Main content area

Simulating Cumulative Fire Effects in Ponderosa Pine/Douglas‐Fir Forests

Keane, Robert E., Arno, Stephen F., Brown, James K.
Ecology 1990 v.71 no.1 pp. 189-203
Larix occidentalis, Pinus ponderosa, Pseudotsuga menziesii var. glauca, algorithms, basal area, fire behavior, fire regime, fire suppression, fires, forest succession, forest types, fuels, fuels (fire ecology), models, mortality, overstory, prediction, saplings, temperature, understory, water stress
A successional process model has been adapted for use with species from ponderosa pine/Douglas—fir (Pinus ponderosa var. ponderosa)/(Pseudotsuga menziesii var. glauca) forests of the inland Northwest. Its design allows modification for application to other forest types. This model, FIRSUM, 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 five different disturbance regimes. This allowed comparison of patterns of basal area by species, of duff and fuel accumulation, and on 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 and 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 200simulations 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, and a sensitive analysis of FIRESUM showed parameters associated with the growth algorithm to be most critical for predicting successional trends.