Forest Structure and Fire Susceptibility in Volcanic Landscapes of the Eastern High Cascades, Oregon

Multidimensional forest health initiatives-those designed to restore and protect forest integrity in the large sense-require, among other things, a better understanding of the relationship between the structure of forests and (1) their susceptibility to wildfires outside the range of natural variability and (2) the level of treatment (if any) required to lower susceptibility into an acceptable range. Within the ponderosa pine zone (Pinus ponderosa Dougl. ex Laws.) on the eastern slopes of Oregon's Cascade Range, an average of 86% of trees >5 cm in diameter at breast height (dbh) were less than 101 years old, the density of young trees correlating negatively with the density of old trees (>150 yrs). Species composition differed significantly between age classes: 72% of trees older than 150 years were ponderosa pine, whereas 83% of trees younger than 101 years were either grand fir (Abies grandis [Dougl.] Lindl.) or lodgepole pine (Pinus contorta Dougl. ex). Loud Crown bulk density calculated to reflect the multiple canopy layer structure of these stands (CBDm) averaged$0.049 kg/m^3$, compared to$0.093 kg/m^3$using the standard approach (p < 0.001), indicating that the latter significantly overestimates risk of active crown fire in stands with complex canopy structure. Nevertheless, modeling with CBDm predicts that under severe but plausible weather conditions (late summer fuel moisture and 48-kph winds) crown kill would exceed 70% on 5 of 14 plots and 50% on an additional 5 plots. Models also predict that thinning trees of