Restoration of plant species diversity of ditch banks : ecological
constraints and opportunities
Blomqvist, M.M.
Citation
Blomqvist, M. M. (2005, February 3). Restoration of plant species diversity of ditch banks :
ecological constraints and opportunities. Retrieved from https://hdl.handle.net/1887/592
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Chapter 3
Restoration of ditch bank plant species richness:
the potential of the soil seed bank.
M. M. Blomqvist, R. M. Bekker & P. Vos
Abstract
Small-scale landscape elements, such as ditch banks, play an important role in preserving plant species-richness in agricultural landscapes. In this study, we investigate whether the seed bank might be useful for restoring the above-ground plant species richness. We studied the vegetation and seed bank composition at six species-rich and six species-poor ditch banks, where agri-environment schemes are running to maintain and enhance ditch bank plant diversity. We show that the number of species in the seed bank was low, regardless of the number of species in the established vegetation. Moreover, the seed bank was always dissimilar to the established vegetation. Target species for nature conservation were occasionally present in the seed bank both at species-poor and species-rich sites, but rarely so if the species was absent from the established vegetation. We conclude that the potential use of the seed bank for restoration of ditch banks is slim, at best. At present, plant species richness seems to be largely controlled by germination opportunities; high biomass and competition appear to hamper germination at species-poor sites. We, therefore, recommend continued nutrient reduction at species-poor sites. Soil disturbance measures and deliberate sowing should also be considered.
Chapter 3 46
Introduction
Intensification of farming practices during the last century has had a detrimental effect on the biodiversity of the agricultural landscape in Europe (McNeely et al. 1995; Stoate et al. 2001). Due to current intensive management of pastures and arable fields (van Strien et al. 1988; van Elsen 2000), small-scale and linear landscape elements including ditch banks, field edges and hedgerows, fulfil an important refuge function (Ruthsatz & Haber 1982; Baudry 1988; Melman & van Strien 1993; Fry 1994; Joenje & Kleijn 1994; Bunce & Hallam 1998). To protect the biodiversity of agricultural areas as a whole, it is necessary to preserve the species richness of these landscape elements.
The Western Peat District in the Netherlands with its 300,000-400,000 km of ditches and ditch banks (Higler 1994) represents this type of landscape. Here, many previously common plant species from wetland, grassland and dry hayfield communities are currently mainly found in ditch banks. Many of these species are nationally (Caltha palustris, Lychnis flos-cuculi), or internationally important (Cirsium dissectum, Myosotis discolor) (Clausman & van Wijngaarden 1984; Westhoff & Weeda 1984, Melman 1991). However, during the last decades, the ditch banks have been under increasing pressure, resulting in a decline of plant species richness in this habitat as well (Provincie Zuid-Holland 2002; Blomqvist et al. 2003)
Research into these ditch bank systems has, until now, focused on exploring types of management by which the species richness can be maintained or increased (Melman & van Strien 1993). Lower fertilisation levels, extensive grazing and mowing regimes and lower ditch cleaning frequencies were associated with high plant species richness, but when implemented these management practices did not guarantee an increase in species richness (Kleijn et al. 2001; M. Blomqvist, unpubl.). This lack of success may be related to high nutrient levels preventing successful establishment. ‘Target’ species may also be lacking in the soil seed bank or in nearby source populations (Prins et al. 1998; Bakker & Berendse 1999).
Sod cutting or removal of the topsoil layer, is often practised as a means of restoring (semi-) natural grassland vegetation in the Netherlands. Ideally, soil nutrient levels are reduced, giving remaining seeds in the subsoil the opportunity to re-establish in the vegetation (Chippindale & Milton 1934; Bakker 1989; van der Valk & Pederson 1989). In practice, sod cutting has occasionally been successful (Oomes & van der Werf 1996; Jansen & Roelofs 1996). However, more often viable seeds of endangered and other ‘target’ species are absent, or present only at very low densities (Bakker 1989; Maas & Schopp-Guth 1995; Dutoit & Alard 1995; Hutchings & Booth 1996; Jensen 1998; Bakker & Berendse 1999).
ditch-bank, sod-cutting experiments (Melman 1991; Melman & van Strien 1993), broad terraces contained significantly more species than untreated ditch banks after three years. Our objective is to determine whether the ditch bank seed bank might be useful for restoring the above-ground plant species richness, by comparing the seed bank species richness and composition between sites with species-poor and species-rich vegetation. In order for the seed bank to be potentially useful for restoration, we either need a species-rich seed bank, or in case of seed banks with few species, it should contain those important target species absent from the vegetation. To explore other restoration strategies, we investigate how the community pool (vegetation + seed bank, Zobel et al. 1998) differs ecologically between species-poor and species-rich sites.
Material and Methods
Research area and site selection
The study area (51°52’N - 52°07’N and 4°47’E - 5°05’E) is situated in the Province of South-Holland in the Western Peat District in the Netherlands. The dominant soil types are peat or peat with clay. Most land is utilised as pasture for dairy cattle and sheep. Typically, the pastures have 0.5-1.5 m wide field edges (ditch banks) of varying species richness. Ditch water levels are artificially controlled: winter levels are normally some 10-15 cm lower than summer levels. As a rule, ditches never dry out and the lower parts of the ditch banks remain moist throughout the season.
The composition of the seed bank may be influenced by a number of factors, including present and past vegetation, proximity to other seed sources and present and past management. To avoid the confounding effects of different management regimes, we selected six farms where similar, ‘nature-friendly’ ditch bank management (see introduction) has been practised at least since 1992. Based on previous (unpublished) vegetation data from 1993-95, we selected six species-poor (mean 25.1 ± 2.4) and six species-rich (mean 40.3 ± 2.6) ditch banks (from the bottom and top end of the species richness spectrum). For a detailed design, see Fig. 1a.
Seed bank sampling, seedling emergence and vegetation
Chapter 3 48
The soil samples were concentrated using the bulk reduction technique (ter Heerdt et al. 1996). The samples were washed out over a coarse sieve (mesh 2.0 mm) and a fine sieve (mesh 0.21 mm), filtering out larger vegetative parts and particles, while retaining all but the smallest seeds (e.g. of orchids). We filled germination trays with a 5 cm deep layer of sterile soil, covered by a 1 cm thick layer of sterile sand. The concentrated residue was spread out evenly in a 2-3 mm thick layer on top of the sand. We placed the soil samples in a glasshouse with 16 h light at 25 °C and 8 h darkness at 15 °C. Due to restricted space, the germination took place in two phases. Soil samples from farms F1 and F4 (Fig. 1a) were placed in the glasshouse immediately, the rest were stored in +4 °C for 6 weeks before germination. We monitored seedling germination in all soil samples for 13 weeks; samples from F1 and F4 were monitored for an additional 6 weeks. Number of species and number of seedlings (indicative of the number of viable seeds in the samples) were recorded. Four trays, containing sterilised soil only, were placed randomly among the ditch bank soil samples to control effect of sterilisation and possible contamination by wind borne propagules from outside the glasshouse. No germination was observed in these trays, so our samples were not contaminated.
During one week in mid-May 2000, we made an inventory of the vegetation in the field (nomenclature follows van de Meijden 1996). The presence and percentage cover of each plant species were recorded in the same 50 m plots where the soil samples had been taken. Percentage bare soil was also estimated. In addition, five replicate above-ground biomass samples (50 x 50 cm2) were collected at regular intervals in those
plots still unaffected by grazing at the time of sampling (n = 9). Biomass is expressed as g dry weight / m2.
Target species and ecological characteristics
We defined target species according to two criteria: ‘nature-value’ of a species (Clausman & van Wijngaarden 1984) and ‘valuable ditch bank plants’ (van Harmelen et al. 1997). The nature-value of a species is a composite index, based on regional (Provincial), national (Dutch) and international rarity (see also van Strien 1991). Species with a nature-value higher than average in ditch banks of South-Holland are defined as target species (see also Chapter 2: Appendix 1). Those species occurring on a list of 25 valuable ditch banks plants in the area were added to these target species. The number of target species was counted in the vegetation and the seed bank.
Table 1. Description of ecological traits: n = number of records (species) per ecological variable. For aggregate taxa we
used an estimate based on the possible species present in the study area.
Ecological variables n Range or no. classes
Description Source Nutrient requirements (N) 104* 2-9 nutrient poor - nutrient rich 1
Minimum light requirement (L) 105* 4-9 shade tolerant - light demanding 1
Moisture (F) 105* 4-11 dry - wet 1
Acidity (R) 105* 3-8 acid - alkaline 1
Mean plant height (hgt) 104 4.5-165.0 low - high (cm) 2 Begin flowering time
(fl-begin) 105 1-7 month: 1 = January - 7 = July 2 Seed bank type
(sb-type lit) 97 1-3 transient (1), short-term persistent (< 5 years) (2), long-term persistent (≥ 5 years)(3) 3 Germination time (germ) 92 1, 2-5 directly (1), spring (2), late spring (3), summer (4),
autumn (5); 1 = germ-direct, 2-5 = germ-late
2 Dispersal (disp) 100 6 water (1), wind (2), unspecialised (3), agricultural (4),
animal (5), mixed (water + animal & water + wind) (6) 4 Selfing (self) 105 2 not selfing or unknown (0), selfing (1) 2 Clonality (clo) 101 2 non-clonal (0), clonal (1) 5 Life duration (dura) 105 2 annual (0), perennial (1) 2 * mean value used for indifferent species
1) Ellenberg et al. (1992); Wiertz (1992); Hill et al. (1999) 2) CBS (1997)
3) Thompson et al. (1997), modified according to Tamis et al. (2000) 4) Grime et al. (1988); van Dorp (1996)
5) Klimes & Klimesova (1999)
50 Chap
ter
Ecological species characteristics with an ordinal or interval scale were converted to ‘environmental factors’ by calculating unweighted mean species values per plot for all ordinal ecological characteristics (Melman et al. 1988; Hill & Carey 1997; Ertsen et al. 1998; Schaffers & Sykora 2000). The nominal variables (dispersal vector, selfing, life duration, clonality) were expressed as percentages (e.g. % perennials per plot). ‘Germination time’ was divided into two variables: a nominal part ‘germ-direct’ = percent directly germinating species and an ordinal part ‘germ-late’ = average germination time between early spring and autumn. All these values are intended to reflect the relative differences between average ecological species types represented in a specific plot, rather than absolute environmental values.
In addition, we compared the seed bank classification based on data from the present study (sb-type own) with that found in the literature (sb-type lit, Thompson et al. (1997), modified according to Tamis et al. (2000)). Sb-type own was calculated using the dichotomous key of Thompson et al. (1997) as follows: 1) transient: species found only in the vegetation or upper layers only, 2) short-term persistent: species found more frequently in upper than in deeper layers, 3) long-term persistent: species found as or less frequently in upper than in deeper layers. Species with fewer than three seeds in the dataset were excluded. Species with contradictory classifications derived from species-poor and species-rich sites were also omitted from this comparison.
Data analysis
Prior to analyses, we lumped some taxa, which are difficult to identify at the seedling stage. These species groups were: Agrostis spp., Callitriche spp., Cardamine spp., Hypericum spp., Juncus without septa, Juncus with septa, Myosotis spp. and Oenanthe aquatica + O. fistulosa. Azolla filiculoides, Lemna spp. and mosses were excluded from the analyses, resulting in a list of 105 taxa (referred to as ‘species’).
We tested for differences between species-poor and species-rich sites (in the number of species in the vegetation and the seed bank and number of seeds in the seed bank in different layers) with two-way ANOVAs followed by Tukey HSD when differences where significant (statistical package SPSS 8.0). Furthermore, to determine which ecological factors were associated with high species richness, we used one-way ANOVAs to test for differences in the ecological characteristics of the community pool between species-poor and species-rich sites. Biomass was ln-transformed and %-values were logit transformed to ensure homogeneity of variances. Since we performed multiple ANOVAs on the same dataset, we applied a Bonferroni correction to the results (Haccou & Meelis 1992).
Chapter 3 52
The Sørensen coefficient of similarity (Ss)
(Kent & Coker 1998) was used to describe the similarity of species-poor and species-rich sites in terms of vegetation and the seed bank: (Spoor-rich): S = 2a
/ (2a+b+c), where a = number of species common to poor and species-rich sites, b = no. sp. at poor sites, c = no. sp. at rich sites. Furthermore, we com-pared the similarity of the upper and deeper layers of the seed bank (Sup-deep) and
the similarity of the established vegetation and seed bank at species-poor and rich sites (Sveg-sb).
Fig. 2. Mean number of species found in the vegetation (veg) and the seed bank (sb) at species-poor and species-rich ditch banks. Dotted line indicates the mean number of species common to the vegetation and seed bank. Bars indicate standard deviation. Two-way ANOVA followed by Tukey HSD: site species richness: F1,23 = 36.567, p
< 0.0001, data category (veg/sb): F1,23 = 30.484, p
< 0.0001, interaction: F1,23 = 58.134, p < 0.0001.
Letters indicate significant differences between the means.
Results
Species richness at species-rich and poor sites and ecological correlates
The mean number of species in the established vegetation was significantly greater at species-rich than at species-poor sites, still the mean number of species in the seed bank did not differ between species-poor and species-rich sites (Fig. 2). The seed bank was thus equally poor at all sites, regardless of vegetation species richness.
The number of species per ditch bank in the soil seed bank varied with depth, but not with site species richness (Fig. 3a). The upper soil layer (0-5 cm) contained, on average, more species than the deeper soil layer (5-10 cm). Likewise, the number of seeds per ditch bank was highest in the upper layer, but did not differ with site species richness (Fig. 3b, Table 2). There were no differences in the mean total number of seeds per ditch bank between species-poor and species-rich sites (11730 and 11781 seeds / m2
respectively, F 1,23 = 0.001, p = 0.977).
Fig. 3. (a) Mean number of species and (b) mean number of seeds found at different depths (0-5 and 5-10 cm) in the seed bank of species-poor and species-rich ditch banks. Seed numbers expressed as numbers per ditch bank (volume = 2 litres; area = 0.0393 m²; 1 seed corresponds to 25.5 seeds / m²). Bars indicate standard deviation. Two-way ANOVA: (a) site species richness: F1, 23 = 0.636, p = 0.435, depth: F1,23 = 6.601, p =
0.018, interaction: F1, 23 = 0.385, p = 0.542; (b) site species richness: F1, 23 = 0.001, p =
0.980, depth: F1,23 = 4.696, p = 0.042, interaction: F1, 23 = 0.204, p = 0.656. Letters
Table 2. Species found in the vegetation (veg) and the seed bank (sb). Frequency (number of plots) in the vegetation, seed bank and
vegetation + seed bank at species-poor, species-rich and all sites indicated per species. Total number of seeds (sum seeds) per ‘treatment’ and layer (volume = 12 l, area = 0.2356 m2) is indicated for the upper 0-5 cm and lower 5-10 cm layers of the seed bank (1 seed
corresponds to 4.24 seeds m-2). Species are divided into four groups according to presence in the species poor and rich vegetation and
within each group species are subdivided according to presence in the seed bank. T = Target species; sb-type lit = seed longevity from literature: 1 = transient, 2 = short-term persistent, 3 = long-term persistent; sb-type own = seed longevity inferred from present data (see methods), two values indicate that different classifications were derived from species-poor and species-rich sites; S = species is present in the surrounding vegetation (within the next 50 m in the same ditch bank).
Sum seeds in sb Frequencies Ecology
(per treatment & layer) sb veg veg+sb veg sb veg+sb target sb- sb-
poor poor rich rich poor rich poor rich poor rich all all all sp type type
Species upper deeper upper deeper *) *) *) lit own
Not in veg
Sb poor Alnus glutinosa 2 1 1
only Atriplex patula 1 1 1 3
Callitriche spp. 92 127 3 3 T 2 3 Epilobium ciliatum 1 1 1 2 Gnaphalium uliginosum 1 2 1 1 T 3 3 Hypericum sp. 1 1 1 3 Lamium purpureum 1 1 1 3 Persicaria lapathifolium 1 1 1 3 Rubus fruticosa 1 1 1
Sb rich Atriplex prostrata 2 25 1 1 3 3
only Matricaria dioscoidea 2 2 2 3
Mentha arvensis 1 1 1 T 3
Plantago major 1 1 1 3
Sonchus arvensis 1 1 1 2
Veronica chamaedrys 5 1 1 T 3 1
Veronica serpyllifolia 3 1 1 1 T 3 2
Sb all Alisma gramineum 3 1 4 1 1 2 T 1/3
Table 2. contd.
Sum seeds in sb Frequencies Ecology
(per treatment & layer) sb veg veg+sb veg sb veg+sb target sb- sb-
poor poor rich rich poor rich poor rich poor rich all all all sp type type
Species upper deeper upper deeper *) *) *) lit own
Veg poor only
Not in Arrhenatherum elatius 1 1 1 1
sb Bidens cernua 1 1 3 1
Galium aparine 1 1 1 1
Heracleum sphondylium 1 1 1 1
Ranunculus ficaria ssp. bulbilifer 1 1 1 1
Rorippa microphylla 1 S 1 1
Sb poor Lythrum salicaria 30 7 1 1 1 1 T 3 2
only Persicaria mitis 1 1 1 1 1 1 1 T
Sb all Alisma plantago-aquatica 2 1 1 1 1 1 2 3
Capsella bursa-pastoris 4 3 2 2 1 1 1 1 3 1 3 2
Persicaria hydropiper 134 74 3 4 4 2 1 S 1 6 3 2/3
Phleum pratense 3 5 8 7 3 3 1 1 6 3 2/3
Veg rich only
Not in Acorus calamus 1 1 1 1
Table 2. contd.
Sum seeds in sb Frequencies Ecology
(per treatment & layer) sb veg veg+sb veg sb veg+sb target sb- sb-
poor poor rich rich poor rich poor rich poor rich all all all sp type type
Species upper deeper upper deeper *) *) *) lit own
Veg rich only contd.
Sb poor Epilobium tetragonum 1 1 1 1 1 3
only Persicaria maculosa 5 2 2 2 2 2 3 2
Sb rich Epilobium hirsutum 1 1 1 4 1 4 1 1 3
only Epilobium parviflorum 4 5 2 1 1 2 3 3
Lotus uliginosus 1 1 6 1 6 1 1 T 3
Poa pratensis 1 1 2 1 2 1 1 3
Rumex obtusifolius 8 2 4 2 4 2 2 3 1
Scutellaria galericulata 5 1 4 1 4 1 1 1 1
Trifolium dubium 1 1 2 1 2 1 1 3
Sb all Bidens tripartita 1 3 1 2 1 1 1 3 1 1
Festuca rubra 1 1 1 1 S 4 4 2 1
Lycopus europaeus 1 1 2 5 1 3 2 2 2 4 2 1 3
Veg poor & rich
Not in Bromus hordeaceus 1 2 3 1 1
sb Cirsium palustre 1 1 2 T 2 1
Elytrigia repens 3 2 5 2 1
Iris pseudacorus 1 2 3 T 1 1
Rumex crispus 1 1 2 3 1
Sparganium erectum 1 2 3 1 1
Sb poor Bellis perennis 3 1 1 1 1 2 1 1 2 1
only Carex pseudocyperus 1 1 2 1 1 3 1 1 T
Oenanthe aquatica + fistulosa 17 13 2 2 4 1 6 2 1 T 1 2
Trifolium repens 4 1 3 3 1 6 1 1 3 1
Sb rich Anthoxanthum odoratum 5 2 1 4 2 5 2 2 2 1
only Carex acuta 1 1 1 3 1 4 1 1 3
Table 2. contd.
Sum seeds in sb Frequencies Ecology
(per treatment & layer) sb veg veg+sb veg sb veg+sb target sb- sb-
poor poor rich rich poor rich poor rich poor rich all all all sp type type
Species upper deeper upper deeper *) *) *) lit own
Veg poor & rich contd.
Sb all Agrostis spp. 18 21 21 24 6 5 5 6 5 5 11 11 10 3 3 Alopecurus geniculatus 4 1 12 14 3 4 2 4 2 6 7 2 2 2/3 Alopecurus pratensis 4 2 3 1 2 6 4 1 2 10 3 3 1 1/3 Cardamine spp. 129 87 237 88 5 6 5 5 5 5 10 11 10 3 2 Cerastium fontanum 23 7 50 15 5 5 4 6 4 5 10 10 9 3 2 Dactylis glomerata 3 3 1 1 4 1 1 2 1 3 5 1 1 3 Galium palustre 2 5 9 7 2 5 1 4 1 4 5 7 5 T 3 2/3 Glechoma hederacea 31 30 22 12 4 6 6 6 4 6 12 10 10 2 2 Glyceria fluitans 13 13 22 15 3 4 6 6 3 4 12 7 7 2 2/3 Glyceria maxima 17 25 22 16 6 4 5 6 5 4 11 10 9 2 2/3 Holcus lanatus 303 160 195 70 6 6 6 6 6 6 12 12 12 3 2 Juncus no septa 784 462 313 129 6 6 5 4 5 4 9 12 9 3 2
Juncus with septa 587 462 1001 761 6 6 3 2 3 2 5 12 5 3 2
Lychnis flos-cuculi 15 2 22 12 3 5 3 5 2 4 8 8 6 T 3 2 Myosotis spp. 37 38 18 8 5 4 4 4 4 3 8 9 7 3 2 Persicaria amphibia 6 6 4 3 1 2 5 2 1 7 4 3 1 3 Poa trivialis 434 321 254 178 6 6 6 6 6 6 12 12 12 3 2 Ranunculus flammula 3 4 3 3 2 1 1 2 1 3 3 1 T 3 3 Ranunculus repens 92 70 171 97 6 6 5 6 5 6 11 12 11 3 2 Ranunculus sceleratus 53 111 43 84 6 4 5 2 5 2 7 10 7 3 3 Rorippa amphibia 45 21 4 3 4 1 5 2 3 1 7 5 4 1 2 Rorippa palustris 126 43 2 1 2 2 1 1 1 1 2 4 2 3 2 Rumex acetosa 58 23 15 7 2 4 5 6 2 4 11 6 6 2 2 Stellaria media 40 32 1 5 1 4 1 4 5 6 4 3 2 Stellaria uliginosa 51 37 302 129 6 4 4 4 4 4 8 10 8 T 3 2 Taraxacum officinale 13 3 2 2 3 2 2 4 2 1 6 5 3 2 2/3 Urtica dioica 19 15 7 2 3 2 2 2 2 1 4 5 3 3 2
Total no. sp = 105 sum sum sum sum max max max max max max max max max total mean total
3248 2275 3519 2033 6 6 6 6 6 6 12 12 12 27 2.3 70+11
*) only those species present in vegetation and seed bank in the same plots are counted
Chapter 3 58
Table 3. Ecological differences of all species present in the ditch bank community pool (vegetation and seed bank (veg+sb)) at species-poor and species-rich sites. F = F-ratio from one-way ANOVAs with site species richness (poor/rich) as fixed factor, p = significance values, n = 12, df = 1 (except for biomass, n = 9, df = 1). Significance-values at the 0.05-level are indicated in a bold letter type. Biomass-Significance-values were ln-transformed and %-values were logit-ln-transformed before analysis; backln-transformed means are given. Biomass-values are expressed as g dry weight / m2. See Table 1 for
information on ecological values.
Ecological value Sp poor sites Sp rich sites One-way ANOVA
(mean per site) veg+sb veg+sb F p
No. species 35.83 45.83 27.356 < 0.0001 * Biomass 228.70 111.73 31.323 0.001 * N 6.10 5.79 4.836 0.053 L 6.93 6.97 0.669 0.432 F 7.18 7.15 0.060 0.812 R 5.66 5.66 0.000 0.992 Mean height 55.40 54.96 0.024 0.881
Begin flowering time 5.21 5.20 0.021 0.888
Seed bank type 2.43 2.31 9.071 0.013
Mean ‘late’ germination 2.49 2.92 34.686 < 0.0001 *
Direct germination (%) 41.91 34.86 19.188 0.001 * Water dispersal (%) 19.09 17.24 0.597 0.457 Wind dispersal (%) 11.22 13.54 1.067 0.326 Unspecialised dispersal (%) 28.29 28.81 0.091 0.769 Agricultural dispersal (%) 16.26 12.06 3.454 0.093 Animal dispersal (%) 16.71 20.51 1.648 0.228 Mixed dispersal (%) 6.20 6.12 0.004 0.953 Selfing (%) 38.87 47.37 0.250 0.628 Clonality (%) 81.63 85.15 5.464 0.042 Perennials (%) 77.76 83.69 3.177 0.105
* values significant after Bonferroni-correction
Species composition at species-rich and poor sites and ecological correlates
In the vegetation ordination (Fig. 4a), a distinction could be made between species-poor and species-rich sites in the species composition. The longest vectors indicate the most important ecological factors correlated with the species composition. Species-poor sites tended to have higher biomass and N-values and to contain more agriculturally dispersing and directly germinating species. Species-rich plots tended to have later germinating species and more species with transient seeds in the seed bank.
Chapter 3 60
The DCA of the lumped vegetation and seed bank data (Fig. 5), showed a clear separation between seed bank and vegetation samples on the first axis. In other words, the seed bank is dissimilar from the vegetation in all ditch banks. The seed bank tends to contain more species with persistent seeds. The vegetation, on the other hand, contains more perennials, more clonal species and fewer selfing species.
Sørensen’s similarity index of the established vegetation of species-poor and rich sites was low (39.8 %, Table 4A), indicating that poor and species-rich sites have few species in common. The similarity of the seed bank of species-poor and rich sites, was also low (40.4 %). The established vegetation and the seed bank were ‘equally’ dissimilar both at species-poor and species-rich sites (similarity 40.4% and 39.3%, respectively, Table 4B). Similarity of upper and deeper soil layers was also low (42.7% at all sites, Table 4C), indicating that different species are present at different depths.
Table 4. Total numbers of species present: A) at species poor, rich and poor + rich sites in the vegetation and the seed bank, B) in the vegetation only, seed bank only and vegetation + seed bank at different sites and C) in upper layers only, deeper layers only and both soil layers at different sites. The total number of species (sum sp) and Sørensen’s similarity index for poor and rich sites (Spoor-rich), the vegetation and the seed
bank (Sveg-sb) and for upper and deeper soil layers (Sup-deep) are also indicated.
A
Plots Type data Sp poor only Sp rich only Poor + rich Sum sp Spoor-rich
all veg 12 31 42 85 39.8%
all sb 17 19 38 74 40.4%
B
Plots Site sp richness Veg only Sb only Veg + sb Sum sp Sveg-sb
11-32 sp. poor 17 18 37 72 40.4%
41-62 sp. rich 31 15 42 88 39.3%
all poor + rich 31 20 54 105 40.4%
C
Plots Site sp richness Upper only Deeper only Upper + deeper Sum sp Sup-deep
11-32 sp. poor 11 6 38 55 45.0%
41-62 sp. rich 16 6 35 57 43.2%
all poor + rich 20 10 44 74 42.7%
Occurrence of target species and seed bank classification
Out of 105 species, 27 can be defined as target species (Table 2). Of these, 11 target species were found only in the vegetation, while 10 species were present both in the vegetation and in the seed bank. Only six species (Alisma gramineum, Callitriche spp. Gnaphalium uliginosum, Mentha arvensis, Veronica chamaedrys, V. serpyllifolia) were found (at low frequencies) in the seed bank while absent from the (nearby) vegetation. Of these, only two were present at species-poor sites. Thus, the seed bank does not contain many target species absent from the established vegetation.
Chapter 3 62
Discussion
Differences in vegetation and seed bank species richness and composition
To be useful for restoration, the seed bank should contain more species than the vegetation or valuable target species (absent from the established vegetation) should be present. We found that, although the above-ground vegetation species richness was much greater at species-rich than at species-poor sites, the number of species in the seed bank was low at all sites (Fig. 2). In terms of species number alone, the seed bank is not useful for restoration of ditch banks.
However, the seed bank composition is always dissimilar from the vegetation (Fig. 5, Table 4A), results which are well in correspondence with previous studies (e.g. Dutoit and Alard 1995; Bekker et al. 2000). Also, a large number of species are present in the seed bank only (Table 4B). Potentially, this dissimilarity could be an advantage for restoration, if the seed bank contains target species absent from the established vegetation.
Unfortunately, this appears not to be the case. In all ditch banks, we found 27 target species, 16 of which were found in the seed bank (Table 2). At any given ditch bank, we have a reasonable chance to find at least one or a few target species in the seed bank (Fig. 4b). However, only six of these species were found in the seed bank only. The chance of finding a new target species in the seed bank is therefore very small. We must concur with previous observations that the seed bank tends to be poor source of target species of grassland communities (Bekker et al. 1997).
This seems to be in contrast with the findings of Melman & van Strien (1993) and Melman (1991), who found more species and more target species in ditch banks after sod cutting. Their studies also indicated that terraces contained more species than sites with soil stripping. In other words, deep soil layers contained more species than upper layers. Our findings indicate that the number of species decreases with depth (Fig. 3a). Perhaps terraces contain much deeper and older seed bank layers than our ‘deep’ layers (5-10 cm). Moreover, since the sod cutting experiments were carried out in the mid-1980s, it is possible that species with long-term persistent seeds had still survived in the soil during the agricultural intensification process that started in the 1960s (van Burg et al. 1980; Chancellor 1986; van der Valk & Pederson 1989). Changed abiotic conditions of the terraces (less competition and higher moisture levels) may also have created a suitable environment for (target) species sprouting from remaining vegetation fragments and seeds dispersing from the surrounding vegetation.
Methodological issues
bank and 13 seeds m-2 per treatment (species-poor vs. rich sites) (Thompson et al. 1997).
The chance of finding a rare species at ditch bank level was thus small. On the other hand, Hutchings (1986) recommends a soil sample of 1-1.2 litres in grassland if the objective is to determine species composition (and not species densities). We were mainly interested in determining species composition, which means that the sample size should have been large enough. Moreover, even if we may have missed species after all, it is safe to say that the numbers present would be so low that it would not contribute to restoration in a major way.
Another potential source of error might result from some species not germinating in the glasshouse. However, the additional germination period of samples from one species-poor and one species-rich farm, yielded more individuals (average 8.1% of total seedling numbers), but no new species. Moreover, even if some species may not have germinated, we would not expect any systematic differences between species-rich and species-poor sites.
Processes determining ditch bank species richness
Although previous studies have indicated a low similarity between seed bank and vegetation, the low number of species in the seed bank at species-rich sites was surprising. After all, we were comparing sites within the same habitat-type, where the source population at species-rich sites is much larger. The low seed bank species richness at all sites and the low similarity of seed bank and vegetation (Table 4B) indicate that, although the seed bank may be important for the maintenance or dynamics of more common species, it does not seem to be the driving force behind plant species richness (Bekker et al. 2000; Kalamees & Zobel 2002).
The dissimilarity of the established vegetation and the seed bank may be a result of high disturbance rates (trampling, mowing) in ditch banks. This is supported by the fact that we see more transient and short-term persistent species in our seed bank classification (sb-type own) compared to that found in the literature (sb-type lit, Table 2). This indicates that the turnover time from seed bank to vegetation is rapid, leaving little time for the seeds to sink into deeper soil layers, thereby reducing the role of the seed bank as a source of species diversity.
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Degree of disturbance may also be important. Species-rich sites contain more species with a transient seed bank. As suggested by Bekker et al. (2000) and Geertsema (2002), this could indicate that these sites represent less dynamic, later successional stages. Although dynamics is needed to create germination opportunities, species-poor sites may be too dynamic for transient species to establish permanent populations. More frequent ditch cleaning and dumping of the sediment onto the ditch bank at species-poor sites (M. Blomqvist, personal observation) could be a contributing factor.
Options for restoration of ditch banks
This study shows that the potential for restoration of ditch bank plant species richness from the seed bank is virtually non-existent. While target species are sometimes found in the seed bank, they are hardly ever present at sites where they are absent from the vegetation. This is an important conclusion in the framework of agri-environment schemes. Farmers can get financial compensation for the reduction of agricultural intensity especially along ditches (less fertilisation, extensive mowing and grazing, careful ditch cleaning) (LNV 1995). Although this might be a suitable strategy for maintaining species numbers in species-rich vegetation (Kleijn et al. 2001), it seems unsuitable for increasing species richness in species-poor ditch banks. When the starting species richness of the ditch banks is unknown, this strategy involves certain risks. A ‘no cure, no pay’ system in species-poor situations (Musters et al. 2001) or a combination form (DLG 2000) may be useful alternatives. However, to increase the species richness or ‘value’ of the vegetation we cannot rely on the seed bank as a source of new (target) species. Other options are needed.
The ecological differences between species-poor and species-rich sites indicate that high nutrient levels, high biomass and the resulting competition are hampering seedling germination and establishment. A first step at species-poor sites will clearly be to reduce the nutrient levels of the ditch bank through an intensive mowing and grazing regime, as already suggested (Blomqvist et al. 2003). Another possibility could include the creation of terraces by sod cutting, since this reduces competition in the first year(s). However, this can only be successful if new seeds are added, either by natural dispersal from nearby species-rich source populations or by deliberate sowing.
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