ABSTRACT Predicting wood decomposition is challenging due to complex successional dynamics among decomposers that colonize and defend wood territory. This starts with saprotrophic fungi that reside latently in healthy wood until trees senesce, but these “endophytes” are rarely considered an endogenous wood trait that might improve predictions for decomposition rates or fates. Here, we used repeated measures to track the decomposition of paper birch ( Betula papyrifera ) and red pine ( Pinus resinosa ), assessing wood properties and microbial succession over 5 years in a northern forest (Minnesota, USA). We compared fungi and bacteria present in sound wood (endophytes) versus those arriving as external colonizers, and we used relevant treatments to vary accessibility for colonizers (ground contact versus aboveground; bark off versus bark on). Over 5 years, accessibility treatments had a significant effect on decay rates and fungal community succession. Wood rot type was unanimously white rot (lignin-degrading fungi), but fungal dominance was treatment-specific. Most dominant fungi could be traced to operational taxonomic units (OTUs) present as endophytes in sound wood, suggesting that treatments affected endophyte competition more than external colonizer success, even in ground contact. Although fungal communities lost diversity (Shannon index) as certain taxa became dominant, bacterial communities converged irrespective of treatment, without notable co-occurrence with fungi and without losing diversity, suggesting a decoupled dynamic. The results imply a strategic benefit for saprotrophic fungi to colonize trees as endophytes, and they support including fungal endophytes along with predictors of their competitive success as “plant” traits to improve predictive models. IMPORTANCE Establishing this persistence and influence for endophytic saprotrophs has not been possible without repeated measures in a long-term study. We believe our findings are significant for two key reasons. First, they link community succession to a “trait” in wood that may be more predictable—governed by the living tree as the “gate-keeper”—compared with predicting assembly history for external colonizers. Second, they highlight a new avenue toward developing a predictable trait for wood decomposition that could improve Earth Systems modeling, which has historically been challenged in predicting carbon sequestered/released by wood, where most of Earth’s aboveground biotic carbon resides.