Combined effects of warming and drying on a temperate-to-boreal forest ecotone exert additive changes on soil microbiome structure and diversity

2020 Fall.Includes bibliographical references.The soil microbial community is an important mediator of many ecosystem functions, so understanding dynamics under climate change. These responses could be more robust in transitional zones such as the temperate-to-boreal forest ecotones, which are poised to experience substantial changes under projected climate change over the next century and beyond. Because these systems are projected to move towards a warmer, drier climate, it is important to understand how the soil microbiome's structure and interactions shift under such conditions. Here, we examined the response of microbial communities to simulated warming and drought conditions using the B4WarmED (Boreal Forest Warming in an Ecotone in Danger) experiment in Minnesota, USA. B4WarmED is a fully factorial blocking experiment which uses in situ experimental 3.4°C warming and precipitation reduction to simulate the projected regional late-21st century climate. Using Shannon-Weaver Diversity and Canonical Analysis of Principled Coordinates, we found that combined warming and drying effects exerted significant effects on the diversity and structure of microbial communities after 8 years of warming, and 5 of drought treatments. Specifically, warming and drying effects appeared to combine additively, rather than exhibiting nonlinear interactive effects, at the community level. Per-taxon linear models revealed a sizeable portion of individual microbes exhibit a significant abundance response to one or both of warming and drying effects. However, co-occurrence network analysis and Dufrene-Legrende Indicator Value characterization revealed a smaller portion of bacterial sub-communities with persistent taxonomical makeup and response profiles across treatments. Within the microbial communities our analysis identified three types of taxon-specific responses to climate change stressors: resistant, opportunistic, and sensitive, with most taxa being resistant to warming and drying effects. However, our results provide strong evidence that combined warming and drought influences will impact soil microbial communities of temperate-to-boreal ecotone forests ("boreal ecotone" hereafter), with potential implications for ecosystem functioning