DISTRIBUTION AND DYNAMICS OF SHRUB ROOTS IN RECENT COASTAL DUNE VALLEY ECOSYSTEMS OF BELGIUM

A chronosequence, up to 60 years old, was investigated in the Westhoek Nature Reserve. Root growth of the shrubs in dune valleys was found to be restricted to the surface A1bi and Bbi horizons which often did not exceed 25 cm. The thickness of these horizons and the amount of litter and humus could be explained by the position along secondary topo-hydrosequences within the chronosequence. The mostly stratified sands underlying the A1bi and Bbi horizons had a higher bulk density. In these relatively dense Cd horizons root growth of new colonizing shrubs seems to be possible only along old root or stem channels of the previous vegetation of dune building plants such as Ammophila and Salix which grew in pace with the accumulating dune. It is concluded that root growth in these sandy soils is restricted by the high penetration resistance of the closely packed sands in the subsurface horizons. The root penetration problem in these sandy soils explains the uniform vegetation physiognomy better than other climatic or edaphic factors.