Wet meadow restoration in Western Europe: A

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non for fen meadow restoration (Grootjans and Van Diggelen,. 1995; Pfadenhauer and ..... This is in agreement with projects on dry mesotrophic grasslands ...
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Wet meadow restoration in Western Europe: A quantitative assessment of the effectiveness of several techniques Agata Klimkowskaa,b,1, Rudy Van Diggelena,c,*, Jan P. Bakkera, Ab P. Grootjansa a

Community and Conservation Ecology Group, Centre for Ecological and Evolutionary Studies, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands b The Institute for Land Reclamation and Grassland Farming, Falenty, Al. Hrabska 3, 05-090 Raszyn, Poland c University of Antwerp, Department of Biology, Ecosystem Management Research Group, Universiteitsplein 1c, B2610 Antwerpen, Belgium

A R T I C L E I N F O

A B S T R A C T

Article history:

Techniques such as rewetting, topsoil removal, diaspore transfer or combinations of these

Received 10 August 2006

are increasingly applied in fen meadow and flood meadow restoration in Western Europe.

Received in revised form

In this paper, we present a quantitative assessment of the effectiveness of the commonly

25 August 2007

used meadow restoration methods. We use the change in ‘saturation index’ to evaluate

Accepted 28 August 2007

the degree of success. The index reflects the completeness of restored communities in comparison to regional target communities. Meadow restoration has limited success in most cases, with an average increase in species richness below 10% of the regional spe-

Keywords:

cies pool. Restoration success was partly determinated by the starting situation. The

Restoration success

more species-rich the starting situation, the higher the saturation index after restoration

Rewetting

but, at the same time, the smaller the increase in the number of target species due to res-

Top soil removal

toration. Top soil removal and diaspore transfer were found to contribute most to resto-

Diaspore transfer

ration success. A combination of top soil removal and diaspore transfer and a

Saturation index

combination of all three techniques appeared to be the most effective measure and resulted in an increase in the saturation index of up to 16%. Rewetting alone had no measurable effect on restoration success.  2007 Elsevier Ltd. All rights reserved.

1.

Introduction

Between 50% and 90% of wetland ecosystems on organic soils have been lost in Europe (Joosten and Clarke, 2002). Particularly fens were transformed into meadows, pastures or arable land. Recently there is increasing concern to conserve and restore fens and fen meadows for their high biodiversity and characteristic plant species (Tallowin and Smith, 2001). Much knowledge in the field of semi-natural meadow restoration

already exists in Western Europe, but a quantitative assessment of the project outcomes is often missing. The main causes of fen degradation are drainage and associated peat degradation (Wild, 1997; Chapman et al., 2003; Rupp et al., 2004), acidification (Grootjans et al., 2002) and eutrophication (Wassen et al., 1998; Lamers et al., 2002). Rewetting, therefore, is considered a conditio sine qua non for fen meadow restoration (Grootjans and Van Diggelen, 1995; Pfadenhauer and Klo¨tzli, 1996). A second essential

* Corresponding author: Address: Community and Conservation Ecology Group, Centre for Ecological and Evolutionary Studies, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands. Tel.: +31 50 3636133; fax: +31 50 3632273. E-mail addresses: [email protected], [email protected] (A. Klimkowska), [email protected] (R. Van Diggelen), [email protected] (J.P. Bakker), [email protected] (A.P. Grootjans). 1 Tel./fax: +48 22 628 37 63. 0006-3207/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2007.08.024

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condition for fen meadow restoration is a reduction in trophic status. A radical technique to lower nutrient availability is to remove the entire top soil. This measure proved to be quite successful on mineral soils (Verhagen et al., 2001; Tallowin and Smith, 2001; Ho¨lzel and Otte, 2003), but little experience exists for organic soils (but see Pa¨tzelt et al., 2001; Beltman et al., 2001). However, only restoring environmental conditions may not be enough. A sufficient number of viable propagules of target species and appropriate conditions for germination and establishment should be present (Verhagen et al., 2001; Ho¨lzel, 2005). Many authors showed that the soil seed bank is often an insufficient source for re-colonisation (Maas and Schopp-Guth, 1995; McDonald et al., 1996; Jensen, 1998). Seed dispersal is often too limited to compensate for this deficiency (Poschlod and Bonn, 1998; Ve´crin et al., 2002; Middleton et al., 2006). Seed dispersal of fen or flooded meadow plants, measured in the field was proven to be very limited (Van Dorp et al., 1996; Bischoff, 2002; Middleton et al., 2006), despite adaptations such as high buoyancy (Van den Broek et al., 2005). Therefore, addition of diaspores by the transfer of hay has been introduced as a technique in meadow restoration (Ho¨lzel and Otte, 2003; Kiehl and Wagner, 2006). Successful restoration was also achieved by other variants of species addition: seeding, transplanting or transferring soil from donor areas (Klo¨tzli, 1987; Bruelheide and Flintrop, 2000; Bank et al., 2002; Ve´crin and Muller, 2003). The present paper aims at a quantitative assessment of restoration efforts in semi-natural wet meadows. We focus on the effects of three, afore mentioned, restoration techniques: rewetting, topsoil removal and diaspore transfer. These techniques are compared with respect to the effect they have on the plant community composition. A systematic, in-depth meta-analysis is not possible due to lack of sufficient data.

1.1.

Research questions

We started the analysis by measuring the degree of resemblance in species composition between restored meadow communities and regional reference vegetation types (a). Next we investigated the relation between degree of change in plant richness and the initial plant community composition (b). Then we analysed the relative effect of the three restoration techniques on the overall restoration success (c). In a limited number of cases we had more detailed information on the depth of top soil removal and type of diaspore material used for transfer. In these cases, we analysed whether removal depth and type of transfer material affect the degree of success (d).

2.

Materials and methods

2.1.

Selection of systems

We focus on three semi-natural meadow types. Mesotrophic fen meadows (MFM, 1) of the Junco–Molinion (or Molinion) community occur in nutrient-poor sites, often after longterm hay making without fertilization. Eutrophic fen mead-

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ows (EFM, 2) (Calthion palustris) occur often on deep peat soils, along fen margins and brooks or after moderate drainage and fertilization of fens. Flood meadows (FM, 3) of the Lolio-Potentillion anserinae, Alopecurion pratensis or Cnidion alliances develop in regularly flooded sites. All three meadow types occur on peat soil (O), on mixed organic–mineral or mineral soils. The latter two soil types were combined into one category (M/O).

2.2.

Data compilation

Unfortunately, there are no electronic databases available with records of wet meadows restoration and we, therefore, had to obtain the data through professional networks and experts (e.g. Society for Ecological Restoration). We used peerreviewed, published sources and records of conservation agencies of projects performed in Western Europe, located on organic or mixed mineral-organic soils. Data on plant species composition from before and after the intervention came from permanent plots or releve´s taken for monitoring purposes. No independent reviewer check on data quality could be performed. We obtained information on project location, meadow type (MFM, EFM, FM), soil type, techniques applied, land use before restoration (meadow or arable field) and species composition of the vegetation. Whenever possible we included information on the period since restoration (age), approximate surface area, top soil removal depth and diaspore material in the analysis. Concerning rewetting, the specifications of the adopted technique such as sources of water, extent of water-level rise or fluctuation patterns are known to affect the outcomes of restoration (e.g. Lucassen et al., 2005). Unfortunately, those details were often missing. As we aimed at a general evaluation, we defined ‘rewetting’ as any type of activity leading to an increase in wetness of a site. Similarly, the factor ‘diaspore transfer’ comprises various activities, all aiming at bringing new species to a site.

2.3.

Comparison method

Various studies differed in their objectives and methodologies, and data differed in quality. Some studies were set up as scientific experiments, whereas others were poorly documented projects. We quantified the effect of the restoration by comparing the local vegetation composition with the regional species pool (Zobel et al., 1998) for a particular meadow type and expressed it as a saturation index (SI). This saturation index concept (Wolters et al., 2005) enables to quantify and compare the success of restoration experiments in different countries and on different sites. SI expresses the proportion of the regional species pool that is actually recorded at a given site and consists of a value between zero and one. The saturation index estimates the ‘completeness’ of the community, but does not give an idea about the ‘nature conservation interest’ of the vegetation. It shows the similarity of the recovering community to the typical community of a particular meadow type for a given region. We used presence-absence data to calculate this index.

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Table 1 – Reference vegetation types per meadow type and geographic region (Schamine´e et al., 1996; Rodwell, 1992; Oberdorfer, 1977) Meadow type

The Netherlands (used also for Eastern France)

Germany (used also for Switzerland)

Mespotrophic fen meadow

Junco–Molinion alliance 47

Juncion acutiflori, Molinion alliances 81

Eutrophic fen meadow

Calthion alliance 41

Calthion palustris alliance 47

Flood meadow

Lolio-Potentillion anserinae, Alopecurion pratensis alliances 50

Cnidion alliance 36 Agropyro– Rumicion alliance 15

UK (used also for West Norway) Juncus subnodulosus–Cirsium palustre M22, Juncus effusus/acutiflorus–Galium palustre M23, Molinia caerulea – Cirsium dissectum M24, Molinia caerulea – Potentilla erecta M25 71 Cynosurus cristatus–Caltha palustris MG8, Holcus lanatus–Deschampsia cespitosa MG9, Holco-Juncetum effusi MG10, Filipendula ulmaria–Angelica sylvestris M27, Iris pseudacorus–Filipendula ulmaria M28 80 Alopecurus pratensis–Sanguisorba officinalis MG4, Festuca rubra–Agrostis stolonifera–Potentilla anserina MG11, Agrostis stolonifera–Alopecurus geniculatus MG13 56

The number of species included in the regional species pool is given in bold.

We selected three geographical regions (The Netherlands, Germany and the United Kingdom), differing in the number and type of species, and three meadow types. The regional species pools of the three types of wet meadows were derived from frequency tables in the literature. For The Netherlands we used Schamine´e et al. (1996), for Germany Oberdorfer (1977) and for the United Kingdom Rodwell (1992) (Table 1). Species were included in the species pool if they occurred with a frequency exceeding 20% in the description of a particular vegetation type. We refer to these species as ‘target species’. We used changes in saturation index before and after restoration (DSI) as an indicator of restoration success: DSI = SIafter restoration SIstart situation, expressed as absolute value. We used data from before the restoration or data from control plots to calculate DSI. When we could not obtain data on the initial community composition, we made the following assumptions. If the restoration was conducted on former arable land we assumed that the starting situation SI = 0. If the restoration was conducted on agriculturally improved grassland we estimated the starting SI on the basis of species lists of agricultural grasslands in the region. One study from three sites in The Netherlands (Kolham, Dannemeer, Woudbloem), showed that there were no significant changes in the total number of species (t = 1.32, p = 0.2; n = 12) in the starting situation and after 3 years in the control plots (no treatment applied). The overlap in species composition was above 60%. The appearance of one or two species per sample from the regional species pool in the control treatment contributed little to the saturation index. We, therefore, assumed that changes in the community composition are also in other areas the result of the applied restoration measures, especially when we consider a short period of time.

2.4.

Data analysis

Prior to analysis, the indices (SI after restoration and DSI) were transformed with the function: SItransf. = ln(SI + 1) to improve the normality of the distribution. If for a given location and restoration technique plots were replicated, we averaged

individual SI’s and used one value. The number of observations with different restoration techniques applied was in some cases somewhat unbalanced (Table 2). Most of the data originated from The Netherlands, while studies from other countries were under-represented. In our data some combination of techniques might be more often used than others, thus we checked for strong inter-correlation between techniques, using variance inflation factors. We assumed that the data originating from scientific studies with replicates and registered start situations or controls are of better quality than other data. Due to strong dichotomies in the results of the analysis of both sub-sets (revealed in the sensitivity analysis, not presented here), we used only better quality data for further analysis. We used a Student t-test to investigate the questions (a) and (d). For question (b) we used regression analysis. We performed a main-effect ANOVA, to determine the factors influencing restoration success (c). Prioi to the analysis we tested whether the criteria of normality and of homogeneity of variance were met. Due to the limited number of data we could not test for any interaction effects between the different restoration techniques and with meadow type. Our main intention in the analysis was to evaluate the effectiveness of different restoration techniques, including their com-

Table 2 – Number of observations per restoration technique Rewetting No rewetting No rewetting Total Rewetting Rewetting Total Total

Top soil removal

No diaspore transfer

Diaspore transfer

Total

No Yes

(92) 10 10 16 23 39

6 13 19 7 17 24

6 23 29 23 40 63

49

43

92

No Yes

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binations. Therefore, we re-arranged the data, grouped them according to the restoration techniques applied and tested the effect of combined restoration measures with the Kruskal–Wallis non-parametric test. For preliminary data exploration we used histograms. All analyses were performed with the Statistica 7 package for Windows.

3.

Results

3.1.

Data overview

Data from 36 sites were collected, resulting in 119 individual observations (Appendix A). Due to the lack of reliable records on the initial situation in some observations we could only use 92 observations for further analyses. The majority of the observations came from The Netherlands, usually 3–5 years after restoration and from smallscale (