Interacting effects of invasion and soil microbes on Douglas‐fir seedling response to drought

Global change is affecting native species and communities through multiple anthropogenic drivers which likely interact, complicating our ability to predict the net effects of global change. In the Pacific Northwest region (USA), Cytisus scoparius (L.) Link (Scotch broom) invasion has dramatically altered many ecosystems, including postharvest timber lands. Simultaneously, the intensity of summer drought conditions associated with climate change is making successful reforestation increasingly difficult. We investigated how Douglas‐fir ( Pseudotsuga menziesii (Mirb.) Franco var. menziesii ) responds to the multiple stressors of drought and Cytisus invasion. We further evaluated whether the soil microbial community, including mycorrhizal fungi, ameliorates or exacerbates Douglas‐fir's response to drought and Cytisus competition. Drought and the presence of the invader both increased stress (measured by chlorophyll fluorescence) and decreased survival of Douglas‐fir seedlings, and their combined effects on stress were more than additive. Douglas‐fir grew bigger in live than in sterile soil, but this effect was strongly reduced in the presence of Cytisus ; we also found that mycorrhizal colonization was reduced in the presence of the invader. Surprisingly, however, Douglas‐fir survival in live soil was lower than in sterile soil, especially in the presence of Cytisus . Our results suggest that the impact of drought on Douglas‐fir seedlings is likely to be exacerbated by the invasion of Cytisus . Our results further suggest that in a warming climate, the presence of impactful invasive species can affect whether soil microbes have a net positive or negative effect on native plant performance. Our results illustrate the value of studying multiple stressors simultaneously to understand their interactions and combined impacts on native species.