Effects of prescribed burning on mortality and resin defenses in old growth ponderosa pine (Crater Lake, Oregon): Four years of post-fire monitoring

Abstract

Forests dominated by old growth ponderosa pine (Pinus ponderosa Dougl.) at Crater Lake, Oregon, have been viewed as good candidates for restoration via prescribed burning. Previous burn experiments in this ecosystem observed that ponderosa pine typically survived burning treatments but suffered high post-fire mortality from bark beetle attacks. This paper describes the results of four years of post-fire monitoring of ponderosa pine mortality and resin flow in areas subjected to low intensity spring burning (SB), moderate intensity fall burning (FB), or no burning (unburned controls, UC). Crown vigor estimates, correlated with ring width indices, were also included as a factor in mortality and resin flow analyses. Burn treatment was significant in both ponderosa pine mortality and resin flow, as follows: FB > SB > UC. These results suggest that resin defenses overall did not protect trees from post-fire beetle attacks. Crown vigor was positively related to both survival and resin flow. The relationships between burning, tree vigor, and resin defenses in this study suggest a complex web of interactions. Although some physiological mechanisms are still unconfirmed, these findings suggest that beetles may have been attracted to ponderosa pine following burning, perhaps via the release of volatile resin compounds. Following attraction, resin defenses appear to have been important for protecting trees from beetle attacks, with greater defenses in trees with higher crown vigor and higher growth rates. Management recommendations, including a gradual and incremental approach to fire restoration in these stands, are suggested.