Veranderingen in het milieu en de vegetatie in de Biesbosch door afsluiting van het Haringvliet

In November 1970 the Haringvliet-sluices were put into operation. These sluices are an integral part of the Deltaworks which aim at protecting the south-west of Holland against inundations. As a result of this closing of the Haringvliet the fresh-water tidal area of the Biesbosch was nearly completely cut off from tidal influences of the sea. The tidal movement in this delta of the rivers Rhine and Maas decreased from over six feet to only under one foot. In this thesis the consequences for the vegetation in the Biesbosch are described. An extensive survey was held from 1970 till 1975.The Biesbosch originated from dike-bursts in 1421 A.D. The old soils were washed away by the sea and over the centuries new sediments were deposited by the rivers Rhine and Maas. In 1970 the Biesbosch, from the south-west to the north-east, consisted of large stretches of water, low-lying sandy flats, rushmarshes and creeks of varying width, reed-marshes and willowcoppices. In most willow-coppices the hydrology was controled by low dikes and drainage systems. The coppices in which sedimentation had progressed farthest had been reclaimed and put into use for agriculture. In the east and north of the Biesbosch the agricultural polder-land covers nearly the whole area.The marshes, because of their structure, are highly characteristic for the geomorphology of fresh-water tidal areas. Along the creeks natural levees are found which are relatively high and sandy. Behind the natural levees are lowlying backswamps of clayey soils which had usually hardly ripened because of bad drainage. Due to the closing of the Haringvliet the vegetative environment of the areas outside the dikes strongly changed. Flooding of the soil only seldom occurred, in stead of twice a day formerly. The ground-water level sank from just below the surface to the height of the new water-levels. Therefore a quick ripening of the soil took place in the only young sediments.For the original vegetation the differences in influence of tidal flooding twice a day were of great impact. The ecology of this vegetation is frequently characterized by a twodimensional diagram in which on the one hand altitudes of soils and on the other drainage (of natural levees or backswamps) are indicated as variable factors. As regards altitudes of soils the Mean High Water (MHW) level formed a frame of reference for the possibilities of species and plant- communities to occur. For most species sharp limits were found concerning their tolerance for flooding. In this investigation the data of several earlier investigations have been brought together. This resulted in the distinction of 17 groups of earlier described plantcommunities: 10 groups of marsh-vegetation and 7 groups of willow-coppices vegetation. These groups are indicated by a combination of the letters of the most important species groups.The changes in vegetation have yearly been investigated by making records of the vegetation on permanent sample plots. Frequent mapping out of larger and smaller parts of the area took place as well. The main outstanding features of changes were the following:
- Each year the wet-soil preferring species decreased in favour of herbage preferring relatively dry soil.
- In the changes of vegetation, stages occured that were connected with different stages of ripening of the soil.
- The strongest changes took place in the backswamps of marshes and in the relatively low-lying willow-coppices.
- The smallest changes were to be found on the high natural levees and willow-coppices of high level. Here the milieu had already been relatively dry in former times and the soil was nearly entirely ripened.
- In the new riparian zones, where the influences of flooding, flow, wash, and where the structure of the soil are strongly varying, the vegetation also varies to a rather large extent.
- On higher grounds the differentiation of vegetation decreased, but the number of species has in general become larger.The new differentiation of vegetation has been described in a circular diagram. The differentiation between various environmental factors has also been indicated in this figure. The basic form of this diagram reflects a small island, the highest part of which is situated in the middle and round which different environmental influences are expressed in the variations of riparian vegetation.As a result of observed separate elements in the developments of vegetation and the environmental factors, a number of causal relations can be established between environment and vegetation. Various factors in these new circumstances influence vegetation selectively and differentiatingly. The variation in altitudes of soil is still of great importance; not so much because of differences in levels of flooding, but because of differences in ground-water levels. In the mutual relations of plants whithin the world of vegetation the principle of primarity (being first) ranks as an important factor. Hence not all species have gained similar chances to establish themselves in appropriate new environments. Competition for space and light is subsequently the cause of continuing changes in the vegetation when in more or less stabilized environments.