Effects of crown scorch on ponderosa pine resistance to bark beetles in Northern Arizona

We conducted a study linking mechanistic relationships among fire injury, tree physiological condition, and bark beetle (Coleoptera: Scolytidae) susceptibility. Crown scorch of 40 Pinus ponderosa trees was estimated in a naturally regenerated stand that was thinned and prescribed burned in winter and early spring 2000. Net photosynthetic rate (P_n) of undamaged foliage was higher in heavily and severely than moderately and lightly scorched trees. This trend was more pronounced in the dry season (June) than the wet season (August). This suggests the mechanism underlying the increase in P_n was an improvement in water relations. This was also supported by differences observed in predawn water potential and stomatal conductance (g_s). Constitutive resin volume did not differ among crown scorch classes in June, but was negatively related to crown scorch intensity in September. This suggests that constitutive resin was formed and stored before fire injury, and therefore not strongly affected by crown scorch levels in June. Induced resin production (resin produced after depletion of constitutive resin) generally decreased with increasing crown scorch in both June and September. Induced resin appeared to be related to carbon available for resin synthesis. The proportion of successful colonization attempts by Ips spp. and Dendroctonus spp. was low throughout the season. Colonization of Ips sp. and Dendroctonus spp. was not spatially separated in host trees. Colonization attempts were generally positively related to intensity of crown scorch in both pheromone and no-pheromone treatments. This finding suggests that pheromones attracted insects to the tree, but host physiological condition or other factors ultimately determined host colonization. Our results also suggest that intensity of crown scorch may affect colonization attempts of Ips and Dendroctonus spp. on P. ponderosa trees.