Long-term change in understorey plant communities of conventionally managed temperate deciduous forests: effects of nitrogen deposition and forest management

Questions: The understorey vegetation of temperate forests may serve as an indicator of long-term change in site conditions. We investigated vegetation change of the herb and moss layers during the last 50 yr in managed European beech (Fagus sylvatica) and sessile oak (Quercus petraea) forests under chronic N deposition. We tested the hypotheses that (1) the species richness of the understorey vegetation has increased in intensively managed forests because species gains due to increased light and nutrient supply are greater than species losses, and (2) shifts in species richness and community composition differ between beech and oak stands due to contrasting light regimes. Location: Forests of the Solling Mountains, central Germany. Methods: We re-sampled 95 relevés of the herb and moss layers after 46/47 yr (1966/67 vs 2013) to analyse changes in cover and species composition. In the beech forests, two widespread types of production forest (selection cutting vs shelterwood management) were compared with unmanaged forest reserves. Trends in vegetation change were related to a long-term record of N deposition and soil chemistry, shifts in EIV and changes in canopy structure. Results: A decrease in cover of the uppermost tree layer and emerging lower tree and shrub layers have increased the heterogeneity of canopy structure and indicate a more heterogeneous light distribution on the forest floor of the beech and oak forests. Plot-level species richness has increased in both forest types (except for beech stands under shelterwood management). Under chronic N deposition (∼40 kg N·ha−1·yr−1 before 1990, 20–25 kg·ha−1·yr−1 since 1990), the organic layer N pool more than doubled between 1966 and 1993 with a decrease thereafter, and the EIV-N scores have increased. The N score increase was not related to changes in the canopy. Conclusions: This study presents evidence of an N deposition effect on the understorey vegetation, which acts independently from the recorded changes in canopy structure. We conclude that altered nutrient supply due to atmospheric N deposition, together with changes in light regimes and management-related disturbance, are the main drivers of vegetation change in the studied stands.