Effects Of Host Stored Resources On Bark Beetle Fungal Conifer Interactions

Bark beetles and their associated fungi are among the greatest natural threats to conifers worldwide, but the degree to which host stored resources influence tree-beetle-fungal interactions has not been investigated. In western North America, the range of the mountain pine beetle (Dendroctonus ponderosae) has expanded from lower elevation Pinus contorta (lodgepole pine) forests into high elevation Pinus albicaulis (whitebark pine), a presumed superior host. I investigated whether stored resources in tree sapwood change after D. ponderosae attack, and whether this change relates to fungal colonization and beetle performance. I also studied how phloem and sapwood resources vary with elevation and tree diameter and examined the effect of tree species and diameter on D. ponderosae host selection. Following beetle attack and fungal colonization, sapwood non-structural carbohydrates (NSC), lipids, and phosphorus declined in attacked trees relative to un-attacked trees. Resource declines were related to the degree of fungal colonization, suggesting a direct benefit to fungi in both host species. In P. contorta, beetle performance was also positively related to stored resources. The concentration of stored resources was generally higher in P. albicaulis than in P. contorta and increased with elevation and tree diameter, suggesting a potential increase in host quality for D. ponderosae and/or fungi. Beetles preferred larger diameter hosts, and although stored resources did not affect beetle performance in P. albicaulis, beetles were more likely to attack P. albicaulis even when larger P. contorta were available. In a parallel system in Norway, phloem NSC and sapwood lipids also declined in Picea abies trees inoculated with the fungus Ceratocystis polonica relative to trees attacked by the bark beetle Ips typographus (which vectors C. polonica) or control trees, again indicating that stored resources enhance fungal colonization. Overall, my results suggest that host stored resources influence the interaction between bark beetles, fungi, and conifers primarily by enhancing fungal growth. Fungal access to stored resources may also benefit beetles in some host tree species. A better understanding of the trophic interactions between beetles, fungi, and conifers may improve our ability to predict bark beetle dynamics and range expansion.