Vegetation change in a South German raised bog: Ecosystem engineering by plant species, vegetation switch or ecosystem level feedback mechanisms

The main results of a long-term ecological study from 1957 to 1994 are pres ented. Aspects of vegetation dynamics and hydrology of a South German ombro genous bog were analysed in different spatial scales and temporal periods. Vegetation change from 1957 to 1992 was documented by permanent plots and c omparison of vegetation maps. Pinus sylvestris dominated forest stages, bog forests and Calluna vulgaris hummocks increased by replacing Sphagnum lawn s and hollow structures in the course of succession. Fundamental changes in hydrology occurred in the same period of time. Month ly water level means dropped by approximately 10-15 cm and duration and ext ent of the maximum water levels were reduced. Discharge of the catchment ar ea remained almost constant during the winter months, but summer discharge significantly declined. Important climatic factors (air temperature, monthl y and annual precipitation) showed a common high variability during the sam e period, but no significant trend could be found. At the same time, trend analysis indicates a 350 mm increase in the total yearly evapotranspiration . Numerous interdependences between vegetation development and hydrology are hidden behind the long-term dynamics of this ecosystem. The complexity of t he bog ecosystem prevents a simplistic separation of causes and effects. Bu t this long-term study can help to clarify the debate about the effectivene ss of vegetation switches or species acting as ecosystem engineers. The invasion of tree species of the genus Pinus and Betula contributes to s tructure important hydrologic features that change abiotic conditions for o ther species within the ecosystems. However, such processes of interaction between biotic and abiotic ecosystem properties cause additional positive o r negative feedback mechanisms. Succession in raised bogs might be a good w ay of showing that discrimination between biotic and abiotic ecosystem proc esses requires an interdisciplinary approach.