands and conifers were present in the understory in all except very young or semi-permanent stands. As a consequence, alpine vegetation is close to the ground and consists mainly of Plants have adapted to the harsh alpine environment. The understory conifers, if present, had the potential to overtop and replace the relatively short-lived (100-150 years) aspen in succession, converting stands into mixed conifer forest. These forests cover mostly the southern slopes of the Himalayas, places having high altitude in southern and north east India. Although tree regeneration was uncommon (White 1985), ponderosa pine seedlings established in years when seeds and sufficient moisture were available; otherwise the microsite was colonized by herbs. It typically occurs in small to large patches where crown fire had burned mixed conifer forest, or where other stand-replacing disturbance had occurred (right). Low elevation stands had occasional ponderosa pines and Douglas-firs indicative of transition with mixed conifer forest. Relatively dry stands were generally dominated by a mix of ponderosa pine, Douglas-fir, quaking aspen, and white fir.
Also subalpine fir, blue spruce, and Engelmann spruce on moist sites.Similar to mixed conifer forest at low elevation, but primarily Engelmann spruce, subalpine fir, and blue spruce at high elevation.Likely open to closed canopy with variable herbaceous layer.Likely open to closed deciduous canopy with dense herbaceous layer at young successional age, followed by ingrowth of conifers.Mixed-severity fire regime with frequent, low-severity fires and smaller areas of infrequent, high-severityPoorly known, but likely mixed-severity at low elevation (see mixed conifer forest) and possibly a crown fire regime of infrequent, high-severity fires at high elevation.Initiated by crown fires and thereafter similar to adjacent coniferous forest but likely with lower frequency and intensity.Fire exclusion at low elevations; unknown at high elevation.Increased density of canopy and understory trees followed by decreases in at least someSimilar to mixed conifer forests at low elevation.
The fourth forest type is quaking aspen forest, a deciduous forest that grows in areas of past disturbance, primarily at the elevation of mixed conifer forest.Relatively low elevation; generally flat to moderately steep topography.Mid elevation; often moderately steep, sometimes dissected topography.Ponderosa pine, along with pinyon pine and junipers in understory at low elevation and Douglas-fir, white fir, and quaking aspen at high elevation.Ponderosa pine, Douglas-fir, white fir, and quaking aspen on dry sites.