Plant responses to rising water tables and nutrient management in calcareous dune slacks

Published source details Bakker C., van Bodegom P.M., Nelissen H.J.M., Ernst W.H.O. & Aerts R. (2006) Plant responses to rising water tables and nutrient management in calcareous dune slacks. Plant Ecology, 185, 19-28.
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Summary

In the Netherlands and elsewhere in Western Europe, plants of oligotrophic (low-nutrient) wet dune slacks have dramatically decreased as a result of desiccation and eutrophication. On the west coast of the Netherlands, a large-scale restoration project was initiated with an aim being to restore dune slack vegetation through a combination of reduction of groundwater extraction (leading to recovery of a high water table) in combination with topsoil removal or mowing management. This study focussed on the effects of this management on abiotic factors (i.e. concentrations of available N and P, soil Fe²⁺, and average groundwater level during summer) on survival and biomass production of four dune slack plant species.

Study area: The study was carried out in Zuid Kennemerland National Park, a 25 kmÂ² calcareous dune area (52º23'- 52º28' N, 0º10'- 0º15' E) on the west coast of the Netherlands. The area had been utilized for drinking water extraction for over 130 years (14 million m³/year up to 1997). The groundwater table rose from 1997-2000 as water extraction reduced, in combination with exceptionally high rainfall in 1998 and 1999. Groundwater tables had annual amplitudes of 30 - 80 cm (highest in winter and lowest in late summer).

Restoration management and experimental sites: Eight sampling locations were selected: four with a large groundwater level rise (i.e. >75 cm increase in the year round average ><1997-2000) and four with a small rise; two of each set of four were in slacks where the top 20 -50 cm of the soil was removed in 1999 or 1992; and the other two where vegetation in old dune slacks was mown annually (in October or November).

At each location, three transects (average 15 m long) were established along elevational gradients (average height difference 60 cm between the highest and lowest point). The lowest transect point was positioned where it was calculated that it would just fall dry in late summer, and the highest point 10 cm above the highest winter groundwater level. A third point was established at an intermediate elevation.

Soil data: Piezometers were installed at sampling points and groundwater level was recorded every 2 weeks. Soil cores (10 cm deep, 5 cm diameter) were collected four times during the growing season in 2000 (the first series on 2 May, the last on 5-7 September). Samples were analysed for soil N availability, P availability and Fe²⁺.

Plant study species: Four plants that frequently occur in Dutch oligotrophic, wet dune slacks were selected as 'phytometer' species: Carex flacca, Molinia caerulea, Samolus valerandi and Schoenus nigricans. The first two species were vegetatively propagated from material from a nearby source population; the latter two were grown from seed. For each species, test plants (2-5 individuals planted/sampling point/species) were selected and planted in mid-June at each transect sampling point. Ant dead plants were replaced two weeks after planting, but not thereafter. After three months plants were harvested (between 16 and 20 September 2000). Survival and biomass were recorded.

Effect of groundwater level rise and management on nutrient availabitiy: At mown sites with a large rise in groundwater level, N availability increased and there were higher reduced iron concentrations than at the other sites (i.e. mown with a small groundwater level rise; topsoil removal sites with small or large groundwater level rises). No effect of groundwater level rise on P-availability was apparent. Topsoil removal on average lowered N availability by 13%, P availability by 65% and Fe²⁺ by 56%.

Plant responses: All four species survived better in mown dune slacks than in those with topsoil removed, and their survival was lower at sites with a large groundwater level rise. Survival of S.nigricans and S.valerandi was lowest at sites with a large groundwater level rise with soil removed; their survival was also lowest at the low sampling points in locations with a large groundwater level rise. Biomass of C.flacca was conspicuously higher at mid-elevation sampling points with soil removed and a large rise in groundwater level. Biomass of M.caerulea was unrelated to any of the management parameters.

Conclusions: The authors considered that in the short term, losses of small populations of dune slack plants may be exacerbated by rewetting and topsoil removal. However, reinstatement of higher, more natural groundwater levels along with reduction of nutrient levels are a prerequisite to re-establishment and maintenance of vegetation characteristic of oligotrophic wet dune slacks in this degraded dune system.

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