Tree defenses against bark beetles decline with increasing elevation

Rapid warming has promoted unprecedented bark beetle epidemics in high elevation, subalpine forests of western North America. In the U.S. Rocky Mountains, the mountain pine beetle (Dendroctonus ponderosae) historically attacked trees below 2700 meters (asl), but can now be found infesting forests above 3500 meters in elevation. Bark beetle epidemics are hypothesized to select for enhanced tree defenses, leading us and others to hypothesize that trees found at higher elevation are less defended against bark beetles than trees at lower elevation due to elevational gradients in bark beetle pressure. We tested our hypothesis by characterizing the resin defenses of three widespread pine species (lodgepole, ponderosa, and limber pine) along a 1500 to 1700 meter elevational gradient for each tree in the Colorado Rocky Mts. We found little variation in resin chemistry within any of the pine species across the elevational gradient, but found that the number of vertical resin ducts (structures where resin is produced and stored) tended to decline with increasing elevation for all three pines. While the decline in resin ducts with increasing elevation appears to support the hypothesis that higher elevation trees are less defended against bark beetles than lower elevation trees, the pattern appears to be more likely due to difference in stand structure than due to gradients in insect pressure. Our ongoing work investigates the processes behind the pattern of defenses across elevation with a focus on genetically controled traits. Regardless of patterns in tree defenses across elevation, the presence of poorly defended trees in high elevation forests has likely promoted recent bark beetle epidemics.