Biotic homogenization of herb layer composition between two contrasting beech forest communities on limestone over 50 years

Abstract

AbstractQuestionsHow do vegetation changes in a limestone beech forest community on dry and nutrient‐poor slopes differ from changes in a community on moist and nutrient‐rich soils over the past 50 yr? Have temporal changes led to a further differentiation between communities?LocationAncient, beech‐dominated forest region on limestone in central Germany.MethodsWe compared vegetation relevés sampled between 1955 and 1960 and resurveyed between 2009 and 2012 on quasi‐permanent plots in the Carici‐Fagetum primuletosum (n = 26) on dry, nutrient‐poor and sun‐exposed slopes, and in the Hordelymo‐Fagetum allietosum (n = 25) on nutrient‐rich plateau sites with mesic soils in spring. We concentrated on changes in species richness, vegetation structure and vegetation composition, identified winner and loser species and calculated pair‐wise dissimilarity indices within and between communities to test for differentiation or homogenization over time.ResultsOur results show a more pronounced change in community composition in the Carici‐Fagetum primuletosum within 50 yr compared to the Hordelymo‐Fagetum allietosum. Despite no significant increase in total canopy cover, shade‐tolerant, mesophilous, nutrient‐demanding species expanded, whereas light‐demanding, drought‐tolerant, oligotrophic, often Red List species decreased. Changes in the Hordelymo‐Fagetum allietosum were more random. Overall, vegetation dynamics led to a homogenization between the two contrasting communities caused by a loss of specialized species from the Carici‐Fagetum community and a gain in generalist species in both communities.ConclusionThe drastic changes in the Carici‐Fagetum community indicate the higher susceptibility of marginal sites – that often have a high conservation value – to local environmental changes. In particular, N deposition and the abandonment of former (more traditional) forest management, such as coppice‐with‐standards and wood pasture, are responsible for this development. On the other hand, the increasing temperatures from climate change are not sufficient to secure the survival of the once frequent thermophilous species. Therefore, adequate forest management and conservation measures, such as the removal of competitive or exotic tree species or the re‐establishment of coppice‐with‐standards management, are necessary to counteract the on‐going biotic homogenization of beech forest communities and should focus on marginal sites.