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Background:Plant diversity can influence numerous ecosystem processes, including plant–animal interactions, which, in turn, will affect plant growth and fitness. At present, little is known on how plant–animal interactions in forests respond to gradients in tree and herb-layer diversity. Aims:To quantify how invertebrate herbivory, pollination-dependent seed production and post-dispersal seed predation vary along a gradient of tree and herb diversity in a semi-natural temperate deciduous forest. Methods:Potted individuals of the understorey herbsLathyrus vernusandPrimula elatiorwere exposed in 40 forest plots along a natural gradient of tree and herb diversity for 3 months to record seed production and leaf damage caused by invertebrate herbivores. In half the plants, pollinators were experimentally excluded to test if seed production depended on insect pollination. InLathyrus vernuslinkages between below- and above-ground herbivory were tested by inoculating plants with root-feeding nematodes. To study seed predation, we measured seed removal from seed depots that were selectively accessible to different seed predator groups. Results:Herbivore damage decreased with increasing tree diversity inP. elatior.InL. vernus, above-ground herbivory was higher in nematode-treated plants than in control plants. Seed production inL. vernus, which strongly depended on insect pollination, showed a positive relationship with tree diversity. Seed predation was positively related to herb diversity inL. vernus, but only weakly so inP. elatior. While both vertebrates and invertebrates acted as seed predators ofL. vernus, seeds ofP. elatiorwere mainly predated by invertebrates. Conclusions:The fitness of understorey plants is linked to tree and herb diversity via changes in plant-invertebrate interactions. However, species-specific responses of study plants underline the importance of species identity effects in addition to effects of biodiversity per se. [ABSTRACT FROM AUTHOR]

Abstract: In temperate deciduous forests, the herb layer contributes most to total vascular plant species richness. The diversity of the tree layer can influence herb layer diversity by modifying resource availability and environmental conditions relevant to herb layer plants. Here, we explore the relationship between tree layer diversity and herb layer species richness and cover. Also, we address the question how different environmental factors that are potentially modified by the tree layer influence herb layer characteristics. Our study area is located in the Hainich National Park, one of the largest continuous stretches of broad-leaved deciduous forest in Central Europe. We recorded herb and tree layer composition on 79 plots selected along a tree diversity gradient ranging from two to nine tree species. In addition, canopy cover, soil pH, mass of the humus layer, soil C:N ratio, soil moisture and distance to the forest edge were determined. We used generalized least square models to analyse effects of tree diversity, environmental variables, and spatial plot positions on herb layer species richness and cover. Species richness and cover of the herb layer increased with increasing tree diversity. In addition, both species richness and cover showed a negative response to increasing canopy cover and mass of the humus layer. Herb layer species richness was also positively related to increasing soil pH and the distance to the forest edge. The proportion of forbs increased with increasing tree diversity, whereas the proportion of tree saplings decreased and the proportion of graminoids was not affected. The proportion of true forest species increased with increasing canopy cover. We conclude that forest stands with a high tree diversity feature a more diverse herb layer and a higher herb cover. Furthermore, the environmental variables humus layer mass, light availability and pH also strongly affect herb layer species richness and cover. [Copyright &y& Elsevier]