The secret life of woody species : a study on woody species establishment, interactions with herbivores and vegetation succession

Abstract

Woody species are generally known to be among the most successful plant strategist in the world. They play a prominent role in vegetation dynamics because of their size, longevity and ability to survive under stressful conditions. Nevertheless, the establishment stage, involving seed dispersal, germination and seedling survival is very critical. The main objective of this study is to investigate the biotic and abiotic factors that limit woody species establishment. Furthermore, I investigated the role of woody species in plant-animal interactions, i.e. the direct and indirect interactions between woody species and herbivores.Since woody species do not form seed banks, they strongly depend on the production and dispersal of seeds. During research on secondary succession on abandoned fields, after 30-45 years, woody species became dominant depending on habitat productivity. On rich soils, the colonisation rate of woody species was slower than on poor soils. This difference can be explained by the higher abundance of perennial herbs and grasses on the rich soil type, which inhibited woody species establishment through competition for light, water and nutrients. Establishment chances for late successional 'forest' woody species were found to be higher in early successional communities than in intermediate or late successional communities due to a lower granivory pressure. In early successional communities, abiotic stress was the key factor in woody species establishment of both pioneer as well as mature forest species. During the intermediate and late successional stages, seed predation by small rodents had a negative effect on the establishment of woody species. In the late successional stage, forest species establishment is reduced due to a severe parent tree effect through seedling defoliation by host-specific insects. Grazing exclusion of large herbivores in former, heavily grazed pine woodland resulted in a fast recovery of graze-tolerant shrub species, while grazing introduction in pine forest and oak-beech forest did not have significant effects on the shrub layer. While graze-tolerant woody species were able to recover rapidly following release of grazing, this was not the case for graze-sensitive species like deciduous tree species. Small rodent densities and seed predation intensities were higher in vegetation communities excluded from grazing than in those introduced to grazing. Grazing by large herbivores caused a decrease in small rodent densities. Small rodents depend on vegetation cover for shelter and food. Differences between the effects of introduction and exclusion of grazing by large herbivores on small rodent communities can be explained by differences in vegetation structure development. The recovery of heavily browsed understory vegetation following release of grazing by large herbivores proceeded faster than the understory degradation due to grazing introduction. Introduction of red deer ( Cervus elaphus L.) in former roe deer ( Capreolus capreolus L . ) habitats triggered a significant decrease in roe deer performance in these areas. Shifts in habitat use as well as a lower reproduction success of the latter species pointed to competition for woody forage (browse) between red deer, as superior competitor and roe deer as inferior one. Within the context of vegetation dynamics, woody species play a prominent role in important plant-animal interactions. The establishment stage of woody species can be considered as a significant bottleneck in vegetation succession