Symbiosis and Biodiversity’s Roles in Pine Beetle Epidemic

There have been suggestions that climate change has played a role in the epidemic outbreak of mountain pine beetle in Western Canada.  University of Alberta’s Justine Karst ‘s research on symbiotic fungi sheds light on another factor which makes lodgepole pines more vulnerable. It also serves as another reminder of the importance of biodiversity. What follows is a “transcript” from last week’s interview with Karst by host Bob Macdonald on Quirks and Quarks. After listening to the podcast, I edited the conversation, filling in some details and adding some references.

BOB:   Exactly how does the pine beetle actually kill a mature pine tree?

KARST: When reaching an epidemic population, the females attack pine trees and then send out pheromones into the air. These invite males. After mating eventually the females burrow into the bark and lay eggs, which hatch and feed off the sugar-containing tissues of the tree (phloem). The beetles also carry fungi that attack and plug the xylem (water-conducting tissues). The combination of these events is what kills the trees.

BOB:  So the beetles literally starve the tree by eating its energy source out of its veins?

KARST: Yes, and the fungi interfere with water flow, which aside from preventing photosynthesis also compromises its defense mechanism.

BOB:  And what impact has it had, so far, on the forest?

KARST: There has been widespread loss of pine in British Columbia and parts of Alberta where entire landscapes have lost pines.

beetlerange(Natural Resources Canada points out that since the current Mountain Pine Beetle outbreak started in British Columbia in the early 1990s,  50% of the total volume of commercial lodgepole pine in the province have been killed. A 2006 outbreak in BC fuelled a massive migration into Alberta. Worse, the beetle is now also reproducing in jack pine, the dominant pine species of the boreal forest.)

BOB:  What was your interest in studying the pine beetle?

KARST: I am not an entomologist but interested in mycorrhizal fungi, which colonize roots of most trees in the boreal forest, including pines. These organisms rely on a living tree host, which supplies them with sugars. When I arrived in Alberta and saw entire sections of the forest dying, I wondered what was going on with the associated fungi below the ground and also about the next generation of pines.

BOB: If the fungi is relying on the tree, what does the tree in return get from the fungi?

KARST: These fungi, having small, web-like tissue growing around the fine root tips, are able to access nutrients, which otherwise would be unavailable to the tree. They are also good at fetching water from pores of the soil–that would also not  be accessible to the roots. So it’s typically a mutualism: nutrients and water in exchange for sugars made through photosynthesis.

BOB: How did you go about  studying what was happening below the surface?

KARST: There were two components to this study: first we sowed pine seeds of lodgepole pine in disturbed pine stands and in areas not affected by the beetle. The fate of those seeds was tracked for two years. What we found is that seeds growing in the beetle-killed strands had a lower survival rate than those growing in an undisturbed forest. That was the first stage.

We also noticed that the mycorrhizal community changes after widespread loss of trees. Not only is there a loss in the number of fungi but there ‘s an impact on their biodiversity. So in the second stage of our research, we took samples of soil from both affected and healthy forests and brought them back into the greenhouse. We used this to inoculate seedling grown in pots. After letting them grow for 8 months, we measured monoterpene levels to see if the source of the soil and their associated fungi affected the seedling defense –response.

BOB:  What did you find?

KARST: The seedlings grown in soil from beetle-infested forests had much lower defenses (secreted less monoterpenes) than those from unaffected forests.

BOB:  So what unusual thing is happening in the soil of affected forests?

MS. KARST:  The community changes; species are lost. There is a  certain class of fungi — Leotiomycetes. — belonging to the Helotiales order that live on decaying matter. Their increasing numbers  are probably responsible for the lower defense mechanisms.

BOB:  Where did these fungi come from?

KARST: They were already present but were not as abundant as what you’d find in a forest with widespread mortality.

BOB:  What implications does that have for the regeneration of a forest?

KARST: It might imply that seedlings growing in such forests need some kind of intervention. By triage we might have to identify which strands of the forest have high concentrations of this particular fungus. They have to be inoculated with some other type of mycorrhizal fungi to help them through that window of time when they are  more vulnerable to attack.

BOB:  Without intervention, could other species of trees move in to take over where pine seedlings are struggling?

KARST: White spruce seem to be less sensitive to the species-composition of mycorrhizal fungi. So a shift in tree composition is possible in the future. In fact, that might be a good reason for planting a diverse forest. So in case of another insect outbreak, other tree species can carry the necessary mycorrhizal fungi through to prevent complications in subsequent seedling growth.

BOB:  Biodiversity. Strengthening biodiversity.

KARST: Yes, I think that’s a good message.

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