Underappreciated influencers: How do beetle-fungal mutualisms influence fungal communities and wood decay in Pinus ponderosa?

Background/Question/Methods

Pine mainstems contain some of the oldest tree tissues and host many fungal endophytes. Some of these endophytes are latent saprobes that will begin the decomposition of dead and dying trees, facilitating nutrient turnover in forest ecosystems. Secondary bark beetles also colonize these trees, introducing their own beetle-associated fungi. These fungal mutualists aid beetles by translocating nutrients from deep within the sapwood to the phloem where it is more accessible for beetle larvae, but perhaps less accessible to fungal decomposers. In this study, we aim to evaluate the effect of bark beetles and their fungal mutualists on fungal endophyte and decomposer communities and how this influences downstream wood decomposition and nutrient turnover in forest ecosystems.We conducted a beetle exclusion experiment on ponderosa pine (Pinus ponderosa) trees in Northwest Montana. A healthy tree was felled and cut into six log sections that were made either beetle-accessible or inaccessible. We collected multiple tree cores from each log over a series of four time points: at felling, after beetle colonization, after beetle emergence, and one year later. We used Illumina sequencing to monitor changes in fungal community composition and analyzed nutrient content in sapwood and phloem tissues.

Results/Conclusions

Secondary bark beetle colonization shifted fungal communities over time. Beetles increased the abundance of saprobes, specifically rot fungi, in the pine logs a year after colonization. After beetle emergence, beetle mutualist blue-staining fungi were highly abundant in beetle colonized logs but decreased over the following eleven months. In addition, nutrient composition changed due to beetle colonization, with a depletion of phosphorus in the inner sapwood and nitrogen enrichment in phloem tissues. Overall, our results suggest that secondary bark beetle colonization changes the tree fungal and saprophyte communities and may influence wood decomposition and forest nutrient cycling. Further studies will investigate how these differences affect wood decomposition over time.