Multiple soil factors explain eutrophication signals in the understory vegetation of temperate forests

Prepared by Marina Roth, Hans-Gerhard Michiels, Heike Puhlmann, Carina Sucker & Markus Hauck

Vegetation field survey in a typical low-productive oak forest on acidic soil without signs of eutrophication. The photograph shows one of the study sites located in the Black Forest, southwest Germany. Photo credit: Marina Roth

Eutrophication due to increased nitrogen concentrations is known to alter species composition and threaten sensitive habitat types. In addition to nitrogen, forest understory vegetation is also strongly determined by the availability of light, water and other nutrients. In our study, we identified drivers behind the eutrophication in the ground vegetation of nine potentially nitrogen-sensitive temperate forest types in southwest Germany. More specifically, we examined whether nitrogen is the most important factor for the decline of nitrogen-sensitive species in understory communities, as measured by average Ellenberg N values and the share and number of nitrogen-sensitive character species. Ellenberg N values are a proven measure in vegetation ecology, which rate the preference of individual species for nutrients.

We conducted vegetation surveys and measured comprehensive soil data in 135 forest stands across nine temperate forest types with low productivity. In every forest type, we covered a gradient ranging from forests with no visible signs of eutrophication to forests where untypical and nutrient-demanding plants replace parts of the original vegetation.

Our results show that the increase of Ellenberg N values and the decline of nitrogen-sensitive character species is controlled by nitrogen (N) and phosphorous (P) availability in relation to carbon (C) content, soil pH and base saturation. Typical species of nutrient-poor forest communities decline at sites with higher N or P availability and/or higher base saturation and soil pH. These conditions disadvantage species adapted to nutrient-poor conditions but favour fast-growing ubiquitous species in the competition for light and other limiting factors.

We found considerable differences in the importance of different soil and environmental variables between broad-leaved forests on carbonate soils, broad-leaved forests on acidic soils, and coniferous forests on acidic soils. In general, the relationship of the soil C/N ratio and vegetation composition was stronger in forests on acidic, as compared to forests on carbonate soil. In forests on carbonate soils, eutrophication was strongly related to high canopy closure and low soil C/P ratio. Our results show that eutrophication in understory forest vegetation is not monocausal. The variance in the importance of soil and environmental variables among the forest types has important implications for evaluating the impacts of environmental changes. For example, the addition of nitrogen by atmospheric deposition can differ in its effects, dependent on microclimate, acidity, light, historical forest use and natural nutrient availability of the affected sites. Understanding this context-dependency may contribute to target eutrophication and the loss of nitrogen-sensitive species, which still remains a major challenge for nature conservation in the 21st century.

This is a plain language summary for the paper of Roth et al. published in the Journal of Vegetation Science (