Pattern of tree species co-occurrence in an ecotone responds to spatially variable drivers

Context: Ecological structure in ecotones, defined by how species from adjacent systems co-occur, affects ecosystem functions and climate change responses. Ecotone structure can vary spatially, yet variability in broader-scale ecotones is poorly understood. In Wisconsin (USA) the Tension Zone is an ecoregional ecotone, separating northern and southern ecosystems. Objectives: Characterize ecotone structure in the Tension Zone, examine how structure varied spatially, and identify how environmental drivers affected structure. Methods: Using historical (1800s) tree occurrence data, we examined co-occurrence of northern and southern species at multiple scales (1.0 km to 7.5 km) at different locations in the Tension Zone, identifying the finest scale at which co-occurrence was detected. We assessed relationships between co-occurrence and environmental variables. Results: Co-occurrence emerged at different scales, related to interacting climate and soil variables and location within the ecotone. Northern and southern trees co-occurred at broader scales near ecotone center and at locations with higher climatic water availability and sandier soils; they co-occurred at finer scales in locations with higher climatic water availability and richer soils. Sites with xeric tree species were associated with broader-scale co-occurrence. Conclusions: We detected spatially variable structure within the Tension Zone, resulting from multi-scale processes among underlying environmental drivers. Finer-scale co-occurrence may have resulted from competition in high-resource environments, while broader scale co-occurrence may have been driven by fire and associated feedbacks. Characterizing structure in an ecoregional ecotone adds to a growing body of evidence that finer-scale factors play a role in defining the characteristics, functions, and responses of broader-scale ecotones.