oxygenation, but may lead to low availability of water. However, typical dune
slack species are adapted to both anaerobic conditions and temporary desiccation.
For instance, some early successional stage species such as Littorella uniflora and
Schoenus nigricans are capable of radial oxygen loss (ROL), a mechanism of
pumping oxygen into the soil. This oxygen pumping helps them to neutralize
anoxic conditions around their roots, thus eliminating toxic concentration of
reduced sulphide, iron, and manganese (Adema et al. 2005). Early-stage dune
slack species have developed adaptations such as a very low nutrient demand (L.
uniflora, Centaurium pulchellum, and Radiola linoides) or the ability to retain and
recycle nutrients within their living and senescing tissues (S. nigricans). Later
successional species such as Carex nigra and Salix repens are not capable of ROL.
They can only win the competition with Littorella or S. nigricans when the
nutrient availability increases. But Littorella, in particular is very successful at
avoiding rapid accumulation of organic matter. Because of its low litter production
and high mineralization conditions (due to ROL) this pioneer vegetation can
dominate the vegetation for a long time. Adema et al. (2002) showed a case on the
island of Texel in which Littorella was able to dominate the vegetation for more
than 80 years.
The hydrology of dune slacks is dynamic and its effect on dune slack vegetation is diverse. Its effect depends on the nature of the soil and the source of
water: groundwater, surface water or precipitation (Zunzunegui et al. 1998;
Munoz-Reinoso 2001; Clarke and Sanitwong Na Ayuthaya 2010). Dune slack
soils are usually calcareous because they often originated from recently deposited sand that is rich in shell materials. When dune slacks are formed on a beach
plain that has a low initial lime (\0.3 %) content, they are acidic. When dune
slacks are fed by groundwater that comes from deep layers, the calcium-rich
water buffers the acidity of the soil, which keeps the accumulation of organic
matter at a low level. However, in the past few decades, this has increased
considerably in many dune slacks, owing to high atmospheric deposition, but
also the lowering of groundwater levels in the surroundings of the slacks.
Groundwater tables may have further dropped because of increased growth of
vegetation in the whole dune area. Consequently, less groundwater is available
for dune slacks, leading to a positive feedback loop, i.e., further accelerating
succession in the dune slacks.
15.2 Approaches to Dune and Dune Slack Restoration
15.2.1 Mowing and Grazing
Mowing is a traditional management technique in dune slacks, preventing grasses,
willows, and tree species to dominate the vegetation, but it does not prevent
acidification when hydrological conditions are suboptimal. In calcareous dune
15 Restoration of Dune Vegetation
241
slack species are adapted to both anaerobic conditions and temporary desiccation.
For instance, some early successional stage species such as Littorella uniflora and
Schoenus nigricans are capable of radial oxygen loss (ROL), a mechanism of
pumping oxygen into the soil. This oxygen pumping helps them to neutralize
anoxic conditions around their roots, thus eliminating toxic concentration of
reduced sulphide, iron, and manganese (Adema et al. 2005). Early-stage dune
slack species have developed adaptations such as a very low nutrient demand (L.
uniflora, Centaurium pulchellum, and Radiola linoides) or the ability to retain and
recycle nutrients within their living and senescing tissues (S. nigricans). Later
successional species such as Carex nigra and Salix repens are not capable of ROL.
They can only win the competition with Littorella or S. nigricans when the
nutrient availability increases. But Littorella, in particular is very successful at
avoiding rapid accumulation of organic matter. Because of its low litter production
and high mineralization conditions (due to ROL) this pioneer vegetation can
dominate the vegetation for a long time. Adema et al. (2002) showed a case on the
island of Texel in which Littorella was able to dominate the vegetation for more
than 80 years.
The hydrology of dune slacks is dynamic and its effect on dune slack vegetation is diverse. Its effect depends on the nature of the soil and the source of
water: groundwater, surface water or precipitation (Zunzunegui et al. 1998;
Munoz-Reinoso 2001; Clarke and Sanitwong Na Ayuthaya 2010). Dune slack
soils are usually calcareous because they often originated from recently deposited sand that is rich in shell materials. When dune slacks are formed on a beach
plain that has a low initial lime (\0.3 %) content, they are acidic. When dune
slacks are fed by groundwater that comes from deep layers, the calcium-rich
water buffers the acidity of the soil, which keeps the accumulation of organic
matter at a low level. However, in the past few decades, this has increased
considerably in many dune slacks, owing to high atmospheric deposition, but
also the lowering of groundwater levels in the surroundings of the slacks.
Groundwater tables may have further dropped because of increased growth of
vegetation in the whole dune area. Consequently, less groundwater is available
for dune slacks, leading to a positive feedback loop, i.e., further accelerating
succession in the dune slacks.
15.2 Approaches to Dune and Dune Slack Restoration
15.2.1 Mowing and Grazing
Mowing is a traditional management technique in dune slacks, preventing grasses,
willows, and tree species to dominate the vegetation, but it does not prevent
acidification when hydrological conditions are suboptimal. In calcareous dune
15 Restoration of Dune Vegetation
241