Pervasive early 21st-century vegetation changes across Danish semi-natural ecosystems: more losers than winners and a shift towards competitive, tall-growing species.
Abstract
Semi-natural open habitats in north-western Europe are highly prioritized for conservation, and optimization of management planning is essential for continued protection of their diversity. We evaluate whether current management practices, which consist mainly of summer grazing by livestock, are sufficient to maintain plant species composition in a stable state across semi-natural areas in Denmark, or if shifts in functional composition are taking place. Further, we investigate important drivers of any ongoing changes through trait differences between winner and loser species. Using a data set of 6513 annually inventoried (2004-2010) vegetation plots from 202 semi-natural NATURA 2000 protected areas, we assessed changes in cover of all plant species and, using boosted regression trees, whether winners and losers differ in regard to autecological characteristics and traits. There were consistent changes in cover for 123 out of 603 species across the 6-year study period, with more species losing than winning (78 vs. 45). Winner and loser species were strongly differentiated by competition-related traits, but traits related to soil nutrients and soil moisture also had some importance. Generally, our results indicate a trend towards taller and more competitive species. Synthesis and applications. Despite conservation actions in order to maintain a so-called favourable conservation status, vegetation in NATURA 2000 protected areas is still undergoing compositional changes. Overall, autecological characteristics and trait differences between winners and losers match expectations given the contemporary anthropogenic pressures, especially reduced livestock grazing. Atmospheric nitrogen deposition and increased soil moisture due to reduced drainage might also be involved in observed changes. Current management (i.e. summer grazing by livestock) is insufficient to preserve the diversity of less-competitive, low-statured and stress-tolerant herbaceous species, potentially leading to species losses. Such patterns can thus be expected across semi-natural ecosystems in north-western Europe and other places where traditional land use (i.e. previous widespread extensive livestock grazing) have been replaced with various degrees of conservation management. A greater focus on returning large grazing herbivores to conservation areas is recommended for the protection of plant species diversity and to counteract ongoing changes.