Among the ecological factors shaping microbial community assembly, historical contingencies (i.e., the order and timing of past events) play a critical role in a diverse array of study systems. Because traditional abiotic indices (e.g., temperature, moisture, plant chemistry) often fail to capture local variation in rates of wood decomposition, historical contingencies and their effect on fungal communities may be a principal governor of decay rates. In wood, historical contingencies likely begin with endophytes (i.e., fungi that are able to colonize living plant tissues asymptomatically), as these initial colonists are able to gain early resource access and alter the establishment of succeeding species. In a four-year field experiment, we investigated the extent to which endophytes influenced fungal community composition and subsequent wood decomposition, comparing the decay of sterilized and non-sterilized birch (Betula papyrifera) logs using high-throughput sequencing and wood physiochemical analyses (i.e., decay rate and rot type). We hypothesized that the presence of endophytes would lead to competitive interactions with later-arriving saprotrophic fungi. Given that resource requirements of endophytes and wood decay fungi are largely similar, we also expected the presence of endophytes to have greater effects on community composition relative to rates or types of wood decay.
The presence of endophytes significantly altered initial fungal species composition during the first two years and enhanced mass loss over the duration of the experiment. Support for this claim comes from evidence that the presence of endophytes significantly altered the taxonomic composition of fungi colonizing wood after 7 months and 19 months (PerMANOVA; F1,16 = 2.7 - 5.2; P < 0.001), but not significant at 42 months (P = 0.20). Specifically, the presence of endophytes suppressed saprotrophic fungal guilds (ANOVA; P = 0.017) following seven months of colonization. Endophytic initial colonizers also enhanced overall wood decay rates in our study (repeated measures ANOVA; F1,10 = 12.2, P = 0.005), which is likely due to the time lag of fungi recolonizing sterile stem sections; whereas, decomposition immediately commenced in the presence of endophytes. Sterilization treatment had no significant effect on lignin loss: density loss ratios wood (P > 0.05), indicating that the selective removal of lignin by fungi (i.e., white rot) was independent of endophyte presence. Collectively, our findings demonstrate that wood endophytes can affect early community assembly and subsequent decay rates, suggesting that historical contingencies in wood may have important implications for carbon gain and loss in forest ecosystems.