Hints for alternative stable states from long‐term vegetation dynamics in an unmanaged heathland

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Abstract

Abstract 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 Nørholm 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.

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