Plant diversity in a water-meadow landscape: the role ...

Plant Ecol DOI 10.1007/s11258-017-0744-8

Plant diversity in a water-meadow landscape: the role of irrigation ditches Melanie Meier . Rebekka Gerlach . Jens Schirmel . Constanze Buhk

Received: 14 October 2016 / Accepted: 21 June 2017 Ó Springer Science+Business Media B.V. 2017

Abstract Species-rich, extensively managed grasslands are among the most threatened ecosystems in Europe. The traditional technique of meadow irrigation, formerly practised in large parts of Europe, may be a viable management option that meets both ecological and economical objectives. While studies on plant diversity of the meadow matrix exist, the role of other landscape components for biodiversity conservation is unknown. We quantify the role of irrigation and drainage ditches compared with field paths, and analysed the local and landscape plant species composition and diversity in a water-meadow

Communicated by Dr. Anne Bonis.

Electronic supplementary material The online version of this article (doi:10.1007/s11258-017-0744-8) contains supplementary material, which is available to authorized users. M. Meier  R. Gerlach  C. Buhk (&) Geoecology/Physical Geography, Institute of Environmental Sciences, University of Koblenz-Landau, Fortstr. 7, 76829 Landau, Germany e-mail: [email protected] R. Gerlach Environmental Economics, Institute of Environmental Sciences, University of Koblenz-Landau, Fortstr. 7, 76829 Landau, Germany

landscape. The meadows are located in the river Queich catchment (Upper Rhine valley, Germany). We recorded plant species cover on 18 meadows, adjacent ditches and field paths and analysed alpha, beta and gamma diversity within and among these landscape components. Irrigation and drainage ditches were more species rich than meadows and more heterogeneous in their species composition compared with meadows and paths. Ditches contributed onethird of the landscape’s plant species numbers, habitat for wetland species and refuges for high nature value grassland species. Fertilization of the adjacent meadow had no negative impact on the ditch diversity, while well-maintained ditches were more plant species rich than ditches filled with sediments. Traditional flood meadow irrigation should be taken into account when sustainable grassland management techniques are discussed that meet economical and ecological needs. A major contribution to landscape richness arises from the ditch structures as crucial part of the traditional irrigation technique. Keywords Alpha diversity  Beta diversity  Ditch management  Edge structure  Grassland conservation  Landscape diversity  Traditional flood irrigation

J. Schirmel Ecosystem Analyses, Institute of Environmental Sciences, University of Koblenz-Landau, Fortstr. 7, 76829 Landau, Germany

123

Plant Ecol

Introduction The intensification of agricultural practices threatens species-rich grasslands all over Central Europe (Blackstock et al. 1999; Wesche et al. 2012). Much grassland has been either abandoned or transformed into agricultural fields (Isselstein et al. 2005). On remaining grasslands, the increasing use of mineral fertilizer favours the dominance of few highly competitive species, reducing plant species richness and potentially resulting in a loss of ecosystem functionality (Zechmeister et al. 2003; Isselstein et al. 2005; Tscharntke et al. 2005; Isbell et al. 2011, 2015). European legislation calls for action to stop further biodiversity decline (European Commission 2011). To mitigate the trend of biodiversity loss and biotic homogenization due to agricultural intensification (Wesche et al. 2012), conservationists seek for new or improved old techniques to reconcile ecologically sustainable land use while maintaining a reasonable income for farmers (Tscharntke et al. 2012). One possibility might be the traditional irrigation of meadows via a wide network of irrigation and drainage ditches, a practice formerly widely used in Europe (see detailed description and a scheme of such irrigation systems in Leibundgut and Kohn 2014a). The traditional lowland irrigation system is based on damming a river two to three times a year. Sluices help to transport the water via a principal irrigation ditch to smaller ditches with smaller intermediate sluices (Leibundgut and Kohn 2014a). If the water table is high enough, the water diffuses to the adjacent meadows and is drained at the other side with drainage ditches returning the water to the river. If the first part of the meadows is well soaked with water, the interim sluice is opened and the next part of the meadow area is reached by the irrigation water. Accordingly, the water penetrates the soil slowly reaching the surface depending on the micro-relief. There is usually no surface flow of water. After two to three days, the primary sluice in the main stream is opened again and the water table reduces slowly on the meadows and in the ditches. Traditional meadow irrigation can fulfil several important functions such as soil moistening, fertilization or rodent control, and secure high yields of hay especially in dry years (Leibundgut and Kohn 2014a). Biodiversity on traditionally managed water-meadows as compared to non-irrigated meadows has recently

123

been studied (Schirmel et al. 2014; Mu¨ller et al. 2016a,b). However, these studies focused on the meadows themselves and did not include the characteristic ditches or other landscape components. Especially in intensely managed landscape matrices, landscape components such as edge structures are important for biodiversity conservation (Fahrig et al. 2011; Kleijn et al. 2011). Edge structures are generally characterized by lower disturbance than adjacent managed habitats and may therefore serve as refuge habitats (Kark and van Rensburg 2006; Herzon and Helenius 2008). Characteristic landscape components in traditionally managed water-meadow landscapes are not only irrigation and drainage ditches but also field paths. Here, we extend the previously published analysis of within vegetation composition and diversity on irrigated versus non-irrigated meadows (Mu¨ller et al. 2016a, b) to other landscape components to quantify the effect of traditional meadow irrigation on plant species diversity at the landscape scale. Further, we study the influence of meadow fertilization on the adjacent landscape component (ditch or field path), as fertilization is known to influence plant species diversity (Manhoudt et al. 2007). Vegetation compositions in edge structures may differ considerably from those of the landscape matrix: Field paths are characterized by disturbance processes such as trampling, which shifts species community composition towards ruderal plants (Roovers et al. 2004). Ditches serve as irrigation and drainage channels and are characterized by temporal water flow. Because conditions within a ditch alternate between wet and dry as well as nutrient-rich and nutrient-poor over time and in space, different microhabitats develop and lead to a patchwork of vegetation (Konold 2007; Karim and Mallik 2008). Over time, ditches accumulate sediments and therefore have to be maintained regularly to secure their hydraulic functions, which causes regular disturbances (e.g. Sabbatini et al. 1998; Manhoudt et al. 2007). Ditch profiles can be classified according to their morphology (e.g. box-shaped, trapezoid-shaped), and may have distinct plant communities (Seiffert et al. 1994; Kramer 2011). Further, vegetation composition in irrigation ditches might differ from that in drainage ditches. To our best knowledge, no studies exist that compare site conditions and vegetation in irrigation versus drainage ditches. We expect irrigation ditches to be richer in nutrients than drainage ditches, as the water

Plant Ecol

discharged from the stream to the irrigation ditch may transport organic material into the ditch that builds up sediments when the water drains. We hypothesize that (i) ditches have a higher alpha diversity of plants than adjacent meadows and field paths, (ii) alpha diversity in ditches is influenced by ditch type, their role as irrigation versus drainage ditch and the fertilization level of the adjacent meadow and (iii) ditches are highly beta diverse and contain a distinct species composition compared to adjacent meadows and field paths, largely contributing to the landscape diversity (gamma diversity). Based on our findings, we discuss the options for reintroducing water-meadow irrigation systems to counteract the loss of (high nature value) grasslands. Finally, we propose ditch maintenance methods that seem best suited to rise biodiversity and the value of species composition.

Methods Study area and site selection The study area is located in the lowland of the river Queich between Landau and Zeiskam (Germany, Rhineland-Palatinate; 49°110 52.900 N, 8°70 34.200 E) within a NATURA 2000 conservation area. Mean annual temperature is 10.5 °C (Geiger 2008), and the average annual precipitation is 667 mm (station Landau, German Weather Service). Soils are sandy to loamy (Geiger 2008). The practice of irrigation for the purpose of hay making can be dated back to the Middle Ages and follows the lowland irrigation type with about three inundation periods per year in spring and summer (Keller 2013; Leibundgut and Kohn 2014a; Mu¨ller et al. 2016a). The studied meadows are used extensively to semi-intensively (Dierschke and Briemle 2002) with one to two cuts per year and an annual mineral fertilizer application of up to about 80 kg N/ha for hay and silage production (personal communication with the farmers). We selected 18 meadows including their adjacent edge structures (irrigation or drainage ditch, field path) distributed across the whole study area (Fig. 1). Half of the meadows were fertilized (50–78 kg N/ha), while the others were not. Not all meadows were surrounded by both a ditch and a field path. Accordingly, some meadows are only compared to either their

adjacent irrigation or drainage ditch(es) or a field path, while other meadows could be studied together with both ditch types and a field path (see Fig. 1 indicating the position of meadows with ditches and or field paths and Table S1 with additional information on the studied sites and replicates). Vegetation survey The vegetation survey was carried out in spring 2014 prior to mowing. On the meadows, vegetation was sampled on three randomly chosen 2 9 2 m2 plots. Vegetation in the adjacent ditches and field paths was sampled on four plots each with a plot size of 4 m2. Plot shape was adapted to the width of the ditch or path: The plots in the ditches or field paths reached from the middle of the structure (either lowest point in a ditch or central green line on a field path) up to 50 cm into the adjacent meadow. To reach a plot size of 4 m2, the length of the plot was adjusted depending on the width of the structure (see small graphic in Fig. 1), creating rectangular plots. The widths of the plots varied between 2 and 0.5 m—the length accordingly between 2 and 8 m (Table S3). As the main environmental gradients in ditches and on paths are to be expected across the structures (horizontally) and not along the structure (vertically), we believe that the effects of increased plot length along the direction of the structure did not play a relevant role for species turnover and results are therefore comparable. In contrast to the other ditches and field paths with four plots per meadow, one irrigation ditch was only studied in two plots due to limited space. This resulted in a total of 160 plots on 18 meadows, eleven irrigation ditches, eight drainage ditches and eight field paths (Table S1). In each plot, higher plant species cover was estimated using the Londo scale (Londo 1976). For species determination, Eggenberg and Mo¨hl (2009) and Seybold (2009) were used. The nomenclature followed Ja¨ger (2011). Morphology and management of ditches For each plot in a ditch, a cross section was outlined according to the schemes listed by Seiffert et al. (1994) to characterize the ditch type. We found the following ditch types in our study area: Well-maintained ditches with a clear form, namely box ditches [box], trapezoid

123

Plant Ecol

Fig. 1 Study area and the studied meadows (including their ID). Symbols show the different constellation of samples: studied meadows with an adjacent ditch and an adjacent field path, meadows with an adjacent ditch only (or two ditches: irrigation and drainage) and meadows with an adjacent field path only. Meadows A and B are located in close proximity, so the symbols overlap strongly. In the separate window on the left, the study design of a meadow with ditch and field path is shown

schematically. While the three meadow plots are quadratic (2 9 2 = 4 m2), the plots along field paths and ditches are rectangular. The plot width covers the distance from the centre of the ditch or field path up to 50 cm into the meadow. The length of the plot is adapted according to the width to reach a size of 4 m2. A summary of the meadows with further information is found in Supplement Table S1, the length of the plots is documented in Table S3

ditches [tr.clea] and stages ditches [stages]; an intermediate type of less-maintained trapezoid ditches slightly filled with sediments [tr.fil] and ditches of undeterminable form strongly filled with sediments [filled-up] due to a long time interval since the last maintenance (Fig. 2). Filled-up ditches were not abandoned but resulted from lower maintenance frequencies. The municipalities in charge of the maintenance of the ditches dredge (excavate) the sediments ‘‘according to necessity’’—which is in large ditches every year (personal communication with farmers and staff responsible for the ditch maintenance), but varies strongly. Regular ditch management also includes mowing (usually once or twice a year) and/or mulching. The mown material is either left at

the rims of the ditches or removed (see maintenance information of municipalities in the Supplementary data, Table S2 and information on the location of the municipalities in Fig. 1).

123

Data analysis Analyses were performed using R statistics 3.2.1 (R Development Core Team 2015), including the packages ‘‘nlme’’ (Pinheiro et al. 2012) and ‘‘vegan’’ (Oksanen et al. 2011) with a = 0.05 for all statistical tests. Model assumptions for the mixed effect models (normality of the residuals and homogeneity of variances) were checked visually using diagnostic plots (Zuur et al. 2010).

Plant Ecol

Fig. 2 Proportion of recorded ditch profile types of the studied ditch plots and their species richness and Simpson diversity. Ditch profile types are ‘‘filled-up’’= filled-up less-maintained ditch, ‘‘tr.fil’’ = trapezoidal ditch, partially filled-up due to a lack of maintenance, ‘‘tr.clea’’ = trapezoidal ditch (well-maintained), ‘‘box’’ = box ditch (well-maintained), ‘‘stages’’ = stages ditch (well-maintained). Information on the maintenance of ditches in the different municipalities is given in the Supplement (Table S2)

Alpha diversity (dependent variables species richness and Simpson diversity in two separated analyses) was related to the landscape component (fixed factor with three levels: meadow, ditch, field path) using linear mixed effects models (LMM; Laird and Ware 1982). The identity of the study site (the ID of the adjacent meadow) was included as a random factor to account for the nested study design thus avoiding pseudoreplication. Comparable analyses were carried out to analyse the effect of meadow fertilization (binary variable: yes/no) on alpha diversity of the adjacent ditches, differences in alpha diversity between irrigation and drainage ditches, the differences in alpha diversity among the different ditch types (box, trapezoid, stages, filled-up trapezoid, filled-up; explanation see above), differences in alpha

diversity between well-maintained ditches versus lessmaintained ditches (ditches with or without accumulation of sediment) and the percentage of nitrophilous species (nitrogen EIV C 6) in different ditches (irrigation versus drainage and maintained versus little maintained). For the latter three comparisons, data were rank-transformed prior to the analyses due to violations of model assumptions. For post hoc comparisons between fixed factors of more than two groups, Tukey HSD was used. Plant species composition was analysed using detrended correspondence analyses (DCA) as the gradient length of the first axis was larger than 4 standard deviations. After the ordination, weighted mean Ellenberg’s indicator values (EIV, Ellenberg et al. 2000) per plot for moisture (F), nitrogen (N), light (L) and soil reaction (R) were correlated with the axes of the DCA of the species composition to visualize environmental gradients in the ordination plot. The fitting was done post hoc with the method envfit in the R package vegan with 999 permutations to calculate p values. EIVs were weighted by multiplying the single species values with their cover before the mean value per plot was calculated. Beta diversity (species dissimilarity) of meadows, ditches and field paths was assessed using the multivariate method ‘‘multivariate homogeneity of dispersions’’ for Bray–Curtis similarity values as found in the package ‘‘vegan’’ (Anderson et al. 2006; Oksanen et al. 2011). Differences between the mean distances to the median of each landscape component were analysed using permutational ANOVA.

Results Plant species richness and Simpson diversity differed significantly among the landscape components when including meadow ID as random factor (Table 1). In a pairwise comparison, Simpson diversity in ditches and field paths was 10–35% higher than in their respective adjacent meadows. Fertilization of the adjacent meadow did not significantly influence plant species richness or Simpson diversity in ditches (Table 1). In irrigation ditches, plant species richness was higher than in drainage ditches. Though the mean difference seems marginal (irrigation 21.36 ± 7.41 versus drainage 21.25 ± 5.23), the difference is significant due to

123

Plant Ecol Table 1 Comparison of plant species richness and Simpson alpha diversity among various fixed factors: landscape components field paths, ditches and meadows; between ditches with and without fertilization of the adjacent meadow; between Explanatory variable

irrigation versus drainage ditches; between well-maintained cleared ditches versus filled-up less-maintained ditches; between different ditch profiles

Species richness

Simpson diversity

Effect size

F

Landscape component

–

3.395

field path–meadow

-0.9

–

ditch–meadow ditch–field path

-2.3 -1.4

– –

P

Effect size

F

P

0.0360*

–

4.742

0.0139*

0.7377

-0.05

–

0.016*

0.0303* 0.3510

-0.02 ?0.03

– –

0.0448* 0.6326 0.4173

Fertilization (yes/no)

-1.04

0.003

0.9549

-0.03

0.668

Irrigation/drainage ditch

-0.11

40.321

\0.001***

?0.02

0.187

0.6674

Cleared/filled-up ditch

-10.29

48.195

\0.001***

-0.11

35.004

\0.001***

Ditch profiles

–

13.856

\0.001***

–

7.457

\0.001***

filled-up–box

?10.5

–

\0.001***

?0.13

–

\0.001***

stages–box

-1.38

–

0.1264

-0.02

–

0.93851

tr.fil–box

?10.34

–

\0.001***

?0.08

–

0.01591*

tr. clea–box

?0.57

–

0.9448

-0.01

–

0.99817

stages–filled-up

-11.88

–

0.0385*

-0.15

–

0.03488*

tr.fil–filled-up

-0.16

–

0.4933

-0.05

–

0.69376

tr.clea–filled-up

-9.93

–

\0.001***

-0.14

–

\0.001***

tr.fil–stages

?11.72

–

\0.001***

?0.1

–

0.27780

tr.clea–stages

?1.95

–

0.7420

?0.01

–

0.99076

tr.clea–tr.fi

-9.77

–

\0.001***

-0.09

–

0.00295**

Meadow–ID was included as random factor to account for the nested design. Tukey HSD post hoc tests were performed where more than two factors were compared. Significant results are marked with asterisks * p \ 0.05, ** p \ 0.01 and *** p \ 0.001. Effect size was calculated as mean difference. Ditch profiles are abbreviated as in Fig. 2 and described in the methods chapter

the inclusion of meadow ID as random factor (Table 1) as it takes into account that some ditches are located pairwise adjacent to the same meadow and are not independent from each other. Further, the ditch type significantly affected plant alpha diversity: Species richness and Simpson diversity in cleared ditches (box, tr.clea, stages) were significantly higher than in filled-up ditches (tr.fil, filled-up): cleared, wellmaintained ditches contained about twice as many species than less-maintained filled-up ditches (Fig. 2; Table 1). Community variation among plots within the same landscape component significantly differed between field paths, ditches and meadows (permutation-based multivariate ANOVA: p = 0.008). Plots within ditches were significantly more heterogeneous compared with plots within field paths and marginally significantly more heterogeneous compared with plots within meadows (Fig. 3; Table 2).

123

The edge structures contributed half of the whole species pool (73 of 146) basing on 142 of the recorded plots (Fig. 4). Only one-third of the species was found in all structures (46 of 146). A similar percentage of species (45 of 146) was exclusively found in ditches (Fig. 4). Field paths and meadows in contrast contributed only ten and six exclusive species, respectively, to the species pool. Species compositions in ditches were fundamentally different from those in meadows and on field paths. The wide range of environmental conditions found in ditches is revealed in the DCA as species compositions in ditches are spread over large parts along both axes. Species compositions in ditches showed higher Ellenberg’s indicator reaction values (Ellenberg’s R correlated with first axis R2 = 0.47, p \ 0.001), higher moisture values (Ellenberg’s F correlates with first axis R2 = 0.19, p \ 0.001) and a trend towards higher nitrogen values (Fig. 5). The

Plant Ecol

Fig. 4 Venn diagram of species numbers of ditches, field paths and meadows and species’ overlap. The number of samples in ditches is restricted to the first three replicates for this analysis to reduce the number of replicates to a comparable number as recorded on meadows (N meadows: 54; N ditches: 56; N field paths: 32) Fig. 3 Plant species beta diversity (multivariate distance to median) within field path plots, within ditch plots and within meadow plots differ significantly (p = 0.008; Table 2) Table 2 Proportion of species indicating nitrogen-rich conditions in irrigation versus drainage ditches and well-maintained cleared ditch profile types versus less-maintained filledup profile types Effect size

F

P

Proportion of nitrophilous species Irrigation/drainage ditch

-0.11

5.448

0.0231*

Cleared/filled-up ditch

?11.58

5.978

0.0167*

Distance to median (multivariate dispersion) Landscape component

–

4.881

0.008**

Ditches vs. meadows

-0.022

–

0.058(*)

Ditches vs. field paths

-0.040

–

0.004**

Meadows vs. field paths

-0.019

–

0.166

Comparison of distance to median (beta diversity) within meadows, ditches and field paths Significant results are marked with asterisks (*) p \ 0.1, * p \ 0.05, ** p \ 0.01 and *** p \ 0.001. Effect size was calculated as mean difference

second axis is correlated positively with Ellenberg’s light values (R2 = 0.16, p \ 0.001; Fig. 5). Well-maintained cleared ditches (48.6% ± 12.5) contained a significantly lower proportion of nitrophilous species than less-maintained filled-up ditches

Fig. 5 DCA of species abundance data (gradient length first axis: 4.93 standard deviations). Ditches separate clearly from meadows and field paths. The first two axes were correlated post hoc with the EIVs of moisture (F), nitrogen (N), soil reaction (R) and light (L)

(60.2%; Table 2). Nitrophilous species occurred in significantly higher proportions in irrigation ditches (56.1% ± 16.2) than in drainage ditches (45.6% ± 9.4; Table 2). Among species only found in ditches, but not in meadows or field paths were explicit wetland species such as Bidens cernua, Callitriche palustris, Caltha palustris, Cirsium palustre, Epilobium parviflorum,

123

Plant Ecol

Galium palustre, Lycopus europaeus, Lysimachia vulgaris, Mentha aquatica, Scirpus sylvaticus and Veronica anagallis-aquatica, species indicating variable water supply such as Alisma plantago-aquatica or Equisetum fluviatile and also ruderal species such as Medicago sativa or Silene latifolia. Although species compositions in ditches differed from those in meadows, no red list species could be found in the ditch plots. However, several species restricted to field paths and especially to the ditches are relevant indicators of high nature value grasslands in the region (BFN Federal Agency for Nature Conservation 2014). Of those, Caltha palustris, Epipactis helleborine, Lysimachia vulgaris, Succisa pratensis and Valeriana officinalis were restricted to ditches, while Geranium pratense and Lychnis flos-cuculi could be found in ditches as well as—less frequently—on field paths. Neophytic species were very sparse and did not build monodominant stands in any of the plots.

Discussion The higher alpha diversity in ditches and field paths than in meadows can be explained with the ‘‘edge effect’’ (Odum 1953; Kunin 1998): Transition zones between two landscape components, are suspected to contain more species than fields (or meadows) because site conditions are more heterogeneous but less prone to disturbance than field centres (Smart et al. 2002; Gabriel et al. 2006; Simmering et al. 2008; Gaujour et al. 2012; Rusˇina et al. 2013). As hypothesized, ditches had a higher alpha diversity than meadows, which can be attributed to their specific shape consisting of micro-habitats with different moisture and nutrient conditions where a small-scale patchwork of vegetation can develop. Alpha diversity of ditches strongly depended on the ditch profile and management (regular dredging) of the ditches. We found higher plant alpha diversity in deep, cleared ditch types (box-shaped ditches, stages ditches, trapezoidshaped ditches) than in filled-up ones. In filled-up ditches, sedimentation leads to less diverse micro-site conditions and to nutrient accumulation (Janse and Van Puijenbroek 1998), in turn causing lower species numbers compared with well-maintained (cleared) ditches (Krause et al. 1993), because competitive species are favoured. When sediments are excavated from time to time, competitive exclusion by dominant

123

species is avoided, allowing light demanding superior and inferior species to coexist as described within the context of the intermediate disturbance hypothesis (Connell 1978). A negative effect of nutrient input on phytodiversity in ditches by fertilization of adjacent meadows, as observed by Mu¨ller et al. (2016b) in meadows in the same region, could not be found. This may indicate that no considerable amounts of nutrients leach from the meadows into the ditches, most likely because the amount of mineral fertilizer applied on the studied meadows is relatively small (50 kg N/ha) compared to fertilizer applications on intensively managed grassland that often exceed 150 kg N/ha (Dierschke and Briemle 2002). Further, chemical analyses of water collected in five pairwise samples of irrigation and drainage ditches during irrigation in spring 2015 confirm that nitrate does not leach from the meadows into the ditches. In contrast, analyses even show a tendency of marginal significance of nitrate filtering by the meadows (mean nitrate in inflowing water: 5.8 mg/l versus outflowing 3.7 mg/l; pairwise Wilcoxon test p = 0.068; Buhk unpublished data). The higher proportion of species indicating nitrogen-rich conditions in irrigation than in drainage ditches in our study underpins these results. While the sediment fraction of irrigation water used to be of major importance as fertilizer in historical times (Leibundgut and Kohn 2014a), it may be negligible today because of the improved water quality of rivers. Therefore, we suggest that the nutrient content in ditches is mainly related to biomass accumulation on the ditch banks and within the ditches. Biomass accumulates when material from mowing or mulching is left on the ditch rims and when organic substances such as leaves or other debris are transported into the irrigation ditches with the incoming river water during spring irrigation. Seeds or other types of diaspores carried with the river water might also explain the higher species richness in irrigation than drainage ditches which would otherwise be difficult to explain as we argued earlier that nutrient enrichment usually tends to reduce species richness. As expected, ditches had a higher community variation than field paths and meadows. The heterogeneity among ditch plots may be influenced by the humidity of the sites, the various ditch profile types and the different management of the ditches. Heterogeneous moisture conditions lead to a co-existence of

Plant Ecol

moisture-loving species and drought-tolerant species (Ha´jek et al. 2008), and may therefore have contributed to the strong heterogeneity among ditch plots compared with the dryer plots of field paths or meadows. Increasing moisture content of the soil has previously been found to enhance beta diversity ˇ arni 2008) as the (Ha´vlova et al. 2004; Zelnik and C gradient of small-scale site conditions associated with the water supply enlarges with increasing water availability. Moreover, spatial heterogeneity in irrigation ditches is enhanced by additional temporally heterogeneous moisture conditions: varying times, frequencies, intensities and durations of irrigation pulses create small-scale patterns with areas that remain dry most of the time in close vicinity to areas that are wet during certain times of the year and areas where humidity remains high all year around according to the relief and the location within the irrigation network. Other factors fostering beta diversity are the individual methods of management leading to different disturbance regimes and ditch profile types. The studied water-meadow landscape belongs to different municipalities, and therefore ditches in the area are maintained by different staff and machinery, making mowing, mulching and clearing methods manifold. The strong impact of mowing time and frequency on species composition is well studied (e.g. Hobbs and Huenneke 1992; Geertsema and Sprangers 2002; Manhoudt et al. 2007), and temporal diversity of disturbances was highlighted to be of special relevance for regional species diversity in agricultural landscapes (Buhk et al. 2007). In the study on hand, varying frequencies as well as intensities of ditch maintenance between the municipalities probably contribute to the high beta diversity among ditch plots. Differences in alpha and beta diversity increase diversity at the landscape scale. While both of the edge structures studied enlarge the species pool in the study area, especially the ditches seem to be responsible for the high gamma diversity (146 species) found in the landscape. Without the ditches, almost one-third of species (45) would be missing in the study area (Fig. 4). Depending on heterogeneity of humidity, succession stage of the ditch and intensity and frequency of maintenance, ditches are not only characterized by a distinct species composition but also constitute important habitats for rare species and species relevant for

conservation (Garniel 2000; Herzon and Helenius 2008). Although no red list species could be found within our study plots, other ditches in the region are known to be home to several rare species such as Gratiola officinalis, Viola persicifolia or Iris sibirica (Keller 2013). However, a plot-based sampling design as used in our study is not appropriate to detect rare species (Thompson 2004). Nevertheless, the restriction of several grassland species of high nature value (HNV species) to ditches proofs the special role of ditches as refuges. The presence of water plants, meadow plants and plants typical of dry ruderal sites underlines the heterogeneous site conditions within and among the ditches from stagnant water to frequently disturbed dry habitats as highlighted in the discussion section on beta diversity. Neophytic species typical for riverine landscapes (Chytry´ et al. 2008) are very sparse and seem not to considerably colonize ditches in the area. This study emphasizes the high ecological value of ditches for biodiversity in water-meadow landscapes. It suggests that recent land-use practices in the Queich lowland go well along with conservation interests such as high phytodiversity at local and landscape scale and the creation of habitat for many high nature value grassland indicator species. Traditional meadow irrigation techniques should be taken into consideration when options for sustainable grassland management are discussed (Leibundgut and Kohn 2014b). Ditches had a higher alpha and beta diversity than field paths and meadows and may additionally be important refuges for several HNV species. However, ditch profile type and management strongly influence alpha diversity of ditches, indicating that well-maintained cleared and deep ditches with diverse micro-site conditions are most valuable for plant diversity. Irregular cleaning of the ditches with differences in timing, partial cleaning or half-site cleaning (Garniel 2000) and a cleaning frequency of 2–3 years (Van Strien et al. 1991) have been shown to maximize phytodiversity at local scales. As nitrogen accumulates especially in irrigation ditches, removing the biomass after mowing may be favourable for species sensitive to nutrient-rich conditions. We stress the importance of diversified management in the area as main driver of landscape diversity. The region is characterized by a patchwork of irrigated and non-irrigated as well as fertilized and nonfertilized meadows, creating diverse habitats for

123

Plant Ecol

various plant species. Our results show that the preservation, re-establishment and correct management of the landscape components, especially of ditches, are crucial for enhancing landscape diversity. Acknowledgements This work was supported by the German Federal Environmental Foundation under Grant Number AZ 31109. Melanie Meier was supported by a scholarship within the research focus AufLand as part of the research initiative Rhineland-Palatinate financed by the Ministry for Education, Science, Further Education and Culture (MBWWK).

References Anderson MJ, Ellingsen KE, McArdle BH (2006) Multivariate dispersion as a measure of beta diversity. Ecol Lett 9:683–693 BFN Federal Agency for Nature Conservation (2014) The High Nature Value farmland indicator in Germany. https://www. bfn.de/0315_hnv?M52087573ab0.html. Accessed 13 Jan 2016 Blackstock TH, Rimes CA, Stevens DP, Jefferson RG, Robertson HJ, Mackintosh J, Hopkins JJ (1999) The extent of semi-natural grassland communities in lowland England and Wales: a review of conservation surveys 1978–1996. Gr Forage Sci 54:1–18 Buhk C, Retzer V, Beierkuhnlein C, Jentsch A (2007) Predicting plant species richness and vegetation patterns in cultural landscapes using disturbance parameters. Agr Ecosyst Environ 122:446–452 Chytry´ M, Jarosˇ´ık V, Pysˇek P, Ha´jek O, Knollova´ I, Tichy´ L, Danihelka J (2008) Separating habitat invasibility by alien plants from the actual level of invasion. Ecology 89:541–1553 Connell J (1978) Diversity in tropical rain forests and coral reefs. Science 199:1304–1310 Dierschke H, Briemle G (2002) Kulturgrasland: Wiesen. Weiden und verwandte Staudenfluren. Ulmer, Stutgart Eggenberg S, Mo¨hl A (2009) Flora Vegetativa. Ein Bestimmungsbuch fu¨r Pflanzen der Schweiz im blu¨tenlosen Zustand, Haupt Ellenberg H, Weber HE, Du¨ll R, Wirth V, Werner W (2000) Zeigerwerte von Pflanzen in Mitteleuropa. Erich Goltze, Go¨ttingen European Commission (2011) Our life insurance, our natural capital: An EU biodiversity strategy to 2020. Committee on the Environment, Public Health and Food Safety, Brussels Fahrig L, Baudry J, Brotons L, Burel GF, Crist T, Fuller RJ, Sirami C, Siriwardena GM, Martin J (2011) Functional landscape heterogeneity and animal biodiversity in agricultural landscapes. Ecol Lett 14:101–112 Gabriel D, Roschewitz I, Tscharntke T, Thies C (2006) Beta diversity at different scales: plant communities in organic and conventional agriculture. Ecol Appl 16:2011–2021 Garniel A (2000) Schutzkonzept fu¨r gefa¨hrdete Wasserpflanzen der Fließgewa¨sser und Gra¨ben Schleswig-Holsteins. Teil C. Gra¨ben. Landesamt fu¨r Natur und Umwelt SchleswigHolstein, Kiel

123

Gaujour E, Amiaud B, Mignolet C, Plantureux S (2012) Factors and processes affecting plant biodiversity in permanent grasslands. A review. Agron Sustain Dev 32:133–160 Geertsema W, Sprangers JTCM (2002) Plant distribution patterns related to species characteristics and spatial and temporal habitat heterogeneity in a network of ditch banks. Plant Ecol 162(1):91–108 Geiger M (2008) Haardt, Weinstraße und Queichtal: Ein GeoFu¨hrer. Pollichia, Neustadt/Weinstraße Ha´jek M, Hajkova P, Sopotlieva D, Apostolova I, Velev N (2008) The Balkan wet grassland vegetation: a prerequisite to better understanding of European habitat diversity. Plant Ecol 195:97–213 Ha´vlova M, Chytry´ M, Tichy L (2004) Diversity of hay meadows in the Czech Republic: major types and environmental gradients. Phytocoenologia 43:551–567 Herzon I, Helenius J (2008) Agricultural drainage ditches, their biological importance and functioning. Biol Cons 141:1171–1183 Hobbs RJ, Huenneke LF (1992) Review: disturbance, diversity, and invasion. Implications for conservation. Conserv Biol 6:324–337 Isbell F, Calcagno V, Hector A, Connolly J, Harpole WS, Reich PB, Scherer-Lorenzen M, Schmid B, Tilman D et al (2011) High plant diversity is to maintain ecosystem services. Nature 477:199–202 Isbell F, Craven D, Connolly J, Loreau M, Schmid B, Beierkuhnlein C et al (2015) Biodiversity increases the resistance of ecosystem productivity to climate extremes. Nature 526:574–577 Isselstein J, Jeangros B, Pavlu V (2005) Agronomic aspects of biodiversity targeted management of temperate grasslands in Europe—A review. Agron Res 3:139–151 Ja¨ger EJ (2011) Rothmaler. Exkursionflora von Deutschland. Gefa¨ßpflanzen: Grundband. Spektrum, Germany Janse JH, Van Puijenbroek PTM (1998) Effects of eutrophication in drainage ditches. Environ Pollut 102:547–552 Karim MN, Mallik AU (2008) Roadside revegetation by native plants: I. Roadside microhabitats, floristic zonation and species traits. Ecol Eng 32:222–237 Kark S, Van Rensburg BJ (2006) Ecotones: marginal or central areas of transition? Isr J Ecol Evol 52:29–53 Keller P (2013) Die Queichniederung—Portrait einer Landschaft. Gesellschaft fu¨r Naturschutz und Ornithologie Rheinland- Pfalz, Mainz Kleijn D, Rundlo¨f M, Scheper J, Smith HG, Tscharntke T (2011) Does conservation on farmland contribute to the biodiversity decline? Review. Trends Ecol Evol 26:474–481 Konold W (2007) Die wasserabha¨ngigen Lando¨kosysteme. Gibt es gemeinsame Strategien von Wasserwirtschaft und Naturschutz zu deren Schutz und Erhalt? Hydrol Wasserbewirtsch 51:257–266 Kramer G (2011) Design, Construction, and Assessment of a Self-Sustaining Drainage Ditch. A thesis at the faculty of the graduate school of the University of Minnesota Krause U, Poschlod P, Kapfer A (1993) Pflege- und Ausbaumaßnahmen an Gra¨ben der Singener Aach-NiederungIhre Auswirkungen auf Vegetation und Diasporenbank. In: ¨ kologie Bernhardt KG, Hurka H (eds) Poschlod P: O semiaquatischer Lebensra¨ume—Aspekte der Populationsbiologie. Natur Wissenschaft, Solingen

Plant Ecol Kunin W (1998) Biodiversity at the edge: a test of the importance of spatial ‘‘mass effects’’ in the Rothamsted Park Grass experiments. Proc Natl Acad Sci USA 95:207–212 Laird NM, Ware JH (1982) Random-effects models for longitudinal data. Biometrics 38:963–974 Leibundgut C, Kohn I (2014a) European traditional irrigation in transition part I: irrigation in times past—A historical land use practice across Europe. Irrig Drain 63:273–293 Leibundgut C, Kohn I (2014b) European traditional irrigation in transition part II: traditional irrigation in our time- decline, rediscovery and restoration perspectives. Irrig Drain 63:294–314 Londo G (1976) The decimal scale for releve´s of permanent quadrats. Vegetatio 33:61–64 Manhoudt AGE, Visser AJ, De Snoo GR (2007) Management regimes and farming practices enhancing plant species richness on ditch banks. Agr Ecosyst Environ 119:353–358 Mu¨ller IB, Buhk C, Alt M, Entling MH, Schirmel J (2016a) a) Plant functional shifts in Central European grassland under traditional flood irrigation. Appl Veg Sci 19:122–131 Mu¨ller IB, Buhk C, Lange D, Entling MH, Schirmel J (2016b) b). Contrasting effects of irrigation and fertilization on plant diversity in hay meadows. Basic Appl Ecol 17: 576–585 Odum EP (1953) Fundamentals of ecology. Saunders, Philadelphia Oksanen J, Blanchet FG, Kindt R, Legendre P, O’Hara RB, Simpson GL, Solymos P, Henry M, Stevens H, Wagner H (2011) Vegan: community ecology package. R package version 1 10:631–637 Pinheiro J, Bates D, DebRoy S, Sarkar D, R Core Team (2012) nlme: linear and nonlinear mixed effects models. R package version 3.1–103, https://CRAN.R-project.org/ package=nlme R Development Core Team (2015) R: a language and environment for statistical computing. R Foundation for Statistical Computing, Vienna Roovers P, Baeten S, Hermy M (2004) Plant species variation across path ecotones in a variety of common vegetation types. Plant Ecol 170:107–119 Rusˇina S, Puspure I, Gustina L (2013) Diversity patterns in transitional grassland areas in floodplain landscapes with different heterogeneity. Tuexenia 33:347–369 Sabbatini MR, Murphy KJ, Irigoyen JH (1998) Vegetation– environment relationships in irrigation channel systems of southern Argentina. Aquat Bot 60:119–133

Schirmel J, Alt M, Rudolph IB, Entling MH (2014) Effects of traditional flood irrigation on invertebrates in lowland meadows. PLoS ONE 9:e110854 Seiffert P, Kiefer H, Konold W (1994) Vegetation der Gra¨ben im Raum Kißlegg. Institut fu¨r Landschafts- und Pflanzeno¨kologie, Universita¨t Hohenheim 3:25–52 Seybold S (2009) Schmeil-Fitschen. Flora von Deutschland und angrenzender La¨nder. Quelle and Meyer, Heidelberg Simmering D, Waldhardt R, Otte A (2008) Steuergro¨ßen der Pflanzenartenvielfalt in Mosaiklandschaften – eine Betrachtung auf zwei ra¨umlichen Skalenebenen. Naturschutz und Biologische Vielfalt. Bundesamt fu¨r Naturschutz 60:67–72 Smart SM, Bunce RGH, Firbank LG, Coward P (2002) Do field boundaries act as refugia for grassland plant species diversity in intensively managed agricultural landscapes in Britain? Agr Ecosyst Environ 91:73–87 Thompson W (2004) Sampling rare or elusive species: concepts, designs, and techniques for estimating population parameters. Island Press, Washington Tscharntke T, Klein AM, Kruess A, Steffan-Dewenter I, Thies C (2005) Landscape perspectives on agricultural intensification and biodiversity—ecosystem service management. Ecol Lett 8:857–874 Tscharntke T, Clough Y, Wander TC, Jackson L, Motzke I, Perfecto I, Vandermeer J, Whitbread A (2012) Global food security, biodiversity conservation and the future of agricultural intensification. Biol Cons 151:53–59 Van Strien AJ, Van der Burg T, Rip WTM, Strucker RCW (1991) Effects of mechanical ditch management on the vegetation of ditch banks in Dutch peat areas. J Appl Ecol 28:501–513 Wesche K, Krause B, Culmsee H, Leuschner C (2012) Fifty years of change in Central European grassland vegetation: large losses in species richness and animal-pollinated plants. Biol Cons 150:76–85 Zechmeister HG, Schmitzberger I, Steurer B, Peterseil J, Wrbka T (2003) The influence of land-use practices and economics on plant species richness in meadows. Biol Cons 114:165–177 Zelnik I, Cˇarni A (2008) Wet meadows of the alliance Molinion and their environmental gradients in Slovenia. Biologia 63:187–196 Zuur AF, Ieno EN, Elphick CS (2010) A protocol for data exploration to avoid common statistical problems. Methods Ecol Evol 1:3–14

123