DETERMINING REFERENCE CONDITIONS FOR ECOSYSTEM MANAGEMENT OF SOUTHWESTERN PONDEROSA PINE FORESTS

The fire disturbance regime and forest structure prior to Euro-America n settlement (AD 1883) of a southwestern ponderosa pine (Pinus pondero sa) landscape were quantified in order to establish reference conditio ns as a baseline for ecosystem management. The mean presettlement fire interval between 1637 and 1883 was 3.7 yr for all fires and 6.5 yr fo r widespread fires, but fire has been excluded from the study area sin ce 1883. Forest density increased under fire exclusion from an average of 148 trees/ha in 1883 (65 pines, 80 oaks, three other species), an open forest dominated by relatively large ponderosa pines, to 1265 tre es/ha in 1994/1995 (720 pines, 471 oaks, 74 others), a dense forest ch aracterized by relatively small and young trees. Species composition h as shifted toward greater dominance by Gambel oak (Quercus gambelii) a nd conifers less adapted to frequent fires: white fir (Abies concolor) and Douglas-fir (Pseudotsuga menziesii). The reference presettlement conditions can be applied to management of this ecosystem in two ways. First, reference conditions are a benchmark against which to evaluate contemporary conditions and future alternatives. The comparison shows that the contemporary forest is well above the range of presettlement variability in forest density, and both live and dead fuel structures have developed that can support high-intensity wildfire. Second, refe rence conditions can serve as a goal for ecological restoration treatm ents.