Mountain pine beetle outbreaks in whitebark pine: Do sapwood carbohydrates influence beetle success?

Background/Question/Methods

The mountain pine beetle (Dendroctonus ponderosae) is one of the most destructive native insects in North America, with periodic large scale outbreaks in western forests. Recent expansion of the mountain pine beetle northwards and upwards in elevation has also allowed it access to relatively new host trees like whitebark pine (Pinus albicaulis), a keystone sub-alpine species in the Rocky Mountains. Our recent work suggests that host tree nutritional quality may play an important role in mountain pine beetle preference for whitebark pine over lower elevation lodgepole pine. Numerous studies have shown that tree mobile carbon pools (non-structural carbohydrates and lipids) increase with elevation, and we previously observed a dramatic depletion of sapwood non-structural carbohydrates in whitebark pine after beetle attack. This observation was surprising because 1) such a large depletion of sapwood carbohydrates is rare, 2) the mountain pine beetle is assumed to be hampered by excess carbon in its diet, and 3) the mountain pine beetle only inhabits the phloem of the tree. Beetles have no direct access to sapwood nutrient resources, although their symbiotic fungal partner grows in the phloem and sapwood. We had two questions: 1) Is carbohydrate depletion due to beetle or fungal consumption of tree tissue? and 2) Why does carbohydrate depletion occur?

Results/Conclusions

Sapwood samples were obtained from a whitebark pine/lodgepole pine stand in the early stages of a mountain pine beetle outbreak, in the Pioneer Mountains, MT in 2008. A significant depletion of sapwood non-structural carbohydrates occurred in attacked versus healthy whitebark pine trees (p < .001, ANOVA). However, this depletion only occurred in trees with extensive fungal colonization of the sapwood; trees with minimal fungal colonization of the sapwood did not differ from healthy trees. A similar trend occurred for lodgepole pine. These data suggest that depletion of carbohydrates requires the presence of fungal hyphae in the tree sapwood. It is not known whether sapwood carbohydrates directly benefit beetle performance, or indirectly benefit the mountain pine beetle by supporting the growth of its symbiotic fungal partner. The use of nitrogen and other nutrients in beetle and fungal growth is also poorly understood and is under current investigation. Our observations thus far indicate that the mountain pine beetle and its fungal symbiont benefit from sapwood carbohydrates, and that tree nutritional quality may influence beetle preference for host trees and the dynamics of mountain pine beetle outbreaks at high elevation.