The time it takes before vegetation will develop is a key question in many depoldering and managed realignment projects. It is often clear that in tidal area’s with sufficiently high sediment concentrations in the water column, marshes will eventually develop (Balke et al. 2016). However, for nature goals and recreational activities in the area, it is desirable to be able to predict if this will happen on the short term (< 5 years), or if this will only be the case on the long term (> 50 year). Being able to predict the rate of development is desirable to come up with a cost effective design when a specific type of habitat is desired either from nature management perspective, e.g. in case for compensation measures or when public support depends on the type of nature that will develop. Answering this question requires obtaining an understanding of the processes that control (i.e., enable and hamper) the initial establishment and subsequent lateral expansion of pioneer species.
It is well known that the initial vegetation development depends on a number of abiotic and biotic factors (Friess et al. 2011). For example, inundation period, and thereby the elevation, is a major factor controlling seedling survival (Balke et al. 2016) (Bouma et al. 2016). Recently, it was shown that short-term mixing of the sediment due to hydrodynamics (e.g. waves) is another important factor that may limit seedling survival to the higher elevations (Bouma et al. 2016). Much less is however known about how abiotic and biotic ‘sediment’ properties like compaction, drainage, sediment chemistry and bioturbation may affect seedling establishment and lateral expansion. The characteristics and development of the accreting sediment may expected to be an important factor for the vegetation establishment. As schematized in figure 2 differences in drainage and sediment compaction may be the driver of 2 alternative stable states, determining if seedlings will or will not establish (figure 2). From the sediment development following the construction of groins by Waarde, it may be expected that ‘ontpoldering’ areas will experience high rates of sediment accretion, but with low rates of soil compaction.
The Perkpolder study aims to develop generic rules on the factors that control the sediment development and the vegetation development (i.e., including both establishment and lateral expansion), by answering the following questions:
The data and the methods that have been used for the evaluation of the development of the vegetation and sediment are all described in the Perkpolder progress reports: