Mixed Conifer Ecosystem
Warm/dry mixed conifer forests in the Southwest are generally located at the lower elevation of the continuum on predominately southerly aspects. These forests are dominated by fire resistant ponderosa pine (P. ponderosa) and Douglas-fir (Pseudotsuga menziesii) and also includes species adapted to mesic conditions such as white fir (Abies concolor), aspen (Populus tremuloides), and southwestern white pine (Pinus strobiformis Engelm.).
Fire suppression and grazing in the 20th century disrupted the historical fire regime and has increased stand density and fuel loads in these forests. Management is now focused on restoring forest stand characteristics that were historically adapted to frequent surface fire but are now at risk of crown fire.
Restoration Principles
Due to a mixed-severity fire regime, stands were patchy and irregular in size, age structure, and composition. Most stands should be multi-aged and multi-storied. Drier sites (lower elevations, south and west aspects) were more open than moist sites due to the presence of more frequent fire. Retaining stands of old growth is important.
Historical Lines of Evidence
Across the region, past management and lack of fire have allowed presettlement stumps, snags, and logs to persist on the landscape. These legacy structures can serve as a guide for the design of restoration treatments. Reconstructions of presettlement conditions, including tree species and age structure (the distribution and range of tree ages) serve as a basic point of reference central to restoration and management of these ecosystems.
Fire Regime
Fire intervals were 5–25 years with a highly variable pattern of severity. Severe fires were associated with drought conditions. Surface fires burned with sub- to multi-decadal frequency prior to Euro-American settlement (late 1800s) in warm/dry mixed conifer. Most stand-replacing fires occurred at moist locations (higher elevations, north and east aspects), where fire was less frequent but more intense under droughty conditions.
Stand-replacing fires were localized to clumps, groups and stands, and tended not to occur at landscape scales. Aspen was the key vegetative component following stand-replacing fire. Aspen dominated fire-created openings for periods in excess of 100 years while playing a role as a nurse stand for the regenerating conifer species. Almost all regeneration was created due to disturbance activities. Most important of these disturbances was fire, but insects, disease, wind throw, drought and floods, either separately or in combinations, all played a role.
Natural Range of Variability
Natural Range of Variability (NRV) dictates conditions that are functional, sustainable, and resilient ecosystems. Desired conditions are to reduce severity of fire effects, reduce fire hazard and increase the flexibility for managing fires, increase resilience to climate variability and change, insects, and disease.
Forest Structure and Composition
Forest structure consists of uneven aged, grouped, open stands composed of ponderosa pine, Douglas fir, Southwestern white pine, limber pine. Historic conditions varied widely and it is difficult to generalize about historic tree density, fire regime, or forest structure and composition. Trees with interlocking crowns, interspaces between tree groups, high interspersion of age classes were common.
Understory
Understory conditions vary widely from dry, open-canopy forests with grassy undergrowth on open slopes and ridges to moist, closed-canopied stands dominated by numerous herbaceous plants in small openings, riparian areas, and ravines.
Wildlife
Wildlife species in mixed conifer varies widely, and is dependent upon canopy closure, tree size, age, and density, understory composition, snags and coarse woody debris, and more.
Dry Mixed-Conifer LEARN Sites
Pagosa Spring, CO
Mogollon Rim, Coconino Nat’l Forest AZ