Forest herbs show species-specific responses to variation in light regime on sites with contrasting soil acidity: an experiment mimicking forest conversion scenarios

Forest conversion from native deciduous forests to coniferous stands has been performed in many European regions and resulted in dramatic shifts in understorey plant community composition. However, the drivers for changes in specific understorey plant species remained unclear. Here, we experimentally determine the species-specific effects of light availability and chemical soil characteristics, on the vegetative and regenerative performance of five herbaceous forest understorey plants. Topsoil samples from both spruce and deciduous stands at four locations, with two levels of soil acidity, were collected and used in a common garden experiment. Additionally, three different light levels were applied, i.e., 'light deciduous', 'dark deciduous' (extra light reduction during summer) and 'evergreen' (light reduction during winter). In a second experiment we evaluated the germination of two of these species against the acidity and tree species at the site of origin of the soil samples. The light regime affected both the vegetative and regenerative performance of the understorey species: compared to light deciduous, Anemone nemorosa had a significantly lower performance under the evergreen light regime, Convallaria majalis under dark deciduous and Luzula luzuloides and Galium odoratum under both light regimes. The vegetative performance was lower in soil from acid sites for the acid-sensitive species G. odoratum and Primula elatior. Differences between the soils sampled under deciduous or spruce stands had no effect on the vegetative, or the regenerative performance of these species. By contrast, the germination of L. luzuloides and R elatior was higher in soils sampled in deciduous stands and in neutral sites. Species-specific responses in vegetative and regenerative performance of adult plants to a changed light regime and soil acidification could be a reason for the changed vegetation composition in converted stands. Also lower germination and establishment of forest understorey species in spruce stands could influence the species distribution after conversion