Conifer epiphytic phyllosphere bacterial communities respond more strongly to rain exclusion and host species identity than to soil water content

With global warming, the frequency and intensity of drought episodes are projected to increase worldwide, especially in the boreal forest. This represents a serious threat to the boreal forest ecosystem’s productivity and environmental services. It is thus crucial to better understand how drought or water limitation could affect boreal forest ecosystems functioning, and to be prepared to overcome damage caused by drought events. Studies suggest that microbes may mitigate the negative effects of drought or water shortage on plants. However, most of these studies focused on soil microbes and on agricultural ecosystems. Here, we used a rainout shelters and soil irrigation experimental design to study the response to rain exclusion and soil water content of epiphytic phyllosphere bacterial communities associated with four boreal conifer tree species. Our results showed only a weak response of phyllosphere bacterial communities to variation in soil water content. On the other hand, host tree species identity and rain exclusion were the main drivers of epiphytic phyllosphere bacterial communities’ structure and diversity. This suggests that fewer rain events, in the context of climate change, would impact boreal trees phyllosphere microbiome composition. • Rain exclusion has a strong influence on tree phyllosphere bacterial communities. • Tree host species identity is a driver of phyllosphere bacterial communities. • Soil water content has weak effects on tree phyllosphere bacterial communities. • Rainout shelters can contribute to alter phyllosphere bacterial communities. • Lower frequency of rain events can modify tree phyllosphere bacterial communities.