Long-term effects of liming on the species composition of the herb layer in temperate Central-European forests

Abstract

Upon the occurrence of acidic deposition-driven damage to forests at the beginning of the 1980s, large-scale liming has been applied to acidic forest sites in Germany. Due to possible negative consequences on soil chemistry and on (soil) fauna and flora, this measure was discussed highly controversially right from the beginning and is disputed until today. In investigations of liming effects on the forest understory vegetation, very few studies covered a time period of more than two decades with intermittent relevés. We analyzed relevés of the herbal understory on limed and control plots in one pine (Pinus sylvestris), one oak (Quercus robur), two European beech (Fagus sylvatica) and five Norway spruce (Picea abies) stands at permanent observation sites in south-western Germany. These relevés had been repeatedly conducted during approximately 25 years.The number of herbal plant species was generally low. An increase in the herb layer's plant cover with time was found at six out of the nine study sites and was mainly due to spreading of two grass species (Milium effusum, Deschampsia flexuosa) and of the subshrub Vaccinium myrtillus, which might have benefitted from continuous nitrogen deposition. Only two species (Agrostis stolonifera, Stellaria media) with a main distribution in open landscapes were found in the years of investigation and their occurrence was restricted to only four sites and some few years.Across all sites, the average Ellenberg indicator values of nutrient (nitrogen) supply (N-values) were significantly increased on the limed plots ten years after liming, and at Year 10, the N-values of the limed plots were significantly higher than that of the control, presumably due to increased decomposition rates after liming. At the spruce sites, the average Ellenberg indicator values of soil reaction (R-values) of the limed plots 10 years after liming were significantly higher than at the beginning (Year 0) and at the end of the investigation (2013). We found no significant differences among the Ellenberg indicator values of light condition (L-values).In conclusion, liming had a significant, but only transient effect on the composition of the understory species. Therefore, our results do not support arguments against liming of acidic forests that are based on long-term changes in the herbal flora. In managed forests for wood production on acidic soils, liming can continue contributing to buffer the soils against ongoing acidification and accompanying cation leaching caused by anthropogenic deposition of nitrogen.