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

Abstract

Questions: Vegetation in forest understories is determined by a complex interplay of many different environmental factors, including light, water and nutrient availability. Changes in these conditions, especially changes in nitrogen (N) availability, have been widely discussed to cause shifts in species composition and loss of oligotrophic species. We examined whether nitrogen is the most important factor for the decline of nitrogen-sensitive species in understorey communities, as measured by average Ellenberg N values and the share and number of nitrogen-sensitive character species. Location: Nine different oligo–mesotrophic types of temperate forest in southwest Germany. Methods: We combined understorey vegetation and soil data from 135 forest stands. To study the importance of variables in explaining the vegetation gradients, we used random forest analyses and non-metric multidimensional scaling (NMDS) ordinations combined with fitting of environmental vectors. Results: We detected strong differences between three examined forest type groups: eutrophication in the understorey vegetation of broad-leaved forests on carbonate soils was strongly related to high canopy closure and low soil carbon/phosphorus (C/P) ratio. In broad-leaved forests on acidic soil, base saturation played a major role, but the vegetation was also influenced by the C/N and C/P ratios. In coniferous forests on acidic soil, soil pH and organic layer depth were important, as were the C/N, C/P and N/P ratios. Conclusion: Our results show that eutrophication in understorey forest vegetation is not monocausal. Rather, oligotrophic species of forest communities are reduced at sites with higher N or P availability and/or higher base saturation and soil pH. These conditions favor fast-growing ubiquitous species in the competition for light and other limiting factors. Since these factors were of varying importance between the forest types examined, differences shaping the forest communities need to be regarded when evaluating the impact of environmental changes, for example through atmospheric nitrogen deposition.