Long-term changes in the herb layer of a southern Swedish broadleaf forest

The forest herb layer is reacting fast to the changes in the environment due to its sensitivity and short life cycle. The forest overstory has a strong influence over the availability of the environmental factors and also represents a powerful competitor for the available resources. The species of the herbaceous ground flora have different requirements and sensitivities in regard to the environment, therefore each of them has to be treated separately in order to detect the factors that influence and direct the changes in their abundance. Measurements from 30 permanent sample plots extending during the period 1988-2015 in a hornbeam forest in Stenshuvud National park, southern Sweden, were used to quantify the inter-annual variation and long-term trends of the herb layer and its interactions with the environmental factors. Mean Ellenberg indicator values were used to assess the responses of the herb layer community to the variations in environmental factors.Results of paired t-tests show that the species richness is decreasing, while the evenness remained quite stable, probably partly due to the closing up of the canopy which is also reflected by the Ellenberg mean indicator values for light. Ellenberg mean indicator values also indicate that the temperatures have been increasing, while the climate became more oceanic. However, mean indicator values also suggest a decreasing soil moisture, soil reaction and available nitrogen. Overall, the total cover has increased, while some species like Hedera helix and Hepatica nobilis have manifested declining covers through time. Fifteen of the ground flora species present in 1988 were absent in 2015, while only two species colonised the area. Roe deer has probably played an important role in shaping the abundance of several species, due to its rapid increase in population density until early 1990s. The regression analyses revealed that precipitation amounts during spring have quite strong relationships with both total cover and covers of some species