OOS 30-2 - Dominance of coniferous and broadleaved trees shape bacterial associations with boreal feather mosses

Background/Question/Methods
In the dominant coniferous forests of the boreal biome, the ecologically important feather-mosses are a significant part of the understory in terms of cover and biomass and play key ecological roles, such as N₂-fixation. However, boreal forests are increasingly changing in tree dominance from coniferous to broadleaf deciduous forests, due to natural fires, human land use and climate change. The composition of moss-associated bacterial communities seems to be mainly driven by host species but may also be shaped by environmental conditions related to tree-canopy dominance. To determine if the bacterial diversity in the moss phyllosphere (leaf surfaces) is driven by host species or environmental conditions, we used 16S rRNA gene amplicon sequencing to quantify changes in moss-associated bacterial communities as a function of host species (Pleurozium schreberi and Ptilium crista-castrensis) and forest type (coniferous black spruce versus deciduous broadleaf trembling aspen) in eastern Canada.

Results/Conclusions
We show that forest type has a stronger effect on moss bacterial phyllosphere than moss host identity. Bacterial sample-level diversity (α-diversity) was higher in spruce forests, while there was a higher variability in bacterial community composition (β-diversity) and overall diversity (γ-diversity) in aspen forests. Unexpectedly, ecologically important moss-associated Cyanobacteria were significantly more relatively abundant in trembling aspen than in black spruce forests, with Nostocaceae differing the most between forest types. While most studies of moss phyllosphere have focused on coniferous forests, we show that the heterogeneous and nutrient-rich broadleaf deciduous forest is an important habitat for moss-associated bacteria. The effects of forest type as a driver of moss phyllosphere highlights that the increasing change in dominance from coniferous to broadleaf deciduous trees is likely to have significant impacts on the moss phyllosphere and its ecological functions in boreal forests.