Wood characteristics are known to influence decay rate. However, most decomposition studies do not differentiate between bark and wood, even though these tissues differ in structure, nutrient concentrations, and function. We explored if bark influences decay rate via altering fungal and bacterial communities, moisture retention, and nutrient availability. Since terrestrial and freshwater environments can differ in fungal community composition, oxygen and moisture availability, and availability of nutrient sources, we also explored how bark influences wood decay in paired terrestrial and freshwater environments. We conducted a decay experiment in which we removed or retained bark from replicate branches of three tree species placed either in freshwater streams or on land. In addition to calculating wood decay rate, we also characterized fungal and bacterial communities using environmental sequencing, and we measured moisture and wood and bark nitrogen concentrations.
Bark presence increased total mass loss in both terrestrial and freshwater environments. However, with bark present, the increase in mass loss was caused by the fast decay rate of bark and not the bark stimulating the decay of underlying wood. Bark also had higher nitrogen concentrations than wood and increased total moisture on both terrestrial and freshwater environments. We also tested the differences in microbial communities between bark and wood. Bacterial and fungal community composition differed between bark and wood in terrestrial and freshwater environments. Additionally, fungal and bacterial communities on bark were more diverse than on wood. We then tested the impact of bark on microbial communities of underlying wood by comparing wood that was decayed with bark present and bark removed. Fungal and bacterial community composition differed between bark present and bark removed treatments in both terrestrial and freshwater samples. However, only fungal communities differed in diversity between bark present and bark removed treatments. In conclusion, this evidence shows that the substrate differences between bark and wood can influence decay rate and communities of microbial decomposers.