Forest structure and soil conditions influence the potential for drought-induced mortality in Vaccinium myrtillus

Vaccinium myrtillus is a foundation species and of high importance for ecosystem functions and services in many ecosystems of boreal Europe. Climate change now threatens the integrity of these cold-adapted ecosystems, with droughts and heat waves expected to increase in both frequency and intensity. In this study we investigated the occurrence of drought damage (desiccated twigs and mortality) on V. myrtillus ramets following several years with spring and/or summer droughts in southern Sweden. From the 26 524 ramets that were classified according to vitality, 21% were either dead or had more than 50% dead biomass. The proportion of damaged ramets varied according to forest structure and soil moisture. The impact of forest density was strongly dependent on tree species composition. While the proportion of damage decreased with forest density in forest dominated by Pinus sylvestris, it increased with density in forest dominated by Picea abies. Additionally, the proportion of damaged ramets was higher on elevated microhabitats such as rocks or stumps, and lower where V. myrtillus co-occurred with hygrophytic bryophytes. Damage was also inversely related to thickness of the humus layer. While the long-term effects on both V. myrtillus and the ecosystem remain largely unknown, our results indicate that climate change may have considerable direct impacts on this species. Importantly, our results indicate an interaction between climate change and forest composition, suggesting that forest management could potentially mediate the effects of droughts by altering tree species composition.