An ecological basis for the management of grassland field margins

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1 Aspects of Applied Biology 108, 2011 Vegetation Management An ecological basis for the management of grassland field margins By S E SPRATT, A COOPER and T McCANN Environmental Sciences Research Institute, University of Ulster, Coleraine, Northern Ireland BT52 1SA, UK Summary Field margin management of improved and semi-improved grasslands is an option of agri-environment schemes in Northern Ireland (NI). Species communities of field margins may differ regionally and as a result of local environment and management factors. This regional scale study aimed to assess the plant species composition of the field margins of managed grasslands to develop an ecological basis for their management. Multivariate analysis showed that the field margin species assemblage has a significantly different species composition to that of the grass field. Its species composition was determined largely by a soil nutrient gradient, in particular soil nitrogen. A management objective to increase the plant species diversity value of field margins should aim to reduce the grass field nutrient input intensity or the influence of nutrient inputs on the field margin itself, while maintaining a grazing regime. Key words: Grassland field margins, plant communities, management, Northern Ireland Introduction The field margins of agricultural grasslands are subject to management regimes associated with field boundary complexes such as hedges, earth banks and ditches which are regarded as relatively species rich habitats of agricultural landscapes (Baudrey et al., 2000; Le Coeur et al., 2002). As a result, the field margin has a distinctive plant community structure (Marshall & Moonen, 2002) and provides a partial reserve for plant species (Smart et al., 2006). There is a large research literature on the community structure of arable field margins, but for grass field margins this is much less (Le Coeur et al., 1997; Feehan et al., 2005; Sheridan et al., 2009; Woodcock et al., 2009). In Northern Ireland, approximately 75% of the total land area is used for agriculture, the equivalent of 1.35 million ha -1 (DARD, 2010). The majority of this is improved and semi-improved grassland, described by the UK Broad Habitats, Improved Grassland and Neutral Grassland. These habitats cover an estimated 804,126 ha -1 (56.8%) of Northern Ireland (NI) (Cooper et al., 2009). Improved Grassland is dominated by Lolium spp., whereas Neutral Grassland is managed less intensively and is more species rich (Cooper et al., 2002). Our paper aims to assess the regional species composition of semi-improved grassland field margins as an ecological basis for their management. Materials and Methods Study area Northern Ireland is a small country (approximately 13,550 km 2 ; Cruickshank, 1997) with a wide range of soils, rocks and relief patterns and types of woodland, wetland, bog and agricultural 45

2 grassland habitats. It has a predominantly lowland landscape, with approximately two thirds of the land below about 150 m elevation (Cooper et al., 2002). The main rock types are basalt, schist, shale, granite and limestone. Glacial deposits, whose composition is largely related to the underlying rock, predominate. Land ownership and field patterns are determined largely by a small family farm structure (Hunter, 1987). Vegetation sampling The Northern Ireland Countryside Survey (NICS) was used as a basis for field margin sampling. The main Neutral Grassland Broad Habitat was selected for study. It comprised mainly the NICS Primary Habitat types A11 and A09 (McCann et al., 2009). These Primary Habitats have a management intensity intermediate between the Improved Grassland Broad Habitat and certain seminatural grassland Broad Habitat types (McCann et al., 2009). From an ecological perspective, it is central in a gradient of grassland species-richness and management intensity. A large proportion of UK Neutral Grassland is within Northern Ireland (Cooper et al., 2002). There was a 32% reduction in the area of Neutral Grassland in NI from 1986 to 1998 and a further decrease of 12.4% between 1998 and 2007, resulting mostly from agricultural conversion to Improved Grassland (Cooper et al., 2009). The NICS Primary Habitat, Species-rich dry grassland (S02), which comprises about 2% of the Neutral Grassland of NI was not included in our study. An area-proportional random sample of field-mapped A11 and A09 grassland patches from ¼ kilometre sample grid squares was selected from across NI. The sample was stratified by multivariate land classes (Cooper et al., 1986). The sampling rate was 1 quadrat (4 m 2 ) per five hectares. A total of 212 quadrats were surveyed comprising 106 field margin quadrats (1 m 4 m) and 106 paired grassland field quadrats (2 m 2 m). There were four additional field margin quadrats without a paired grassland field quadrat. Fieldwork was carried out between late June and early September Within each field, a 2 m 2 m quadrat was randomly located within a randomly selected 100 m 2 (10 m 10 m) sub-sample. The field margin quadrat (1m x 4m) was randomly located next to the nearest field boundary. The quadrat was placed adjacently with the 4m length parallel to the field boundary. If the field boundary was a close-cropped hedge, bank, wall or fence then the field margin quadrat was located 0.25 m from the edge of the widest feature. In the case of hedges with shrubby outgrowths or large overhanging trees, the quadrat was placed 0.25m into the grassland with >25% grass cover. In each quadrat, the percentage cover of plant species was estimated according to the DOMIN scale. Multivariate analysis An ecological transformation of the DOMIN scale was carried out prior to analysis. This reduced the effect of a small number of the most abundant species. The transformed values (1 5) equate to DOMIN scores 1 2, 3, 4, 5 and Mean Ellenberg indicator values for light, nitrogen, ph and moisture (Hill et al., 1999) were calculated for each quadrat by weighted averaging of the species indicator values. The mean Grime et al. (2007) plant ecological strategies of competitor, stress tolerator and ruderal were also calculated. Mean C, S and R values range between 0 and 1 based on their position within the CSR equilateral triangle. The weighted mean Ellenberg and Grime values for each quadrat are plotted passively in ordination diagrams. Field margin quadrats were classified by two-way indicator species analysis using TWINSPAN for Windows 2.3 (Hill & Šmilauer, 2005). Five pseudospecies cut-levels were set, corresponding to transformed DOMIN values. Species with a frequency of 1 or 2 in the data set were omitted on the basis of their statistically trivial contribution. The classification was stopped by inspection to give four groups. Mid-point percentage cover calculated, from the original DOMIN values were used to summarise the species composition of each TWINSPAN group. 46

3 Multivariate ordination was carried out using Canoco for Windows 4.5 (ter Braak & Šmilauer, 2002). Detrended Correspondence Analysis (DCA) was used to check gradient lengths to determine if a unimodal or linear analysis method was appropriate. Subsequently DCA was used to assess the main gradients of species composition. The direct method of Canonical Correspondence Analysis (CCA) with field margin location as an explanatory environment variable was used to test for significant differences in species composition via a restricted split-plot design Monte Carlo Permutation test. Additionally, species which were significantly more abundant in the field margin quadrats compared with the grass field quadrats were identified using the species scores for the t-value biplot. Results Inspection of species lists of the field margin and grass field quadrats gave a combined total of 164 species (excluding trees and shrubs). There were 105 species in the grass field quadrats (14 of which were unique to them) and 150 species in the field margin quadrats (59 of which were unique to them). Species nomencalture follows Stace (1997) for flowering plants and ferns and Watson (1981) for bryophytes. Species significantly more abundant in the field margin were mainly: broadleaf woodland species such as Geranium robertianum, Hedera helix, Primula vulgaris, Rubus fruticosus and Viola riviniana or competitive-ruderal species such as Cirsium vulgare and Galium aparine. The competitive tall-herb Urtica dioica, a high soil nitrogen indicator, Lathyrus pratensis, a common grassland species, Veronica chamaedrys, a creeping species of some tall grassland habitats (Grime Table 1. Species in the grass field and field margin quadrats with a frequency of occurrence > 25% or mean cover > 25% in any one quadrat set Species % Frequency (% mean cover) Grass field Field margin Holcus lanatus 88 (22.6) 96 (24.0) Ranunculus repens 74 (10.9) 63 (7.8) Agrostis stolonifera 70 (15.8) 82 (19.9) Trifolium repens 64 (8.8) 54 (4.6) Lolium perenne 62 (19.5) 50 (9.3) Anthoxanthum odoratum 43 (7.0) 43 (6.7) Cerastium fontanum 42 (0.7) 43 (0.9) Poa trivialis 42 (4.8) 42 (4.9) Juncus effusus 41 (10.5) 51 (7.9) Cynosurus cristatus 38 (5.7) 34 (3.9) Agrostis capillaris 38 (6.1) 33 (6.1) Rumex acetosa 35 (1.5) 41 (1.6) Ranunculus acris 33 (2.1) 31 (1.2) Cardamine flexuosa/hirsuta 27 (0.3) 11 (0.2) Alopecurus geniculatus 25 (3.8) 17 (1.3) Rubus fruticosus 0 (0.0) 40 (1.9) Rhytidiadelphus squarrosus 23 (1.5) 29 (1.8) Urtica dioica 0 (0.0) 25 (3.3) 47

4 et al., 2007) and species of semi-disturbed habitats, for example Vicia cracca and Vicia sepium (Grime, 2001) were also significantly more abundant in the field margins. The main species significantly more abundant in the grass fields were: the sown species Lolium perenne and Trifolium repens, the common competitive-ruderal species Ranunculus repens and Ranunculus flammula (Grime et al., 2007) (both of which can be toxic to grazing animals) and species showing stresstolerant/ruderal strategies including Carex panicea and Cardamine flexuosa/hirsuta. The grass field and field margin species abundance values (Table 1) describe botanical composition of the A11 Primary Habitat grassland resource of NI. Except for Urtica dioica and Rubus fruticosus, recorded only in the field margins, the frequency and cover of the main species were similar. A classification of the field margin quadrats gave four end groups (Table 2). The first division of the classification separated groups 1 and 2 from groups 3 and 4. TWINSPAN indicator species of groups 1 and 2 were Anthoxanthum odoratum (1), Juncus effusus (1), Rhytidiadelphus squarrosus (1), Rumex acetosa (1), Agrostis capillaris (1) and Trifolium repens (1). The only indicator species of groups 3 and 4 was Urtica dioica (1). Groups 1 and 2 had the lowest mean weighted Ellenberg ph/nitrogen and Grime competition values (Table 3) and the highest mean weighted Grime stress tolerance value. They also had the greatest mean number of species per quadrat. Group 2 represents a 50% proportion of the sample (Table 2), with group 1 representing just 13%. Group 1 was characterised by the indicator species Festuca rubra (2), Anthoxanthum odoratum (4), Rhytidiadelphus squarrosus (2), Cynosurus cristatus (3) and Trifolium pratense (2). Informal comparison suggests that group 1 showed a loose affinity with Lolio-Cynosuretum cristatii grassland (MG6) of Great Britain (Rodwell, 1992). Group 2 was characterised by Poa trivialis (1) and Agrostis stolonifera (1). It showed a loose affinity with Holco-juncetum effusi rush pasture (MG10) (Rodwell, 1992). Group 3 was characterised by Ranunculus repens (3) and Lolium perenne (1). Group 4 was characterised by Urtica dioica (1), Dactylis glomerata (1), Rumex obtusifolius (1) and Poa trivialis Table 2. Species with a frequency of > 75% in any one field margin group. TWINSPAN group indicator species are identified with an asterisk. Groups 1 and 2 were separated from groups 3 and 4 by the first TWINSPAN division Species Vegetation Group % Frequency (% mean cover) Anthoxanthum odoratum 100 * (23.5) 58 (7.6) 11 (1.4) 0 (0.0) Agrostis stolonifera 46 (7.4) 84 * (17.1) 94 (28.8) 95 (30.4) Holcus lanatus 100 (26.2) 100 (28.0) 89 (24.2) 91 (14.5) Juncus effusus 46 (7.3) 75 (12.6) 22 (1.0) 14 (1.3) Ranunculus repens 54 (3.3) 58 (5.1) 100* (22.4) 59 (5.6) Rhytidiadelphus 85 * (7.3) 38 (1.7) 0 (0.0) 0 (0.0) squarrosus Urtica dioica 15 (0.4) 5 (0.2) 28 (3.6) 77 * (12.4) Mean no. of species per quadrat Number of quadrats Proportion of sample (%) Note: TWINSPAN indicator species not in the table along with their % frequency (% mean cover) are: Group 1 Festuca rubra 62 (7.4), Cynosurus cristatus 69 (13.3) and Trifolium pratense 46 (1.5); Group 2 Poa trivialis 58 (6.6); Group 3 Lolium perenne 61 (15.4); Group 4 Dactylis glomerata 41 (5.5), Rumex obtusifolius 41 (2.0) and Poa trivialis 45 (7.1). 48

5 (1). These groups showed loose affinity with Lolio-Plantaginion leys and related grasslands (MG7) (Rodwell, 1992). In group 4, the frequency and cover of Urtica dioica, was particularly high. The groups lay along the first axis of the DCA sample ordination (Fig. 1(a)) in the order 1, 2, 3 and 4, with groups 1 and 2 separated by the second axis. Axis 1, with an eigenvalue of accounted for 6.7% variance in the species data and 24.1% variance of the species-environment relation. Axis 2, with an eigenvalue of accounted for an additional 4.5% species data variance at a cumulative variance of 11.2% and accounted for an additional 8.9% of the species-environment relation with a cumulative variance of 32.9%. Axis 3 had an eigenvalue of Table 3. Field margin mean weighted Ellenberg light, water, ph and nitrogen indicator species values and Grime competitor, stress tolerant and ruderal strategies values for each TWINSPAN group Vegetation group Ellenberg and Grime values Mean (standard error) Light 6.9 (0.1) 7.0 (0.0) 6.9 (0.0) 6.8 (0.00) Water 5.7 (0.0) 5.9 (0.0) 6.0 (0.1) 5.7 (0.0) ph 5.4 (0.1) 5.6 (0.0) 6.1 (0.1) 6.4 (0.0) Nitrogen 4.6 (0.1) 5.1 (0.1) 5.7 (0.1) 6.1 (0.1) Competition 0.4 (0.0) 0.5 (0.0) 0.5 (0.0) 0.5 (0.0) Stress tolerance 0.4 (0.0) 0.3 (0.0) 0.2 (0.0) 0.2 (0.0) Ruderal 0.4 (0.0) 0.5 (0.0) 0.5 (0.0) 0.4 (0.0) Mean no. of species per quadrat Number of quadrats Mean Ellenberg soil nitrogen and ph indicator values and the mean Grime competitor values showed a strong positive correlation with the first axis of a DCA ordination (Fig. 1(b)). Mean Grime stress tolerator values showed a strong negative correlation with axis 1. The main ordination axis, therefore, represents an inferred gradient of increasing soil nutrient status. High values of the mean Ellenberg soil moisture is correlated with axis 2. This axis, therefore, separates samples along a soil wetness gradient. An initial DCA resulted in a gradient length of 3.85 indicating a unimodal method of analysis. Canonical Correlation Analysis (CCA) ordination identified the field margin quadrats to have a significantly different species composition compared with the grass field quadrats (F = 2.71). (P = 0.002). The t-value biplot method showed that there were 57 species more abundant in the field margin quadrats of which 12 were significantly more abundant. They were Cirsium vulgare, Galium aparine, Lathyrus pratensis, Rubus fruticosus, Urtica dioica, Veronica chamaedrys, Vicia cracca and Vicia sepium (mainly scramblers and tall herbs of open habitats) and Geranium robertianum, Hedera helix, Primula vulgaris and Viola riviniana (mainly broadleaf woodland species). There were 41 species more abundant in the grass field quadrats, six of which were Note: Species (Δ) labelled as follows: Agrocapi Agrostis capillaris, Agrostol - Agrostis stolonifera, Anthodor - Anthoxanthum odoratum, Cynocris Cynosurus cristatus, Dactglom Dactylis glomerata, Festrubr Festuca rubra, Holclana Holcus lanatus, Junceffu Juncus effusus, Lolipere Lolium perenne, Poa triv Poa trivialis, Ranurepe Ranunculus repens, Rhytsqua - Rhytidiadelphus squarrosus, Rumeacsa Rumex acetosa, Rumeobtu Rumex obtusifolius, Trifprat Trifolium pratense, Trifrepe Trifolium repens and Urtidioi Urtica dioica. Samples grouped according to TWINSPAN groups and labelled as follows: Group 1 ( ), Group 2 (X), Group 3 (+) and Group 4 ( ). 49

6 Fig. 1(a). First and second axes of a DCA sample ordination. Groups of quadrats delimited by a TWINSPAN classification are outlined. Species with a frequency of > 75% in any one group and all TWINSPAN indicator species are labelled. Fig 1(b). First and second axes of a DCA sample ordination with passive overlay of environment variables as follows species diversity (mean number of species per quadrat), Ellenberg indicator values for moisture, nitrogen, ph and light and Grime competitor, stress-tolerant and ruderal life strategies. Species (Δ) labelled as above. Scaling factor of explanatory variables = 6. 50

7 significantly more abundant. These were Cardamine flexuosa/hirsuta, Carex panicea, Lolium perenne, Ranunculus flammula, Ranunculus repens and Trifolium repens, all common species of agricultural grasslands. Discussion Field margin management of improved and semi-improved grasslands is an option of agrienvironment schemes in Northern Ireland (DARD, 2008). The Primary Habitat sampled in our paper largely corresponds to semi-improved grasslands. The assumption in sampling these less intensively managed grasslands, is that their field margins have plant species assemblages of biodiversity interest to the agriculture industry. We have shown that across Northern Ireland as a whole, the field margins are qualitatively and quantitatively different to that of the managed part of the grass field. They are characterised by species covering a wide range of environmental tolerances and life strategies. The high abundance values of Rubus fruticosus and Urtica dioica in field margins, compared with grass fields suggests that woody outgrowth and eutrophication are key features of the margin. Other differences in species composition such as the greater abundance of the competitive ruderal species Cirsium vulgare, highlight field margins as a source of grassland weed species. Also preferential to field margins, are species of biodiversity interest such as those characteristic of broadleaf woodland habitats. This may be a related to the oceanic Irish climate. Groups 1 and 2 of the field margin classification, characterised by stress-tolerant species of low nutrient status soils, had the greatest biodiversity value. Group 1 field margins were associated with species of better-drained soils, such as Festuca rubra and Cynosurus cristatus. Group 2 field margins were associated with species of poorly-drained soils, such as Juncus effusus. The use of an area-proportional sampling system used means that the frequency of field margin quadrats in each class is directly proportional to their occurrence in the countryside. Therefore, we estimate that group 1 represents 13% of field margins and that group 2 represents 50% of field margins. These two groups, in particular group 1, have the greatest potential value for targeting agri-environment prescriptions. The remaining 47% of field margins (groups 3 and 4) have a relatively low plant species diversity, not greatly different from the managed grass field. The key latent environment variable influencing field margin species composition (identified using ordination), is a gradient of soil nutrient status (in particular soil nitrogen). This suggests that the species diversity of field margins is directly related to management inputs. We conclude that a management objective to increase the plant species diversity value of field margins should aim to reduce the grass field nutrient input intensity or the influence of nutrient inputs on the field margin itself. An integral element of this reduced management, however, should be stock grazing, without which competitive species and the build up of vegetation biomass would work to eliminate species and reduce species diversity. Acknowledgements We thank the University of Ulster field surveyors who assisted with vegetation sampling. DARD funded the research through a postgraduate studentship. References Baudrey J, Bunce R G H, Burel F Hedgerows: An international perspective on their origin, function and management. Journal of Environmental Management 60:

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