Colonisation processes in riparian fen vegetation

Abstract

Fen landscapes in The Netherlands are characterised by high biodiversity. Especially the ponds that have been created by peat excavation in former centuries inhabit numerous species. Under relatively nutrient-poor conditions, those fen ponds were rapidly colonised and often, species-rich floating mats developed. However, since the peat excavation stopped, fens have lost species typical for the colonisation of open water and the succession towards floating mats. Besides, most fens became fragments in an intensively used landscape with barriers for dispersal and an impoverished habitat quality. New ponds have been created to restore opportunities for succession towards floating mats. Unfortunately, the characteristic species have often remained absent and the formation of floating mats has hardly ever been observed. In order to increase the effectiveness of restoration measures, this thesis aims to determine which mechanisms and conditions are important drivers for the colonisation of open water by plant species growing from the bank. Colonisation starts with the arrival and germination of seeds, but for fens, vegetative propagules appeared to be just as important as seeds. Wind speed and direction turned out to be the major determinants of the dispersal of propagules via water. Even over longer periods of time, wind resulted in a directional transport following the main wind direction. This directionality is likely to have consequences for the colonisation of banks with different orientations to the prevailing wind direction, and will determine the effectiveness of connections (e.g. ditches) between water bodies. The wind-induced water currents that drive the dispersal of seeds also cause differences in recruitment conditions between more exposed (downwind) and more sheltered (upwind) banks. This affected germination and establishment there and recruitment was highest at sheltered banks. Most likely, wave action determined this difference. This could imply that the common practice to minimise natural water table fluctuations could impede the establishment of colonisers. With natural water level fluctuations, water tables are higher in the fall, when the majority of seeds disperse, and drop in spring, so that the seeds can germinate and become established without the risk of being washed away. The vegetation development in restored ponds will also depend on the habitat characteristics. A large controlled mesocosm experiment showed that nutrient availability (of N and P) in both the surface water and the bank soil can accelerate the growth of colonisers and their expansion into the water. A large field survey on 62 ponds showed that a high nutrient availability, high sulphate concentrations in the surface water, high turbidity, deep water, a high muskrat density and the absence of the keystone species Stratiotes all impede the development of bank vegetation in Dutch fen ponds. However, the effects of degraded habitat characteristics seemed to overrule the effects of dispersal limitations, as colonisation often failed even in highly connected ponds or when sufficient propagule sources were present. Overall, successful restoration of species-rich plant communities that colonise the water and induce the formation of floating peat mats requires a multifaceted approach to address the various negative effects of land use changes.