Individual study: Restoration of degraded, acidified wet heathland and moorland pools by catchment-scale liming in north and central Netherlands
Dorland E., van den Berg L.J.L., Brouwer E., Roelofs J.G.M. & Bobbink R. (2005) Catchment liming to restore degraded, acidified heathlands and moorland pools. Restoration Ecology, 13, 302-311
During the last century in Western Europe there has been a sharp decrease in the extent of wet heathland with a decline in associated fauna and flora. This has come about due to a combination of factors including drainage and conversion to agricultural land, acidification, eutrophication and urban development. A large proportion of remaining wet heathland in Europe is located in the Netherlands. Here, in areas of degraded heathland numerous restoration attempts have been unsuccessful, particularly in attempts to raise depressed soil pH and re-establish uncommon acid-sensitive plant species.
Two main heathland restoration measures are currently employed in the Netherlands. The first involves the removal of organic matter by sod cutting. This process efficiently removes excess nutrients that have bulit up through lack of management. The second is hydrological restoration, achieved through, for example, blocking drainage ditches to raise groundwater levels. In order to increase the effectiveness of these measures, a procedure to raise soil pH is also required.
Lime addition is a popular method for combating acidification and counteracting biodiversity loss in acidified systems. Although lakes and rivers have been limed directly, the most impressive results have come from catchment-scale liming. A study was undertaken in northern and central Netherlands to determine the effectiveness of catchment liming following sod cutting in the restoration of acidified wet heathlands and pools.
Study sites: Two acidified and eutrophicated heathland sites, both located within nature reserves, were targeted for experimental liming. Site 1 was in Schaopedobbe (98 ha), a reserve in north Netherlands. Site 2 was in the Bieze, a reserve in central Netherlands.
These sites support areas of dry, moist and wet heathland vegetation, with aquatic vegetation present in heathland pools.
Elevation and hydrology varied within sites. Both sites had been sod cut in 1990 (i.e. seven years prior to liming).
Liming: Catchment liming was carried out in November and December 1997 at Sites 1 and 2, respectively. Liming took place on both high and mid-elevation areas at Site 2 but was restricted to high elevation areas at Site 1. Dolokal (an agricultural lime; Ankersmit Maalbedrÿven, Maastricht, the Netherlands) was applied at a rate of 2.4-6.3 tons/ha.
Two transects were established at both sites along a height gradient, each containing three terrestrial sampling points (High, Middle and Low elevation) and one aquatic sampling point in pools.
Soil sampling: Four soil samples (upper 10 cm, 2.5 cm diameter auger) were taken at each of the terrestrial sampling points prior to liming, directly following liming, and annually from 2000-2003. These four samples were pooled within sampling point, and pH and ion content was determined.
Water sampling: Water samples were collected at the aquatic sampling points annually from 1994 during spring and summer, and pH was determined.
Vegetation sampling: Vegetation was assessed in two plots (each 1.5 m²) either side of the terrestrial sampling points in the summers of 2000-2003. Number and percentage cover of all plant species was determined visually using the Braun-Blanquet approach. Plant species were divided into Red List (i.e. nationally endangered) target species, species characteristic of heathlands and non-target species. The distribution of soft-water macrophytes in the pools was also measured before the 1994 and after the 2003 liming.
Abiotic responses: Soil pH increased following catchment liming. However, the magnitude of this rise varied spatially and temporally both within and between sites. For example, at Site 1 the average soil pH at mid and low elevation areas rose from pH 5.0 to 6.0 (i.e. reaching a level characteristic of wet heathland) between 1998 and 2003. The lime was transported downslope by water flow from the higher altitude limed areas to the lower lying areas, hence the increase in pH in the low-lying, unlimed areas. Soil pH remained higher than pre-liming levels six years after liming. At Site 2, long-term positive soil pH trends also occurred but only at higher elevation.
Pool water pH increased following liming. Mean pH after liming (1998-2002), was significantly higher than that before liming (1994-1997) at both sites (ranges: 1994-1997 pH 4.2-4.6; 1998-2002 pH 4.3-5.6 - estimated from figure in original paper).
The most biologically important base cations (Ca2+, Mg²+ and K+) increased in concentration significantly following liming. Al³+ concentrations decreased at both sites, but was only statistically significant at Site 2.
NH4+ concentrations increased at both sites, reaching levels at Site 1 that may be toxic to some target plant species (750 µmol/kg dry soil). This enhanced mineralization could have been due to a reaccumulation of organic matter during the seven-year period between sod cutting and liming. Therefore, elevation of NH4+ levels could possibly be avoided by liming immediately after sod cutting.
Biotic responses: Catchment liming generally provided conditions suitable for target plant species. However, the response of terrestrial Red List and characteristic wet-heathland vegetation was limited (possibly due to elevated NH4+ concentrations or poor dispersal ability). Small positive trends were detected in the numbers of these species but no significant correlations between number of target species and years since liming were detected. A number of Red List species returned or increased in abundance, including: marsh gentian Gentiana pneumonanthe, marsh clubmoss Lycopodiella inundata, white beak-sedge Rhynchospora alba, brown beak-sedge Rhynchospora fusca, oblong-leaved sundew Drosera intermedia and round-leaved sundew Drosera rotundifolia.
In contrast to terrestrial plants, the distribution of macrophytes in the pools changed markedly. Floating club-rush Eleogiton fluitans, bog pondweed Potamogeton polygonifolius and water crowfoot Ranunculus ololeucos at Site 2 increased in distribution considerably. In addition, the number of pools of which these species covered more than 25% increased from four in 1994 to 14 in 2003. Floating water-plantain Luronium natans increased approximately 10-fold at Site 1. These four species feature in the Dutch Red List and are also internationally endangered. Some non-target, acid-tolerant species: bulbous rush Juncus bulbosus and several Sphagnum moss species, present before liming in pools at Site 2, disappeared following liming.
Conclusions: Catchment liming constituted a fairly successful management tool in, at least partial, restoration of these two acidified heathlands. Abiotic conditions were restored, including the non-limed lower-elevation areas, so that there was a noticeable increase in red-listed target plant species. Aquatic plants in the pools responded very well in reponse to less acidic conditions. On going monitoring showed that the positive effects from the liming were moderate to long term, being noticable for at least six years following liming.
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