SIMULATING CUMULATIVE FIRE EFFECTS IN PONDEROSA PINE DOUGLAS-FIR FORESTS

A successional process model adapted for use with species from Pinus ponderosa var. ponderosa/Pseudotsuga menziesii var. glauca forests of the inland Northwest simulates tree establishment, growth, and mortality, along with live and dead fuel accumulation, fire behavior, and fuel reduction on a 400 m2 plot. The modeling contains algorithms for influences on tree establishment and growth including temperature, water stress, light tolerance, and site quality. The model was used to predict 200 yr of forest succession for 5 different disturbance regimes. This allowed comparison of patterns of basal area by species, of duff and fuel accumulation, and of fire intensities among the following scenarios: 1) no fires (fire suppression), 2) consistent fire intervals of 10, 20 and 50 yr, and 3) a natural fire regime of variable intervals reconstructed from fire scarred trees. Frequent fires (10 and 20 yr intervals) were simulated to be of low intensity, resulting in scorch heights of 0.5-3.0 m. These fires prevented Douglas fir saplings from surviving and becoming part of the overstory. Simulation results for the 10 and 20 yr fire intervals were similar to those for the natural fire regime, based on fire occurrence between AD 1600-1900. However, the occasional long fire intervals within the natural fire regime allowed greater regeneration success for ponderosa pine. Fires at regular intervals of 50 yr were more severe and resulted in a decrease of western larch Larix occidentalis after 150 yr, with a corresponding increase in ponderosa pine. Douglas-fir slowly increased in basal area and became established in the overstory after 200 simulation years. The no-fire scenario allowed Douglas-fir to achieve dominance in the understory, and eventually in the overstory, thereby limiting survival of ponderosa pine and western larch regeneration. A test of the model showed predictions to be within 19% of field observations. -from Authors