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Keywords:

  • clonality;
  • conservation;
  • dispersal;
  • diversity;
  • fragmentation;
  • land use;
  • longevity;
  • plant traits;
  • seed bank;
  • seed size

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • 1
    In north European rural landscapes, abandonment of small farms and agricultural intensification have led to a decline in semi-natural grassland, with associated biodiversity loss. Although species richness response to land-use change in rural landscapes is relatively well studied, few have examined its effects on plant species composition over time.
  • 2
    In this study, four life-history traits associated with spatiotemporal dispersal were analysed: seed size, seed dispersal attributes (e.g. awns, wings), seed bank persistence and plant longevity (annuals, perennial with and without clonal ability). I investigated how differences in distribution of these traits among plants in semi-natural grasslands are related to current and historical landscape configuration.
  • 3
    The distributions of two out of the four investigated traits, longevity and seed bank persistence, were correlated with grassland connectivity and area, whereas seed size and seed dispersal attributes were not. The proportion of short-lived plants was positively associated with current grassland connectivity and grassland area, whereas long-lived species, with and without clonal ability, were unrelated to current grassland connectivity and area.
  • 4
    In contrast, short-lived plants were not affected by historical grassland connectivity, but the proportion of long-lived clonal plants was negatively associated with high historical grassland connectivity and large grassland area. In addition, the proportion of species with long persistence in the seed bank was negatively associated with historical grassland connectivity.
  • 5
    The result suggests that there are two main strategies to persist in response to landscape fragmentation: either persist in the seed bank or disperse vegetatively. The higher sensitivity to isolation among short-lived plants and plants without clonal ability calls attention to the importance of considering life-history traits to understand plant community dynamics fully over time. In the long term, reduction in historical connectivity and grassland area will thus create a grassland community dominated by clonal long-lived plants and plants with a persistent seed bank.
  • 6
    This study shows that both spatial and temporal effects of landscape configuration are important factors structuring local plant species composition in grasslands. The results bring important insights to our understanding of large-scale ecological processes, and are also highly relevant to biodiversity conservation in fragmented agricultural landscapes worldwide.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Land-use change, leading to habitat deterioration and fragmentation at various spatial scales, has been identified as a major cause behind declining biodiversity (Hanski 1999; Harrison & Bruna 1999; Eriksson & Ehrlén 2001; Loreau et al. 2001; Balmford et al. 2005). While many species respond to changes in land use instantly, in others the response may be more delayed. Such delay implies that, although the species are still present, the conditions for species persistence are no longer met (Tilman et al. 1994; Hanski & Ovaskainen 2002). Thus, a critical issue to understanding patterns of plant distribution is not only to assess the effects of spatial structure, but also to include a temporal scale of the plant species response to ongoing landscape transformation (Eriksson & Ehrlén 2001; Foster 2002; Lindborg & Eriksson 2004a; Helm et al. 2006). Although the importance of landscape structure for biodiversity is recognized, little is known about the effects of landscape structure on the distribution of life-history traits and species composition.

In Scandinavia, the most extensive land-use changes took place in the agricultural landscape. Historically, semi-natural grasslands covered large areas, due to the need for grazing grounds and for production of winter fodder for livestock (Ekstam & Forshed 2000; Eriksson et al. 2002). Semi-natural grasslands were created by agricultural management (mowing or grazing) and were not fertilized or subjected to ploughing. A major part of the species richness in northern Europe is associated with the rural landscape, especially semi-natural grasslands (Eriksson & Eriksson 1997; Austrheim et al. 1999). During the last century land-use practices in semi-natural grasslands have changed drastically (Ekstam & Forshed 2000), leading to the abandonment of a large number of small farms and, as a result, a decline in grazed semi-natural grasslands. Remaining habitats suffer from deterioration and fragmentation, and species and populations connected to these remnant habitats are declining. In order to prevent extinctions and to recreate a traditional landscape structure, restoration of these habitats has received increased attention in recent decades (Walker et al. 2004; Lindborg 2006).

In fragmented landscapes dispersal is a crucial process that may explain large-scale community patterns (e.g. Tackenberg et al. 2003; Ozinga et al. 2004). Plant species disperse both in space and in time, and trade-offs between seed dispersal and persistence in soil have been suggested (Vanable & Brown 1988; Rees 1993). Other trade-offs have also been recognized, and Ehrlén & van Groenendael (1998) stressed that long-lived plants may have a limited ability to colonize new patches. A positive effect of seed mass on seedling emergence has been documented in several studies (e.g. Ehrlén & Eriksson 2000; Jakobsson & Eriksson 2000), while small seeds have a higher seed bank persistence than large seeds (Thompson et al. 1997). The life-history traits often examined in a dispersal context are those associated with seed production, e.g. seed size (Jakobsson & Eriksson 2000), seed bank persistence (Thompson et al. 1997; Bekker et al. 2000), and seed dispersal attributes and vectors (Hughes et al. 1994; Kiviniemi & Eriksson 2002; Tackenberg et al. 2003; Ozinga et al. 2004). However, clonal reproduction, often overlooked when considering dispersal in plants, may be important for both survival and dispersal at larger regional scales in the long term (Eriksson & Ehrlén 2001; Honnay & Bossuyt 2005). In grassland systems, long-lived plants, often with clonal propagation, and plants with long-lived seed banks, tend to build up remnant population systems in abandoned or isolated grasslands. Here, local populations may persist long enough to bridge unfavourable phases of successional development (Eriksson 1996; Lindborg et al. 2005). Such remnant populations may enhance the persistence of a plant community and help to explain cases of high local species diversity, e.g. the semi-natural grasslands of northern Europe (Eriksson & Eriksson 1997; Austrheim et al. 1999; Eriksson et al. 2002). However, most species in grasslands are not long-lived in the seed bank (Thompson et al. 1997; Eriksson & Eriksson 1997), and studies show that rare species are often under-represented in the seed bank compared with their abundance in the vegetation (Bakker & Berendse 2001). Thus, there is no clear relationship between species abundance in the vegetation above ground and in the seed bank. Indeed, dispersal limitation may play an important role in the patchy distribution of species (Tilman et al. 1994; Ehrlén & Eriksson 2000; Turnbull et al. 2000).

In order to find general explanations to plant community patterns and processes, there is an ongoing effort to identify plant traits related to land use for the purpose of predicting vegetation dynamics (McIntyre et al. 1995; Bullock et al. 2002; Westoby et al. 2002; Verheyen et al. 2003). Several traits have been recognized as important for structuring local plant communities in grazed and formerly grazed vegetation, e.g. seed size, plant height, plant longevity, phenology and specific leaf area (SLA) (Weiher et al. 1999; McIntyre & Lavorel 2001; Lindborg & Eriksson 2005). Although studies suggest that historical grassland connectivity has an impact on present species richness in rural landscapes (Lindborg & Eriksson 2004a; Helm et al. 2005), the effects of habitat connectivity and area on the distribution of plant life-history traits at large spatiotemporal scales have not been not fully investigated. By analysing the relationship of current and historical landscape structure to the distribution of different life-history strategies among species in a plant community, links between plant community patterns and ecological processes can be detected. The main objective of the present study was to investigate if the proportion of different life-history traits in plants among semi-natural grasslands is associated with current and historical landscape patterns. Species-rich grazed semi-natural grasslands were used as target sites, and four life-history traits, suggested to affect species dispersal and persistence abilities, were examined among grassland communities: seed size, seed dispersal attribute, seed bank persistence and plant longevity.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

study sites

The study was conducted in three provinces in the south-eastern part of Sweden: Östergötland, Södermanland and Uppland (57°50′−60°28′N, 15°10′−18°25′E). The study sites consist of semi-natural grasslands located in landscapes dominated by arable fields and managed forests, habitats that are generally inhospitable to most of the plant species in the semi-natural grasslands (Cousins & Eriksson 2001). The field study was conducted during July and August 2001. A total of 25 semi-natural grasslands were investigated. The site areas ranged from 3 to 25 ha. At each site 10 1-m2 plots were randomly distributed, and the occurrence and abundance of vascular plants were recorded in each plot (cf. Lindborg & Eriksson 2004b). In the study areas chosen, dry-mesic semi-natural grasslands are very homogeneous and have a high diversity at a small scale, with a high alpha diversity and relatively low beta and total diversity (e.g. Eriksson & Eriksson 1997; Lindborg & Eriksson 2004a). Hence, the comparatively small-scale sampling used (10 1-m2 plots) will pick up a large fraction of the plant species occurring in semi-natural grasslands, and may therefore be representative of total species richness at a site.

field data

The field data consist of species lists and plot frequency (%) of all vascular plant species detected at each site. The growth-form of the species (grass, herb, tree) was also recorded. Species were analysed with regard to four different life-history traits: seed size, seed bank persistence, dispersal attribute and plant longevity. Seed bank persistence for each species was classified as: (i) transient (< 1 year), (ii) short-term persistent (> 1 year but < 5 years) and (iii) long-term persistent (> 4 years) (Thompson et al. 1997). The species were divided into three major dispersal categories: wind dispersal (pappus, wing), animal dispersal [including endozoochorous dispersal (fleshy fruits), adhesive dispersal (burrs, hooks, awns), ant dispersal] and unassisted (no visible dispersal attribute). Longevity was divided into three groups: perennial plants with clonal ability, perennial plants without clonal ability, and annual plants (representing both strictly annual and biennial plants). All trait data, i.e. seed size, seed bank persistence, dispersal vector data and longevity, were gained from the literature (Eriksson & Eriksson 1997; Thompson et al. 1997; Kiviniemi & Telenius 1998; Dupré & Ehrlén 2002) and ongoing studies in the region (H. Quested, personal communication).

landscape analysis

The landscape surrounding each target site was analysed in a Geographical Information System (ArcView GIS 3.3). For each site two cadastral maps from the Lantmäteriet (The Swedish partner in the European Land Information Service) representing different time layers were analysed: present-day landscape and the landscape approximately 50 years ago (scale 1 : 10 000). The present-day map was delivered digitized, but the map for 50 years ago was digitized and rectified using ArcView. The landscape was analysed with regard to focal patch area and grassland connectivity at two different spatial scales, defined by circles with radii of 1 km and 2 km, respectively, from the centre of each target site. These two spatial scales were chosen to represent (i) the range from an area that surrounds the target site but still includes other potential sites for grassland species (1-km scale: 3.1 km2), and (ii) an area that includes the farm with the target site, and other farms in the surrounding area (2-km scale: 12.5 km2).

Connectivity of each target site, surrounded by k semi-natural grassland sites within the examined circle area, was defined as: Σ exp(–αdj)Aj, where Aj is the area of the jth grassland located at a distance dj from the target site, for j = 1 to k. This measure defines connectivity as the sum of areas with suitable habitat within the analysed circle, weighted by their distance from the target site (Hanski 1999). Dispersal over long distances is difficult to predict with the use of ordinary dispersal curves (e.g. Cain et al. 1998, 2000; Clark 1998; Bullock et al. 2002), although they might be of importance for migration on a longer time-scale. Thus, the value of the constant α, describing how fast the number of migrants declines with increasing distance, was set to 1 for all plant species in the community.

statistical analysis

The actual change in grassland area and connectivity from the 50-year-old landscape to the current landscape was examined with a paired t-test. Before the trait analysis, the proportions of each trait were calculated for each plot. The species found in any of the 250 plots, i.e. 10 plots in each of the 25 sites, were categorized according to the four life-history traits: three discrete variables (seed bank persistence, dispersal attribute and plant longevity), and one continuous variable (seed size). By calculating the number of species belonging to a specific category within a discrete variable in each plot, the proportion (%) of each trait could be analysed. The proportion of traits was arcsine-transformed before analysis. Seed size for each plot was estimated using the median seed size value of all species occurring in the plot. In addition, to detect differences in species composition among sites, growth-form (grass, herb, tree) was also analysed with anova, where the plots were used as replicates and each trait was analysed separately.

To examine the association between traits and grassland area and connectivity, the proportion of each trait was analysed separately in a multiple regression. The mean of each trait for the 10 plots within a site was used as a response variable. The focal patch area and grassland connectivity at 1-km and 2-km scale for each time step were used as predictors using sequential sum of squares (type 1) regression. In order to detect multicollinearity, the variance inflation factors (VIFs) for each predictor variable were computed. Values exceeding 10 in linear regressions were regarded as indications of a multicollinearity problem. All statistical analyses were performed in statistica 7.1.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

From 50 years ago to the present, grassland area declined by more than 50% within the study sites (t = 6.91, d.f. = 29, P < 0.001). Grassland connectivity also decreased drastically over the same 50-year period (> 60%) (t = 8.73, d.f. = 29, P < 0.001). In total, 177 plant species were recorded at the study sites (Table 1). Of the 10 most common species there were seven herbs and three grasses. No difference in growth-form distribution among sites was detected (grass: F25,224 = 0.37, P = 0.88; herb: F25,224 = 0.41, P = 0.96; tree: F25,224 = 1.02, P = 0.76). Twenty species were annuals or biennials (with no clonal ability) and c. 50% of those had a persistent seed bank (Table 1). Twenty-six perennial species were clonal and had seeds with long seed bank persistence. All of the independent predictors were within acceptable ranges according to the collinearity diagnostics (VIF) – present landscape: connectivity 1 km (2.39), connectivity 2 km (2.37), area (1.01); historical landscape: connectivity 1 km (1.02), connectivity 2 km (4.25), area (4.29).

Table 1.  All plant species found in the 25 study sites. The abundance (%) is the mean relative abundance of a species calculated based on the number of occurrences of that species at all sites divided by the total number of occurrences in plots for all species at all sites. Numbers in parentheses are the 10 most common species in the study. All species with an abundance lower that 0.1% are noted < 0.1%. Growth form is divided into tree, herb and grass. Longevity is divided into p = perennials without clonal ability, p & c = perennials with clonal ability and a = annual and biennial plants. Seed bank persistence is categorized into three groups: 1, 2 and 3, where 1 is transient and 3 is the longest seed bank persistence. Dispersal is divided into five dispersal vector groups: wind, endo (endozoochorous), ad (adhesive), ant and other (unassisted)
SpeciesAbundance (%)Growth formLongevitySeed bankDispersalSeed weight (mg)
Acer platanoides< 0.1Treep2Wind10.00
Achillea millefolium (3)5.3Herbp & c2Other0.13
Achillea ptarmica0.2Herbp & c2Other0.20
Agrimona eupatoria0.5Herbp1Ad3.61
Agrostis capillaris (1)9.1Grassp3Other0.06
Agrostis gigantea0.2Grassp & c2Other0.05
Agrostis stolinifera0.6Grassp & c3Other0.02
Ajuga pyramidalis< 0.1Herbp & c3Other1.37
Alchemilla glabra0.2Herbp & c3Other0.49
Alchemilla glaucescens0.9Herbp & c3Other0.49
Alchemilla monticola0.2Herbp & c3Other0.49
Allium oleraceum< 0.1Herbp1Other0.73
Alnus glutinosa< 0.1Treep3Wind1.37
Alopecurus geniculatus< 0.1Grassp & c3Other0.68
Alopercurus pratensis1.4Grassp & c3Other0.68
Anemone nemorosa0.5Herbp & c1Other3.00
Antennaria dioica< 0.11Herbp & c1Other0.05
Anthoxanthum odoratum1.1Grassp2Wind0.29
Anthriscus sylvestris1.8Herbp2Other2.55
Anthyllis vulneraria< 0.1Herba3Other3.31
Arenaria serpyllifolia< 0.1Herba2Other0.06
Arrhenatherum pratensis< 0.1Grassp2Other2.39
Arrhenahterum pubescens0.6Grassp2Other1.38
Betula pendula0.1Treep3Wind0.15
Bistorta vivipara< 0.1Herbp & c1Other3.39
Briza media0.2Grassp2Other0.23
Calluna vulgaris< 0.1Herbp & c3Other0.03
Campanula persicifolia< 0.1Herbp3Other0.10
Campanula rotundifolia0.9Herbp3Other0.05
Cardamine pratensis< 0.1Herbp & c1Other1.3
Carex caryophyllea0.4Grassp1Other0.97
Carex flacca< 0.1Grassp1Other0.88
Carex hirta0.2Grassp & c3Other1.56
Carex nigra0.3Grassp & c2Other0.50
Carex ovalis< 0.1Grassp2Other0.39
Carex pallescens0.7Grassp3Other1.56
Carex panicea0.3Grassp2Other1.73
Carex spicata0.4Grassp2Other2.50
Carlina vulgaris< 0.1Herba1Wind1.44
Carum carvi0.2Herba1Other2.25
Centaurea jacea0.5Herbp2Other1.06
Cerastium fontanum0.9Herbp3Other0.12
Cirsium arvense0.4Herbp3Wind1.20
Cirsium palustre< 0.1Herba3Wind2.00
Convallaria majalis0.4Herbp & c1Endo15.10
Dactylis glomerata1.6Grassp2Other0.51
Dactylorhiza latifolia< 0.1Herbp1Wind0.05
Danthonia decumbens0.3Grassp2Ant1.30
Daucus carrota< 0.1Herbp2Ad0.99
Deschampsia cespitosa1.4Grassp2Other0.30
Deschampsia flexuosa1.0Grassp2Other0.43
Dianthus deltoides< 0.1Herbp3Other0.20
Elymus repens1.2Grassp & c2Other2.02
Epilobium angustifolium0.2Herbp3Wind0.05
Epilobium montanum0.2Herbp3Wind0.12
Epilobium palustre< 0.1Herbp & c3Wind0.13
Erophila verna< 0.1Herba3Other0.03
Euphrasia nemorosa< 0.1Herba2Other0.16
Euphrasia stricta< 0.1Herba2Other0.17
Festuca ovina (2)6.6Grassp2Other0.13
Festuca pratensis0.7Grassp2Other1.25
Festuca rubra0.2Grassp & c2Other0.79
Filipendula ulmaria0.7Herbp & c2Other0.30
Filipendula vulgaris1.3Herbp2Ad0.74
Fragaria vesca0.5Herbp & c2Endo0.35
Fragaria viridis< 0.1Herbp & c2Endo0.44
Galium boreale1.2Herbp1Ad0.78
Galium palustre0.4Herbp3Other1.10
Galium saxatile< 0.1Herbp2Other0.70
Galium uliginosum0.5Herbp2Other0.29
Galium verum (7)2.2Herbp2Other0.66
Gentianella campestris< 0.1Herba1Other0.20
Geranium sylvaticum0.2Herbp1Other5.50
Geum rivale1.2Herbp & c2Ad1.34
Geum urbanum< 0.1Herbp & c2Other1.30
Glechoma hederacea< 0.1Herbp & c3Other0.69
Gnaphalium sylvaticum< 0.1Herbp1Wind0.40
Helianthemun nummularium0.2Herbp2Other0.90
Hieracium pilosella0.4Herbp1Wind0.44
Heiracium sylvaticiformia< 0.1Herbp1Wind0.45
Hieracium vulgatiformia0.3Herbp1Wind0.45
Hypericum maculatum0.3Herbp3Other0.08
Hypericum perforatum0.4Herbp3Other0.04
Hypochoeris maculata< 0.1Herbp1Wind1.10
Juncus articulatus< 0.1Grassp & c3Other0.02
Juncus compressus< 0.1Grassp & c3Other0.03
Juncus conglomeratus< 0.1Grassp & c3Other0.04
Juncus effusus< 0.1Grassp & c3Other0.05
Juncus ranarius< 0.1Grassp & c3Other0.06
Juniperus communis< 0.1Treep1Endo16.00
Knautia arvensis0.1Herbp2Ant2.40
Laserpitium latifolium< 0.1Herbp1Wind10.20
Lathyrus linifolius1.7Herbp & c3Other8.00
Lathyrus palustris< 0.1Herbp & c1Other15.00
Lathyrus pratensis1.5Herbp & c3Other10.10
Leontodon autumnalis0.6Herbp2Wind0.69
Leucanthemum vulgare0.3Herbp & c3Other0.44
Linum catharticum< 0.1Herba3Other0.13
Lotus corniculatus0.8Herbp3Ant1.30
Luzula campestris0.7Grassp & c3Ant0.66
Lychnis viscaria< 0.1Herbp3Ant0.05
Myosotis arvensis< 0.1Herba3Ant0.40
Myosotis laxa< 0.1Herba3Other0.50
Myosotis ramosissima< 0.1Herba3Other0.60
Oxalis acetosella0.2Herbp2Other1.10
Phleum pratense1.2Grassp2Wind0.52
Pimpinella major< 0.1Herbp2Other1.87
Pimpinella saxifraga1.0Herbp1Other1.10
Plantago lanceolata1.4Herbp2Ad1.90
Plantago major0.2Herbp3Ad0.20
Plantago media< 0.1Herbp2Ad0.20
Poa annua0.3Grassa2Other0.26
Poa nemoralis< 0.1Grassp & c2Other0.21
Poa pratensis (8)2.1Grassp & c3Other0.25
Polygala amarella< 0.1Herbp3Ant8.80
Polygala vulgaris0.4Herbp3Ant1.90
Potentilla anserina0.2Herbp & c3Other0.09
Potentilla argentea< 0.1Herbp3Other0.08
Potentilla erecta1.1Herbp3Other0.04
Potentilla reptans< 0.1Herbp & c3Other0.30
Potentilla tabernaemontani< 0.1Herbp3Other0.20
Primula veris0.4Herbp3Other0.90
Prunella vulgaris0.4Herbp2Other0.60
Pyrola rotundifolia< 0.1Herbp2Wind0.08
Quercus robur0.2Treep1Endo20.00
Ranunculus acris (6)3.7Herbp3Other1.38
Ranunculus auricomus0.3Herbp3Other2.50
Ranunculus bulbosus0.2Herbp3Other2.48
Ranunculus reptans0.6Herbp & c2Other1.60
Rhinanthus minor0.3Herba2Wind1.80
Rosa canina< 0.1Herbp3Endo20.00
Rosa dumalis< 0.1Herbp3Endo25.00
Rosa villosa< 0.1Herbp3Endo17.00
Rubus corylifolii< 0.1Herbp & c3Endo1.80
Rubus idaeus0.2Herbp & c3Endo5.00
Rubus saxatilis< 0.1Herbp & c3Endo10.50
Rumex acetosa1.5Herbp & c2Wind0.52
Rumex acetosella< 0.1Herbp3Wind0.45
Rumex aquaticus< 0.1Herbp3Wind0.70
Rumex crispus< 0.1Herbp3Wind0.33
Sagina procumbens< 0.1Herbp & c2Other0.02
Saxifraga granulata< 0.1Herbp3Other0.03
Sedum acre< 0.1Herbp3Other0.03
Sedum album< 0.1Herbp3Other0.04
Sedum reflexum< 0.1Herbp3Other0.05
Sedum sexangulare< 0.1Herbp3Other0.06
Sedum telephium< 0.1Herbp1Other0.07
Silene nutans< 0.1Herbp3Other0.34
Solidago virgaurea< 0.1Herbp3Wind0.52
Stellaria graminea (9)2.0Herbp3Other0.27
Stellaria media< 0.1Herba3Other0.67
Succisa pratensis0.8Herbp2Other1.50
Tanacetum vulgare0.8Herbp2Wind0.39
Taraxacum erythrosperma< 0.1Herbp1Wind0.37
Taraxacum vulgare1.2Herbp1Wind0.50
Thymus serpyllum< 0.1Herbp3Other0.16
Tragopogon pratensis0.2Herba1Wind0.50
Trifolium arvense0.7Herba3Other0.38
Trifolium hybridum< 0.1Herbp3Other0.48
Trifolium medium0.2Herbp & c3Other0.54
Trifolium pratense (10)1.9Herbp & c3Other1.60
Trifolium repens (4)5.2Herbp3Other0.20
Urtica dioica< 0.1Herbp & c3Other0.15
Vaccinium myrtillus0.4Herbp & c1Endo0.30
Vaccinium vitis-idaea< 0.1Herbp & c1Endo0.30
Veronica arvensis< 0.1Herba3Other0.08
Veronica chamaedrys (5)4.5Herbp3Other0.20
Veronica officinalis0.5Herbp3Other0.13
Veronica scutellata< 0.1Herbp3Other0.20
Veronica serpyllifolia0.4Herbp & c3Other0.03
Veronica spicata< 0.1Herbp3Other0.20
Vicia cassubica< 0.1Herbp3Other10.00
Vicia cracca1.5Herbp1Other12.80
Vicia lathyroides< 0.1Herbp3Other15.00
Vicia sepium0.3Herbp3Other16.00
Viola canina0.8Herbp3Ant0.90
Viola tricolor0.2Herba3Ant0.50

seed size and seed dispersal vector

The seed size of focal plants ranged from 0.02 mg to 25 mg. The majority (64%) of species had no visible dispersal attribute, i.e. unassisted dispersal (Table 1). No association between seed size or seed dispersal vector with either current or historical grassland connectivity, or area size, was detected (Table 2).

Table 2.  The effect of grassland connectivity and site area on distribution of the traits seed sizes (a) and seed dispersal vectors (wind, animal, unassisted) (b) in semi-natural grasslands at two different spatial scales and two different time-layers: present and 50 years ago. The area and grassland connectivity at 1-km and 2-km scale for each time step were used as predictors using seq. ss tests (type 1) (d.f. = 21)
(a)PredictorsSpatial scaleSeed size
Time-scaleFPbeta
PresentConnectivity1 km0.010.910.02
2 km0.590.450.15
Area 0.270.600.11
1950Connectivity1 km2.950.10−0.33
2 km0.120.73−0.07
Area 1.880.18−0.27
(b)PredictorsSpatial scaleWindAnimalUnassisted
Time scaleFPbetaFPbetaFPbeta
PresentConnectivity1 km0.060.80−0.050.060.780.080.040.850.04
2 km0.940.340.190.560.450.150.380.54−0.13
Area 0.150.700.080.050.830.040.670.42−0.17
1950Connectivity1 km1.730.200.260.140.70−0.080.330.570.12
2 km0.560.450.150.020.91−0.020.010.970.01
Area 1.580.220.250.010.96−0.010.050.830.05

seed bank persistence

Many of the species (49%) had a seed bank persistence longer than 4 years (Table 1). However, the distribution of seed bank persistence was unrelated to current grassland connectivity and area (Table 3). By contrast, long seed bank persistence was negatively associated with historical grassland connectivity at 1-km and 2-km radii (Table 3). No association between historical area and seed bank persistence was detected (Table 3).

Table 3.  The effect of grassland connectivity and site area on distribution of species with different seed bank persistence in semi-natural grasslands at two different spatial scales and two different time-layers: present and 50 years ago. Seed bank persistence is classified as: (1) transient (< 1 year), (2) short-term persistent (> 1 year but < 5 years) and (3) long-term persistent (> 4 years). The area and grassland connectivity at 1-km and 2-km scale for each time step were used as predictors using seq. ss tests (type 1) (d.f. = 21). Significant values are marked in bold type
Time scalePredictorsSpatial scaleSeed bank 1Seed bank 2Seed bank 3
FPbetaFPbetaFPbeta
PresentConnectivity1 km0.120.73−0.072.050.170.280.020.87−0.03
2 km0.750.39−0.170.030.860.030.490.49−0.14
Area 1.590.21−0.250.010.910.020.520.47−0.11
1950Connectivity1 km0.700.41−0.170.280.580.1136.70.0010.52
2 km0.900.31−0.111.160.290.2130.10.0010.24
Area 0.130.71−0.070.060.810.051.080.31−0.20

plant longevity

The majority (69%) of the species found at all sites were not clonal (Table 1). The proportion of annual plants was positively associated with current connectivity, at 1-km and 2-km radius, as well as the grassland area (Table 4). No such association was detected among perennials with or without clonal ability. Neither annuals nor non-clonal perennials were significantly associated with historical connectivity or area. By contrast, the proportion of perennials with clonal ability was negatively associated with connectivity at both scales, and with grassland area (Table 4). The proportion of perennials without clonal ability was negatively associated with historical connectivity at 1-km scale only.

Table 4.  The effect of grassland connectivity and site area on distribution of plant longevity [annuals, perennials (non-clonal), perennials (clonal)] in semi-natural grasslands at two different spatial scales and two different time-layers: present and 50 years ago. The area and grassland connectivity at 1-km and 2-km scale for each time step were used as predictors using seq. ss tests (type 1) (d.f. = 21). Significant values are marked in bold type
Time scalePredictorsSpatial scaleAnnualPerennial (non-clonal)Perennial (clonal)
FPbetaFPbetaFPbeta
PresentConnectivity1 km5.520.030.440.100.80−0.090.510.42−0.16
2 km5.340.020.430.320.61−0.130.060.91−0.01
Area 8.900.010.530.150.78 0.060.910.33−0.14
1950Connectivity1 km0.140.700.124.220.020.396.540.020.47
2 km1.140.290.230.580.42−0.175.110.040.42
Area 0.250.640.112.830.12−0.356.520.020.45

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Plant life-history traits have been recognized as important for structuring grassland plant communities at local scales (Bullock et al. 2001, 2002; McIntyre & Lavorel 2001). This study examined if the distribution of life-history traits among grassland plants, associated with dispersal and persistence, was related to large-scale current and historical landscape structure. While plant longevity and seed bank persistence were associated with grassland connectivity and area within current or historical landscapes, seed size and seed dispersal vectors were not. Short-lived species were positively associated with current grassland connectivity and grassland area, but not with historical grassland connectivity or area. By contrast, perennials both with and without clonal ability were positively associated with historical landscape configuration. These results are congruent with theories suggesting that short-lived species react quicker to changes in land use and are more susceptible to isolation (e.g. Hanski 1999; Matthies et al. 2004).

Long-lived plants with clonal ability were negatively associated with grassland connectivity and area 50 years ago. This implies that the smallest and most isolated grasslands in the historical landscape have a larger fraction of species with clonal ability than current grassland. This is similar to the results of a study by Lindborg & Eriksson (2004a), in which high historical grassland connectivity was positively associated with present species richness, as the amount of clonal plants was generally higher at sites with low species richness. The importance of dispersal for plants may work at different time-scales (Vanable & Brown 1988; Rees 1993). Short-lived individuals have only one opportunity to disperse their offspring from a local habitat with insufficient quality, while long-lived individuals may have several attempts to disperse their offspring. Studies have shown that there may be a considerable time-lag in species response to changes in land use and fragmentation (Tilman et al. 1994; Cousins & Eriksson 2001; Eriksson & Ehrlén 2001). Because short-lived annual plants disappear more quickly than long-lived perennials, the build-up of an extinction debt is primarily associated with long-lived clonal plants. In addition to local habitat quality, area may be positively associated with plant persistence and plant species richness (Harrison & Bruna 1999; Wiegand et al. 2005), and local factors such as grazing continuity and grassland size may also affect species richness and density (Lindborg & Eriksson 2004b). However, it is usually difficult to distinguish a direct association with area (e.g. Matthies et al. 2004), and isolation from a general deterioration of the remaining habitat (Kiviniemi & Eriksson 2002). In this study, the size of the study grasslands affected the distribution of both annual and perennial plants, but at different time-scales. Annuals were positively associated with current area, the relative distribution of clonal plants was negatively related to historical area, whereas non-clonal plants were not associated with area. From this, we may infer that dependencies exist at different spatial scales. Here, annuals may be more sensitive to habitat size than perennials, and clonal perennials are more resistant to connectivity loss than plants without clonal ability in small managed areas. The population dynamics of plant species with a short life cycle, compared with long-lived species, is often characterized by higher turnover rates, i.e. larger fluctuations in individual number (e.g. Matthies et al. 2004), and the persistence of populations are dependent on frequent recruitment. Short-lived species are therefore more vulnerable and respond more quickly to habitat fragmentation than long-lived species.

Similar to dispersal, persistence in the seed bank contributes to long-term survival. Plant species with seeds that have long seed bank persistence were more common in historically isolated grasslands than in grassland with high historical connectivity. No association with connectivity was detected in the current landscapes. Some studies show that persistence in the seed bank may be especially important for annual and biennial species (cf. Bekker et al. 2000). However, no such association was detected between short-lived species and persistence in the seed bank in this study as very few species had that combination of traits. Large seeds may have a competitive advantage (Geritz 1995), but it is believed that seed size is less important for recruitment in open and disturbed environments than in forested and shady environments (e.g. Westoby et al. 1996). On the other hand, few species in Scandinavian grassland are short-lived, and only approximately half of all plant species in grassland communities contribute seeds to the seed bank (Bakker & Berendse 2001). Thus, changes in distribution of short-lived species contribute relatively little to differences in total species richness. With this in mind, the explanation of present plant diversity patterns should be sought in earlier historical landscape structure rather than in present landscapes. Studies have shown that species richness was somewhat higher, and species composition slightly more homogeneous, in the Swedish landscape a century ago than in grazed and mowed areas today (Löfgren & Jerling 2002). This indicates that since the total amount of semi-natural grasslands has declined, the landscape structure of that time buffered extinction of populations by frequent dispersal in space. Even if the larger extent of grazed areas was the major factor for persistence of species, it is likely that the transportation of hay and livestock was an important mechanism for species dispersal (Poschlod & Bonn 1998). Although the effect of long-distance dispersal is a subject of current debate, few studies have tested the effects of such dispersal (Cain et al. 1998; Bohrer et al. 2005). However, over longer time-scales, long-distance dispersal may be crucial for survival of populations (Bullock et al. 2002; Bohrer et al. 2005).

It is important to determine the appropriate spatial area for an analysis of dispersal. In the present study, there was no difference in the response to connectivity at 1-km or 2-km scale for annuals and clonal perennials, implying that a radius of 1 km from a grassland site may be a sufficient dispersal range for grassland species, both in current and in historical landscapes. However, perennials without clonal ability were negatively related to historical grassland connectivity at 1-km radius, but not at 2-km radius. This may be an effect of high autocorrelations in the smaller areas. It is likely that the distribution of habitats, and also grasslands, is more similar in close vicinity to farms, i.e. the study grassland, than in the more distant surroundings, as the configuration of grasslands and agricultural fields close to farms was quite similar historically. This may also be dependent on how the surrounding grasslands were classified. Habitats that may be suitable for grassland species, e.g. abandoned and secondary grasslands, were not included in this study. However, studies have shown that species may remain in such habitats even if the optimal conditions are no longer met (Eriksson 1996; Lindborg et al. 2005). Even though dispersal range was not explicitly tested in this study, it is clear that quite a large area surrounding a grassland may affect the distribution of life-history traits within the plant community.

The fact that landscape structure affected species composition in grassland communities has important implications for conservation of species and the understanding of long-term species survival. The result suggests that there are two main strategies to survive landscape fragmentation over time: by persisting in the seed bank or by vegetative dispersal. The historically isolated grasslands currently contain a species composition dominated by clonal species, especially if the current grassland area is small. This legacy of historically higher connectivity suggests further losses of perennial plants with clonal ability if losses are not compensated for by new colonizations. The fact that short-lived plants and plants without clonal ability were more sensitive to isolation is important to consider in order to understand plant community dynamics over time. Given these new insights regarding the importance of past landscape configuration on community structure, it is perhaps time to consider more seriously the historical dimension to studies of plant community dynamics in fragmented landscapes.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This study was financially supported by the Swedish Research Council for the Environment, Agricultural Sciences and Spatial planning (FORMAS), the Oscar and Lilli Lamm foundation and the MISTRA programme Management of semi-natural grasslands – Economy and Biodiversity. I also would like to thank J. Ehrlén for discussions concerning statistics, and K. Kiviniemi, O. Eriksson, M. Öster and two anonymous referees for valuable comments on the manuscript.

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  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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