The FraDiv experiment: Biodiversity-ecosystem functioning research meets reforestation practice

Abstract

European ash-rich forest ecosystems are transforming rapidly under the influence of ash dieback, putting many associated species at risk. Forest managers face the urgent challenge to deal with the loss of European ash (Fraxinus excelsior L.) as a key tree species to maintain species richness and ecosystem functioning. The project FraDivexp, located in Schleswig-Holstein, Germany, aims at counteracting detrimental effects of progressive ash decline by testing alternative tree species in a Biodiversity-Ecosystem Functioning (BEF) experiment along a hydrological gradient. Here, we provide insight into how a BEF-approach can be combined with silvicultural management practice to meet the needs for both ecological research and reforestation. At the same time, we present first data on the survival of the planted saplings.In winter 2019/2020, FraDivexp was established with autochthonous tree species considered potential substitutes for the functioning of ash. At 12 sites, plantations including Fraxinus excelsior, Acer platanoides L., Carpinus betulus L., Tilia cordata Mill. and Ulmus laevis Pall. were planted directly underneath the remains of collapsing forest canopies involving all monocultures, and all combinations of 2-, 4- and 5-species mixtures. One year after planting of 25,200 trees, total mortality was at 5% with U. laevis showing lowest mortality while establishment of A.platanoides was least successful. In this early phase of FraDivexp tree mortality was species-specific and driven by initial individual tree size, pH-values in the topsoil and canopy openness, while there was no effect associated with tree diversity. Analysis of further biotic factors showed high mortality of A.platanoides in areas with a high cover of Rubus spp. in the herb layer, indicating species-specific susceptibility to different site conditions. Overall, low mortality (compared to other BEF experiments on arable land) suggests an advantage of establishing a BEF experiment within an existing forest matrix. Simultaneously, this study shows that differences in environmental context dependency among species need to be considered more explicitly, when recommending management strategies. To ensure successful establishment of reforestations with substitute tree species is the first step to efficiently counteract the massive loss of ash trees with the aim of maintaining biodiversity and ecosystem functioning.