Springer 2005
Plant Ecology (2006) 185:19 –28 DOI 10.1007/s11258-005-9080-5
Plant responses to rising water tables and nutrient management in calcareous dune slacks C. Bakker1,2,*, P.M. Van Bodegom1, H.J.M. Nelissen1, W.H.O. Ernst1 and R. Aerts1 1
Institute of Ecological Science, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV, Amsterdam, The Netherlands; 2Stichting Het Utrechts Landschap, Postbus 121, 3730 AC De Bilt, The Netherlands; *Author for correspondence (e-mail:
[email protected]; phone: +31-030-2205555) Received 21 June 2005; accepted in revised form 14 November 2005
Key words: Anoxicity, Groundwater, Nutrients, Phytometer, Rewetting, Survival
Abstract Plant species of oligotrophic wet dune slacks have dramatically decreased as a result of desiccation and eutrophication. The aim of this study was to test in a field experiment the effects of restoration management in oligotrophic, wet dune slacks (groundwater level rise in combination with topsoil removal or mowing) on abiotic variables and on survival and biomass of four plant species. The effect of groundwater level rise on abiotic variables strongly differed between mown sampling locations and those with topsoil removal. At locations with a mowing treatment, a large rise in water tables led to increased N availability and higher reduced iron concentrations than at other locations. Such effects were absent at locations with recent topsoil removal. No effect of groundwater level rise on P-availability was found. Topsoil removal on average lowered N availability by 13%, P availability by 65% and Fe2+ by 56%. All phytometer species survived better in mown dune slacks than in dune slacks that had received topsoil removal. Survival of all species was negatively related to groundwater level rise. On the short term local extinction risks of small populations may be enhanced by rewetting and topsoil removal. On the long-term, however, such measures are crucial to maintain vegetation of oligotrophic wet dune slacks in a degraded dune landscape.
Introduction Plants of oligotrophic wet dune slack vegetation have become increasingly scarce in the course of the 20th century (Grootjans et al. 1998). This is due to a lowering of groundwater levels, that has reduced the area of wet dune slack vegetation (Runhaar et al. 1996), and increased nitrogen deposition that accelerated the succession from oligotrophic pioneer vegetation to more mesotrophic vegetation types (Doing 1988). Recent restoration efforts have tried to counteract these trends by elevating groundwater tables and by
topsoil removal and/or mowing, with the aim of promoting those rare plant species that depend on oligotrophic, wet and scarcely shaded conditions. Although the problem of desiccation is widespread in wetlands (Wheeler 1995), the experience with raising water tables as a restoration tool is limited, especially on mineral soils. The best-documented example of rewetting on mineral soils is the prairie pothole region in central North America, where a sudden rewetting resulted in a dieback of flooding intolerant vegetation (Galatowitsch and Van der Valk 1996). Restored potholes differed from undisturbed potholes both in species
20 pool and soil characteristics (Galatowitsch and Van der Valk 1996; Seabloom and Van der Valk 2003). For dune slacks, research on hydrological restoration has focused on the restoration of seepage patterns, rather than on the rise in water tables itself (Grootjans et al. 1998). Due to the limited experience with rewetting projects on mineral soils, there is no certainty about the factors that steer survival and growth rates of oligotrophic wet dune plants in response to groundwater level rise. When groundwater levels rise, three interacting effects may influence nutrient availability. Firstly, water availability increases and availability of soluble inorganic phosphate is predicted to increase (Etherington 1982; Jansen et al. 1996). Secondly, a large rise in water tables probably leads to the death of flooding intolerant species and a nutrient pulse may be released from the decomposing dead biomass (Grootjans et al. 2002). Thirdly, if the soil becomes submerged, decomposition may become limited by low oxygen supply and nutrient mineralization may decrease (Van Duren and Pegtel 2000). It is uncertain whether the combination will result in a net increase or decrease of nutrient availability. In addition to these effects of macronutrients, oxygen can become depleted when a soil is submerged for a prolonged period and high amounts of reduced iron and manganese may become available (Ponnamperuma 1972; Al-Farray et al. 1984). Both anoxicity and high concentrations of reduced iron may severely hamper plant growth and survival (Armstrong 1982; Snowden and Wheeler 1995). Topsoil removal reduces total nutrient concentrations in the upper soil layer, but in the phase directly following soil removal nitrogen availability is sometimes higher than before soil removal. Nitrogen that is mineralised may accumulate, if it is not taken up by vegetation (Dorland et al. 2003). Also N mineralization may increase upon topsoil removal because of low C:N ratios in remaining root material (Berendse 1990). The above-mentioned effects of rewetting are mediated by vegetation (die back), and by soil organic matter (mineralization). Therefore, a dune-slack where the topsoil has been removed probably responds very differently to water level rise than a dune-slack with a mowing regime and a closed vegetation canopy.
Since the aim of rewetting in combination with nutrient removal is to stimulate species of oligotrophic, wet conditions, it may be expected that these species indeed profit from such management. It has been shown that plant survival and growth depend on nutrient availability and water table depth in both greenhouse experiments and agricultural studies (Marschner 1995; Lambers et al. 1998) as well as in field studies (Begon et al. 1996; Aerts and Chapin 2000). However, the direction and strength of this relation is highly dependent on a multitude of interacting factors and, as a consequence, a physiological optimum for particular plant species often cannot be detected (Wheeler 1999). The aim of the current study was to estimate the effects of rewetting on (1) abiotic conditions and (2) growth of plants of wet, oligotrophic conditions. To this end, we addressed the following questions: 1. How does a rise in groundwater tables affect availability of inorganic nitrogen, inorganic P and reduced iron? 2. Is survival and growth of species of oligotrophic, wet dune vegetation affected by groundwater level rise in combination with soil removal or mowing? 3. Do different plant species of oligotrophic wet dune slacks have the same response to management? To answer these questions, water levels and concentrations of available nutrients and reduced iron were measured in dune slacks, where a combination of water level rise with either mowing or topsoil removal was applied. A phytometer approach (Wheeler et al. 1992; Gaudet and Keddy 1995; Van Duren and Pegtel 2000; Ko¨hler et al. 2001) was chosen to compare plant survival and growth responses over a range of nutrient- and moisture conditions using four species of wet, oligotrophic conditions.
Method Study area The study was carried out in ‘‘Zuid Kennemerland National Park’’, a 25 km2 calcareous dune area on
21 the west coast of the Netherlands (coordinates 5223¢ –5228¢ N, 010¢ –015¢ E). This area has been utilized for drinking water extraction from 1850 onwards, with an extraction volume of 14 million m3 per year up to 1997 (Kuipers 1999). A large-scale restoration project, with the aim to restore vegetation of wet, oligotrophic conditions encompassed the reduction of groundwater extractions in combination with a management of topsoil removal or mowing. In autumn and winter of 1992 and 1999, the topsoil was removed in several dune slacks, to provide new opportunities for pioneer plants in the area. Grasslands in dune slacks with established vegetation were mown annually in October or November. A groundwater table rise occurred in the period 1997 –2000. This was the result of a reduction in groundwater extractions, in combination with exceptionally high rainfall in 1998 and 1999 (Heijboer and Nellesteijn 2002). The rise in water levels varied strongly over the dune area; dune slacks close to former extraction points had a water level rise of up to 170 cm, while dune slacks at some kilometres distance had a water level rise of only 30 cm. Groundwater tables had annual amplitudes of 30 – 80 cm, with highest water tables in winter and lowest water tables in late summer.
groundwater level. A third point was established at an intermediate elevation. Transects were on average 15 m long, with an average height difference of 60 cm between the highest and lowest point. Due to vandalism, the measurements in one of the sampling locations (with the treatment small groundwater level rise and top soil removal) were lost and for that reason, results of only seven locations can be shown. A total of 63 sampling points thus remained (7 locations3 transects3 sampling positions). We distinguished management variables, abiotic variables and plant responses. Management variables refer to the combination of groundwater level rise and nutrient management. Position on a transect was an additional variable as a measure of current water table to distinguish between changes in water level and actual groundwater levels. Nutrient management is either topsoil removal or a mowing treatment. Abiotic variables are concentrations of available N and P, soil Fe2+ and average groundwater level during summer. Nutrient availability was measured as the available pool and not as mineralization, since not only differences in mineralization between the sampling locations, but also differences in uptake may be important for what is available to the plants.
Set-up of field measurements
Collection of soil data
Eight sampling locations were selected to conform a factorial design: Four locations had received a large groundwater level rise in the period 1997 – 2000 (i.e. more than 75 cm increase in the yearround mean of the groundwater level) and four locations had a small rise. Four locations were in dune slacks where the top 20 –50 cm of the soil was removed in 1999 or 1992, while four other locations had established vegetation and a mowing regime. This resulted in two replicates for each combination of rewetting (large or small rise) and nutrient management (soil removal or mowing). At each location, three transects were established along elevational gradients. The selection of locations and the positioning of sampling points on these transects was based on the predictions of a hydrological model (Kuipers 1999). The lowest transect points were established on positions that would just fall dry in late summer and the highest point would be 10 cm above the highest winter
Piezometers were installed at all sampling points and groundwater level was recorded every 2 weeks. Soil cores of 10 cm deep, 5 cm diameter were collected at four times during the growing season. The first series of samples was taken at 2 May 2000, the last series at 5 and 7 September 2000. Samples were stored at 4 C until analysis. Soil N availability was measured as extractable nitrate and ammonium after an extraction with 1 N KCl during 2 h. The extract was measured on an automated flow analyser (Skalar). Because of the high CaCO3 content of the dune sand, pH was in the range 7 –8.5 at all sampling locations. Soil P availability was measured as inorganic POlsen, which is the preferred method at the pH range that we encountered (Murphy and Riley 1962; Cross and Schlesinger 1995). Fe2+ was measured by a 0.5 N HCl extraction and measured by the ferrozine assay (Lovley and Phillips 1986).
22 Selection of phytometer species Four species that frequently occur in oligotrophic, wet dune slacks were selected as phytometer species: Carex flacca Schreb., Molinia caerulea (L.) Moench, Samolus valerandi L. and Schoenus nigricans L. The first two species could be vegetatively propagated from a source population in the dune slack Oceaan, in the area North-Holland Dune reserve. The latter two species were grown from seeds. Samolus seeds were acquired in the valley Houtglop, National Park South-Kennemerland, S. nigricans nutlets in the above-mentioned valley Oceaan. For each species, individual test plants were selected within a limited size range and randomly assigned to one of the 63 sampling points. Depending on the number of available individual plants of a similar size, three to five individuals were planted per sampling point and per species. Dead plants were replaced two weeks after planting, with no subsequent replacement. Plants were grown for three months and harvested between 16 and 20 September 2000. Survival and biomass at harvest will be presented as parameter for plant growth for all species.
organic matter were arcsine-square root transformed before testing. Inspection of the residuals in plots of normalized residuals vs. predicted values confirmed that after these transformations homogeneity of variances was satisfactory and the residuals approached a normal distribution. In two tests, the transformation did not resolve problems of heteroscedasticity and non-normality. This was the case for the residuals in the test with survival of S. valerandi tested against management variables and with the residuals of soil Fe2+ at the fourth sampling interval tested against management variables. The data on which these tests were performed were checked, but the possibility of sampling errors could be excluded. ANOVA’s tend to test too conservatively when residuals deviate from the homogeneity assumptions (Neter et al. 1996). We decided to proceed with the testing but to mark these results where they are presented to alert the reader to this problem. All tests were performed with SPSS version 9.0.
Results Impact of management on abiotic variables
Statistical analyses Concentrations of available N, P and Fe2+ as well as the plant responses survival and biomass at harvest were tested as response variables with groundwater level rise and nutrient management as explanatory variables. Effects of management variables on survival and biomass of phytometers were tested with a nested ANOVA: the effect of position was nested within location, since sampling points share all the attributes that are connected to the location. Groundwater level rise (large or small) and nutrient management (mowing or top soil removal) were used as between-subjects effects. Soil nutrients were tested with a repeated measures ANOVA, with the four sampling dates as within subjects effect and groundwater level rise, nutrient management and position as between subjects effect. Also in this test, the effect of position was nested within location. Plant biomass and concentrations of available N, P and Fe2+ were log-transformed prior to testing to attain homogeneity and normality of the residuals. Survival rates and the fraction of soil
Rewetting resulted in conspicuous dying-off zones at mown sampling locations with a large rise in water tables. Also percent carbon was highest in the soil of mown locations with a large groundwater level rise, indicating the incorporation of dead biomass into the soil (ANOVA interaction nutrient managementgroundwater level rise: P= 0.027). The effect of rewetting on N availability depended on nutrient management and time in the season in such a way that highest nitrogen availability was measured at locations with a mowing treatment and a high rise in water tables at the start of the growing season (Table 1, Figure 1). On average, concentrations of available N were 13% lower at locations that had the topsoil removed, compared to locations with a mowing treatment. At high points on transects, N availability was higher at mown locations than at locations with a topsoil removal, but the reverse was found at low sampling points. Rising groundwater tables did not affect P availability (Table 1, Figure 1), but P availability was reduced by 65% by topsoil removal. Fe2+ availability depended on the combined effects of groundwater level rise and nutrient
23 Table 1. ANOVA results for soil nutrient responses in the course of the growing season (Time) to mowing vs. soil removal (Nutrient management), groundwater level rise and position on elevational transects.
Nutrient management Groundwater level rise Time Nutrient managementGroundwater level rise TimeNutrient management TimeGroundwater level rise Position
N
P
Fe2+
0.011 0.213
