| Following a century of fire exclusion, ponderosa pine (Pinus ponderosa Laws., 'PP') forests of Crater Lake National Park, Oregon, are in a condition of poor forest health. Prescribed fire has been used to restore old PP stands, but often with counterproductive effects, including widespread bark beetle (Coleoptera: scolytidae) attacks observed following fires. A series of experiments was designed to assess the effects of fire on PP oleoresin flow (OF), believed to be the main defense against beetle attacks. The restoration problem provided the experimental setting, while findings were discussed in the context of a modern plant defense theory, the Growth-Differentiation Balance (GDB).;Monitoring of large PP mortality after spring and fall burns suggested that beetle activity and PP mortality were related to fire intensity and tree vigor. Both mortality and OF were greatest in hot fall burns, lower in cool spring burns, and least in controls; OF was also positively related to vigor. Additional experiments on pole-sized individuals nearby tested the effects of mechanical injuries on resin. OF responses after bole charring, but not trenching or pruning, were similar to those after burning. The post-fire response consisted of a short-term (2-4 weeks) post-fire OF decrease, followed by OF recovery by ∼4 weeks and possible increase above pretreatment levels within ∼8 weeks. Mid-summer OF remained elevated after fire for 2-3 years in pole-sized PP and 4+ years in old-growth trees. There was also equivocal evidence after burning for increases in resin volatile emissions.;Physiological influences on OF were discussed in the context of carbon allocation. The GDB hypothesis appeared overly simplistic for explaining the short- and long-term variability in OF in Pinus. Increased OF after fire appears to be a generalized response to bole injury. This study suggests that carbon allocation to resin defenses is complex and not easily assessed by simple OF measurements. Commonly accepted ideas regarding PP resin defenses are invalid in the case of fire-injury, as both OF and mortality increased among burned trees despite heightened resin defenses. Induced or enhanced attraction of beetles following fire is suggested as the main mechanism for explaining post-fire beetle responses. |
