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Questions How do land-use legacies, tree encroachment and distance to the edge interact to determine species richness patterns in the ground vegetation of an unmanaged heathland? What are the direct and indirect effects of these factors? Do different functional groups respond differently to these factors? Location Norholm heathland, southwest Denmark. Methods We recorded the overstorey and ground vegetation along with environmental variables in 129 plots on a 350 ha heathland where management ceased more than 100 yrs ago. We used structural equation models (SEM) to examine the relationship between species richness of functional groups (herbs, dwarf shrubs and tree seedlings) and ground vegetation cover, overstorey development, distance to the edge and nutrient availability. Results The relative importance of the driving factors differed between functional groups. Herb richness increased with increasing nutrient availability from previous land use and decreased with overstorey development. Dwarf shrub richness responded negatively to overstorey development and herb cover. In contrast, species richness of tree seedlings responded strongly and positively to overstorey development but not to environmental variables or ground vegetation. In general, the effect of nutrient availability and distance to the edge were of similar magnitude and were largely mediated by the overstorey and ground vegetation. Conclusions Land-use legacies and tree encroachment can have differential effects on the species richness of different functional groups. Typical heathland dwarf shrubs were able to withstand moderate levels of increased nutrient availability and tree colonization. However, the combination of these factors poses a serious threat, as dwarf shrub richness decreased due to an increase in the cover of herbaceous species and light limitation. The absence of management did not lead to a rapid loss of typical heathland species.

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 Norholm 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.

Questions How does heathland vegetation composition change during ca. 100 yr of succession in the absence of management? Is succession divergent or mono-directional? Do soil conditions and land-use history explain the variation in vegetation dynamics? Is there evidence for alternative stable states? Location Norholm hede, a 350-ha heathland in southwest Denmark that was abandoned in 1895 and left to develop naturally via succession. Methods Permanent vegetation inventory plots were established and have been revisited 11 times between 1921 and 2012. Soil conditions were recorded in 2012. We used clustering, linear mixed models, linear models, ANOVA and multivariate techniques (redundancy analysis and principal response curves) to investigate changes in the plant community and how differences in plant community composition related to soil conditions and disturbance history. Results Ericaceous dwarf shrubs dominated most of the heathland initially. A dominance shift from dwarf shrubs to grasses occurred on about half of the plots. The other half of the plots remained dominated by dwarf shrubs, although Empetrum nigrum expanded at the expense of Calluna vulgaris. Lichen cover decreased dramatically across all plots. The divergent successional pattern was not explained by nutrient concentrations. Grasses mainly expanded in areas where they already had a substantial presence, and this initial presence was largely correlated with historical soil disturbance. Plots where dwarf shrubs remained dominant had a relatively thick O-horizon. Conclusions Vegetation dynamics during heathland succession were not deterministically determined by soil conditions. Grass and tree expansion occurred slowly and dwarf shrub dominance was stable for more than 100 yr on large parts of the heath, even in the absence of management. Management actions that disturb stable dwarf shrub vegetation may enhance grass and tree colonization.

Questions Is there a spatial pattern in the community structure (stem densities, species richness and species composition) of trees and shrubs during more than 100 yr of heathland to forest succession? To what extent is community structure influenced by land-use history and distance to seed sources? Do these effects change in time? Location A 350-ha heathland (Norholm) in southwest Denmark was abandoned in 1895 and left for free succession. Prior to abandonment the heathland was under traditional management for centuries. Method Trees and shrubs were recorded and measured in ten surveys spanning 91 yr (1921–2012). In the first nine surveys, complete censuses were used, whereas 116 randomly placed plots (10-m radius) were used in the most recent survey. We used mixed models and different multivariate techniques (non-metric multidimensional scaling and permutational multivariate ANOVA) to analyse spatio-temporal patterns in stem densities, species richness and species composition and the effects of land-use history and distance to seed sources. Results Tree and shrub densities increased exponentially over time and were consistently lower at longer distance from seed sources. Land-use history affected the rate of increase in tree densities, with lower encroachment rates at previously cultivated soils. Initially, species richness increased rapidly but leveled off in later successional stages. Richness was marginally higher at areas closer to seed sources, whereas it was not affected by land-use history. The succession showed a shift from early to mid-successional species over time; but distance to seed source determined which species were colonizing at early stages. Conclusion Distance to seed source and land-use history can differentially affect the structure and spatial patterns of developing forest communities. Although both factors had long-lasting effects on rates of colonization, the spatial patterns of colonization and species composition were mainly determined by distance to seed sources. The importance of distance to seed sources became less over time, suggesting that dispersal is a stronger driver at early stages. After more than a century since abandonment, forest covers