Indirect effects of land-use legacies determine tree colonization patterns in abandoned heathland

Questions How do land-use legacies and distance to forest patches influence tree colonization at a post-agricultural heathland? Are colonizing species with different life-history traits affected differently by these factors? Is the effect of increased nutrient availability from land-use legacies mediated by the understorey vegetation? Location Nørholm hede, a 350-ha heathland in southwest Denmark developing naturally after abandonment of traditional agricultural practices in 1895. Methods Using 140 plots (0.03 ha), we quantified the vegetation structure, soil properties and natural recruitment of tree/shrubs in the heathland. Further, we used high-resolution LiDAR data to classify the vegetation and identify forest patches. In the analysis, we first used a logistic mixed model to test whether colonization of tree and shrub species differed between areas with different land-use history and whether it was influenced by the distance to forest patches and life-history traits (seed mass) of colonizing species. Then, to determine how different factors influence colonization, we explored the direct and indirect relationships among nutrient availability, density of adult trees, canopy cover, cover of Deschampsia flexuosa and the probability of seedling colonization in each plot using a confirmatory multilevel path analysis. Results The probability of seedling colonization for small- and large-seeded species decreased with distance to forest patches. This response was more pronounced at the previously cultivated area. Multilevel path analysis showed that colonizing species with different life-history traits were affected by different factors. Small-seeded species were negatively affected by increased nutrient availability mediated by an increase in the cover of D. flexuosa. This effect was not found for large-seeded species. Moreover, the density of trees/shrubs (>2 m) increased the probability of colonization of small-seeded species, whereas higher canopy cover had a positive effect on large-seeded species. Conclusions Our analyses demonstrate that the interactions between abiotic factors, biotic interactions and life-history traits of colonizing species can lead to distinct patterns of tree colonization. Land-use legacies can have long-lasting, indirect effects on tree colonization by altering the composition of the understorey vegetation, which in turn can delay tree encroachment and slow down the succession from heathland to forest.