Short-term effects of selection and clear cutting on the shrub and herb layer vegetation during the conversion of even-aged Norway spruce stands into mixed stands

The conversion of even-aged Norway spruce (Picea abies (L.) H. Karst.) stands into more site adapted mixed stands is the main silvicultural goal in Germany and many other European countries. The conversion will primarily be achieved with the help of selection cutting, removing single target diameter trees and creating small gaps. At sites highly susceptible to windthrow, however, small scale clear cutting (up to 1ha) might be an appropriate alternative. Both selection and clear cutting will affect the understorey vegetation, an important component of forest biodiversity and ecosystem functioning. Thus, with the help of a BACI-experiment (before-after/control-impact), the short-term effects of selection and small scale clear cutting on the herb and shrub layer vegetation were studied at two sites in the Solling Hills (Lower Saxony, Germany). Both treatments led to an increase in herb and shrub layer species numbers, due to persistent and invading (mainly pioneer tree species and indicators of disturbance) species and to a shift in plant traits. Most responsible for the changes in vegetation was a higher light availability, and to a lesser degree soil disturbance. The number of annuals and ruderals, however, increased only immediately after clear cutting, whereas on selection cutting plots the numbers remained high. Gaining in number and coverage were competitive perennials (Epilobium angustifolium, Juncus effusus, Rubus idaeus) especially on the clear cuttings, fulfilling a nutrient retention function. The competitives did not deter tree species from regenerating in the clear cut plots, whereas on selection cutting plots the large expansion of R. idaeus appeared to be hindering tree colonisation. Suffering under clear cut conditions were predominantly shade-tolerant, closed-forest species, but no loss of these species could be detected. This indicates the potential for a fast recovery of these forest species, as soon as, pioneer trees expand. Although the vegetation changed in similar directions at both study sites, differences in species numbers and composition were found, indicating the importance of site conditions in the beginning of succession. In conclusion, our study shows no overall negative effects on shrub and herb layer biodiversity and trait composition after clear cutting. It is suggested that small scale clear cutting may be considered an appropriate alternative to silvicultural methods which cause a moderate rate of disturbance, to reach the goal of Norway spruce forest conversion.