Biotic homogenisation and differentiation along a habitat gradient resulting from the ageing of managed beech stands

Abstract

Understanding the processes driving biodiversity changes in forest communities helps to better evaluate sustainable forest management methods and their effects on forest ecosystems. This, in turn, is crucial for the appropriate management of protected areas, in particular those covered under the European protection program Natura 2000. Two datasets from the Polish Eastern Carpathians collected in the 1970s and 2000s were compared to analyse the temporal and spatial changes in the species composition of beech forests in a habitat gradient of soil moisture, acidity and fertility. We found that species favouring fertile and alkaline soils, with low ability for vegetative propagation and without a persistent seed bank became less frequent, whilst frequency of good competitors tolerant to lower soil pH and fertility with a persistent seed bank and a capacity for dispersing seeds over long distances increased. Herb layer vegetation on poor and rich soils became less similar in the past 30years, though two opposite processes occurred. In poor habitats, the biotic differentiation of herb layer vegetation prevailed, whilst biotic homogenisation was more important in fertile habitats. It is suggested that the ageing of the managed beech stands might have contributed to these changes through the accumulation of litter, top soil acidification and the lowering of the concentration of nutrients as well as due to altered disturbance regimes in older stands. The observed relationships between biotic homogenisation/differentiation and the habitat gradient indicate that differences in soil conditions not only shape the spatial pattern of the forest vegetation, but also the spatial and temporal patterns of changes in vegetation composition resulting from the ageing of tree stands. Our results also suggest that in spite of continuous management the studied beech forests have only slightly changed in their specific species composition. Moreover, the ageing of tree stands did not necessarily generate better conditions for the conservation of the species richness of herbaceous plants, at least those species that are considered a measure of naturalness in beech forests.