Habitat specific effects of bark on wood decomposition: influences of fragmentation, nitrogen concentration, and microbial community composition

1.    Identifying the drivers of decomposition is
critical for understanding carbon cycling dynamics in forest ecosystems.
Woody biomass is an important pool of carbon, composed of bark and
underlying wood that differ in structure, nutrient concentrations, and
exposure to the environment. We hypothesized that higher nutrient
concentrations in bark would speed the decomposition of underlying wood,
and that this effect would be greater in streams, where nutrients are less
available to decomposers than on land. 2.     
Replicate branches of three tree species, with and without bark, were
placed in streams and on land in a lowland tropical forest in Panama.
After 3 and 11 months of decomposition, we measured mass loss and nitrogen
(N) concentrations and sequenced the fungal and bacterial communities of
both wood and bark tissues. 3.      While bark
decomposed faster than the underlying wood and had higher N
concentrations, bark presence slowed wood mass loss. Nitrogen
concentration could account for interspecific variation in wood mass loss,
but not bark mass loss. In contrast, bark mass loss, but not wood mass
loss, was faster in streams than on land, suggesting fragmentation is more
important for bark mass loss in streams. Differences in fungal and
bacterial community composition between bark and wood substrates were
significant but small. 4.      Our results
indicate that bark can slow wood decomposition instead of promoting it,
and that at least for branch wood, the primary drivers of decomposition
differ between bark and wood. Differences in the factors driving
decomposition rate between bark and wood suggest that the contribution of
bark to the decomposition of woody biomass may depend on habitat.