Mechanisms of biotic homogenization in Central European forest
Final Report Abstract
Human‐induced alterations of the environment are causing biotic changes worldwide, including the extinction of species and a mixing of once disparate floras and faunas. One type of biological communities that is expected to be particularly affected by environmental alterations are herb layer plant communities of fragmented forests such as those in the west European lowlands. However, our knowledge about current changes in species diversity and composition in these communities is limited due to a lack of adequate long‐ term studies. Here, we resurveyed the herb layer communities of ancient forest patches in the Weser‐Elbe region (NW Germany) after two decades using 175 semi‐permanent plots. The general objectives were (i) to quantify changes in plant species diversity, particularly the degree of floristic homogenization, (ii) to determine shifts in species composition in terms of functional traits and (iii) to find indications on the most likely environmental drivers for the observed changes. The observed herb layer changes in the Weser‐Elbe region were rather complex and not unidirectional. However, several patterns and processes were consistent across the study region: α diversity (species richness) increased by almost four species on average, whereas β diversity tended to decrease. The latter is interpreted as a beginning floristic homogenization. This study is one of the first studies in temperate Europe that demonstrates floristic homogenization of forest plant communities at a larger than local scale The magnitude of change in β diversity depended on the β diversity measure applied. The observed changes were primarily the result of a spread of native habitat generalists that are able to tolerate broad pH and moisture ranges. Independent of taxonomic homogenization, the abundance of 30 species decreased significantly; 12 of these are ancient forest species (i.e., species closely tied to forest patches with a habitat continuity > 200 years). Seven of the 30 species are listed on the Red List of endangered plant species in NW Germany. If these decreases continue over the next decades, local extinctions may result. This biotic impoverishment would seriously conflict with regional conservation goals. The decline of several ancient forest species is discussed as an indication that the forest patches are still paying off their “extinction debt”, i.e., exhibit a delayed response to forest fragmentation. 31 winner and 30 loser species were identified (significant increase or decrease in abundance, respectively). Winner species had higher seed longevity, flowered later in the season and had more often an oceanic distribution compared to loser species. Loser species tended to have a higher specific leaf area, to be more susceptible to deer browsing and to have a performance optimum at higher soil pH values compared to winner species. Multiple logistic regression analyses indicated that disturbances due to forest management interventions were the primary cause of the species shifts. Here, disturbance involved both the opening of the tree canopy and damage of the soil surface. As one of the first European resurvey studies, this study provides indications that an enhanced browsing pressure due to increased deer densities and increasingly warmer winters are important drivers. The study failed to demonstrate that eutrophication and acidification due to atmospheric deposition substantially drive herb layer changes. The restriction of the sample to the most base‐rich sites in the region is discussed as a likely reason.
Publications
- 2010. Habitat specialists and generalists drive homogenization and differentiation of temperate forest plant communities at the regional scale. Biological Conservation 143, 848–855
Naaf, T. & Wulf, M.
- 2011. Floristic homogenization and impoverishment : herb layer changes over two decades in deciduous forest patches of the Weser‐Elbe region (NW Germany). Dissertation at Potsdam University
Naaf, T.
- 2011. Traits of winner and loser species indicate drivers of herb layer changes over two decades in forests of NW Germany. Journal of Vegetation Science 22, 516‐527
Naaf, T. & Wulf, M.
