Bark beetle–fungi symbioses reshape nutrient dynamics, fungal decomposer succession, and decay rates in ponderosa pine

Bark beetles introduce fungi into trees, shaping both beetle fitness and tree physiology. However, the impact of these symbioses on fungal endophytes, the succession of fungal communities, and downstream consequences for tree nutrient dynamics and decomposition remain poorly understood. Yet, these interactions can play an important role in forest nutrient cycling and carbon storage. We established logs colonized and uncolonized by red turpentine beetles to track fungal communities, changes in carbon, nitrogen, and phosphorus in sapwood and phloem, and sapwood density, from pre-colonization to after brood emergence. We observed turnover of endophytic fungi in all logs, but beetle-colonized logs developed distinct fungal communities and experienced greater sapwood density loss. Phloem - where beetles feed - became enriched in N (33.3%), P (33.1%), and C (2.8%) after beetle colonization compared to uncolonized logs, while N in the sapwood became more depleted (15.6%). Isotope tracing of N and C further indicated fungal-mediated nutrient transfer from sapwood to phloem, and ultimately, to beetles. We show that bark beetles can benefit from nutrient provisioning via both beetle-vectored and nonvectored fungi. Moreover, beetle-fungal interactions reshaped decomposer communities and accelerated sapwood density loss, underscoring their broader role in nutrient redistribution and C dynamics in forest ecosystems.