Mountain pine beetle outbreak enhanced resin duct-defenses of lodgepole pine trees.

• Residual lodgepole pine trees favored defense over growth after beetle outbreak. • Higher tree mortality increased resin duct density of healthy asymptomatic trees. • Stand density affected the pre-outbreak growth-defense relationship of pine trees. • Bark beetle outbreaks may select phenotypically-resistant pines to future attacks. Millions of hectares of lodgepole pine trees have been affected by the recent mountain pine beetle outbreaks, which also left significant numbers of live host trees in some areas. Studies have primarily focused on the changes of forest conditions in post-outbreak stands, but whether such changes impact the growth, defense, and their interactions of residual pine trees is generally unknown. In this study, we compared the growth-defense relationship of lodgepole pine trees between pre- and post-outbreak periods by examining annual radial growth rates and xylem resin duct characteristics. We also tested the effects of varying percent host tree mortality and forest stand density on the growth-defense relationship of residual pine trees. We found that resin duct densities of residual pine trees were higher and resin ducts occupied larger areas in the xylem in post-outbreak period than in pre-outbreak period. Following outbreak, the percent host mortality showed a positive relationship with the resin duct density of all healthy residual trees, while stand density did not impact any of the resin duct characteristics. We conclude that bark beetle outbreaks can alter the growth, defense, and their interactions of residual pine trees, especially in stands with greater levels of pine mortality. This study also provides an empirical evidence, suggesting that residual trees might be more resistant to future bark beetle attacks. [ABSTRACT FROM AUTHOR]

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