Effect of geomorphology and nitrogen deposition on rate of vegetation succession in inland drift sands
Questions (1) At what rate does succession take place in active and more stabilized drift sands in regions with low and high N deposition in the Netherlands? (2) What is the present composition of pioneer vegetation in active and more stabilized drift sands in regions with low and high N deposition? (3) What efforts are necessary to conserve bare sand in drift sands? Location Eight inland dunes in the Netherlands, which represent four active and four more stabilized drift sands in regions with low and high N deposition. Methods Aerial photographs from 1950, 1981, 1995 and 2007 were used to compare rates of succession from bare sand towards forest. For each site, a detailed vegetation map was made in 2007. Results In all studied sites, bare sand decreased, especially in regions with high N deposition. This decline was significantly higher between 1981 and 2007 than between 1950 and 1981. The loss of bare sand did not lead to an increase of pioneer vegetation, which remained more or less stable; it led, however, to an increase of heath and forest. In active drift sands, pioneer vegetation was characterized by Corynephorus canescens and Polytrichum piliferum, while lichens, lichen-rich grassland and Campylopus introflexus were more common in stabilized drift sands. In regions with high N deposition, the proportion of Campylopus introflexus to older pioneer stages was significantly higher, and the contribution of forest to the ‘heath/forest’ vegetation class was also higher. The calculated life span of bare sand decreased in all sites. Extrapolation of the results suggests that in the absence of restoration measures, bare sand will vanish by 2035. Conclusions In both active and more stabilized drift sands, a generic loss of bare sand and an increase in forest area was found. The increase was higher in regions with high N deposition. Conservation management should include the annual transformation of 43 ha of forest into bare sand in order to stop the loss of bare sand.