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Keywords:

  • Cirsium;
  • seed production;
  • geographical range;
  • latitudinal gradient;
  • population density;
  • quantitative trait;
  • morphology;
  • clonal reproduction

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  • • 
    Patterns in population density and abundance, community composition, seed production and morphological traits were assessed across the UK geographical range of Cirsium acaule, Cirsium heterophyllum and Cirsium arvense based on the expectation that environmental favourability declines from core to periphery of a species range.
  • • 
    These traits were measured in natural populations along a latitudinal transect in the UK and using botanical survey data.
  • • 
    A significant decline in population density and seed production occurs approaching the range edges of C. acaule and C. heterophyllum. There is no latitudinal trend in these traits in the widespread C. arvense and no latitudinal pattern to variation in morphological traits or community composition in any of these species.
  • • 
    Although seed production is reduced at the range edge of C. acaule and C. heterophyllum, peripheral populations of these species may persist through clonal reproduction. Low seed production may interact with reduced availability of favourable habitat to limit range expansion in these species.

Introduction

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

Species’ range boundaries often show a close association with particular climatic variables (Pigott, 1968, 1992; Conolly & Dahl, 1970; Tofts, 1999). This observation is supported by both correlative approaches linking distributional limits with isometric lines of climate (for example; Salisbury, 1926, Conolly & Dahl, 1970) and identification of the principal factors that limit the spread of particular species (Pigott, 1968; Pigott & Huntley, 1981). Low temperatures may limit poleward spread through their actions on both the vegetative (Woodward, 1990, 1997) and reproductive phases of plant growth (Pigott, 1968; Pigott & Huntley, 1981; Woodward, 1990). The factors limiting spread in the equatorial direction are less clear. High temperature (Conolly & Dahl, 1970) and water availability (Pigott & Pigott, 1993) have been implicated and competitive exclusion may play a key role (Woodward, 1996).

The favourability of a species’ environment is expected to decline from the core to the periphery of a species’ range (Brown, 1984), based on the observation that environmental factors frequently change in a clinal manner (Endler, 1977). This pattern is expected to result in a decline in the favourability of a species’ typical habitat toward the range edge and, consequently, a reduction both in the abundance of individuals within a population and the density of populations within an area (Hengeveld & Haeck, 1982; Brown, 1984). Habitat at the geographical range margins is expected to be ecologically marginal for many species (Lawton, 1993, Lesica & Allendorf, 1995). This is predicted to result in a species occurring in atypical habitat (and hence different plant communities; Crozier & Boerner, 1984; Dibble et al., 1999) in peripheral areas, where the impact of decreased environmental favourability is reduced (Lesica & Allendorf, 1995). Declining environmental favourability is expected to result in a decrease of both plant growth and reproduction toward species’ range boundaries (Parsons, 1991).

Many species exhibit a decline in seed production toward their range boundary (Pigott, 1968; Pigott & Huntley, 1981; Reinartz, 1984b; Eckert & Barrett, 1993; García et al., 2000; Dorken & Eckert, 2001). The reproductive phase of the plant lifecycle shows particular sensitivity to climate (Marshall, 1968; Pigott, 1968; Pigott & Huntley, 1981; Houle & Filion, 1993; Despland & Houle, 1997; Woodward, 1997; García et al., 2000). Variation in vegetative characters such as plant size (Marshall, 1968; Clevering et al., 2001) and differential allocation to above- and below-ground organs (Benowicz et al., 2000) toward a species’ range limit have also been reported. Differences in plant size may ultimately be reflected in the reproductive success of the plant (Reinartz, 1984a,b; Primack, 1987; Wesselingh et al., 1997).

Many studies have reported variation in plant characters along altitudinal or latitudinal gradients (Clausen et al., 1940; Pigott, 1968; Lacey, 1984; 1988; Aizen & Woodcock, 1992; Winn & Gross, 1993; Wesselingh et al., 1994; Jonas & Geber, 1999). Although investigation into the effects of reaching a species periphery is often inherent in studies conducted over latitudinal gradients, there is a bias toward investigating the poleward periphery of a species distribution. Studies that record trait variation toward the equatorial periphery or across the entire geographical range of a species are relatively rare (but see Reinartz, 1984a,b; García et al., 2000). Furthermore, studies usually concentrate on a few closely related traits and can therefore present only a fragmented view of plant performance across a species geographical range. Seed production, for example, may represent an integrated measure of plant productivity. A reduction in seed production may result from a specific physiological limitation on seed development (Pigott & Huntley, 1981), or an overall reduction in plant growth (Reinartz, 1984a,b; Primack, 1987). This study aims to present a more extensive view of variation in plant traits from the core to the periphery of a species’ range.

We report trait variation across the UK geographical range of three Cirsium species (C. acaule, C. arvense and C. heterophyllum, Fig. 1). These are perennial, insect-pollinated species that produce wind-dispersed seeds. In addition to reproduction by seed, all reproduce vegetatively by the production of new shoots from underground root and stem tissue (Pigott, 1968; Moore, 1975; Clapham et al., 1981; Grime et al., 1989). Cirsium acaule reaches a northern distributional limit in the UK, in Europe it extends from northern England and Estonia southwards to southern Spain, Serbia and south-east Russia (Tutin et al., 1976). Cirsium heterophyllum occurs at low altitudes in northern Europe and the eastern part of the former USSR, and reaches a southern distributional limit in the UK (Fig. 1). High altitude populations of C. heterophyllum occur in the mountain ranges of Europe, southwards to the Pyrenees and Transylvania (Tutin et al., 1976). Cirsium arvense is a widespread species used for comparison. It occurs almost throughout Europe and is absent only from Svalbard in the extreme north and Crete and the Azores in the south (Tutin et al., 1976). Previous studies have reported a close association between both the northern range boundary of C. acaule and the southern boundary of C. heterophyllum with isotherms of summer temperature (Pigott, 1968; Conolly & Dahl, 1970), implicating climate as a major factor in determining the distribution of these species.

image

Figure 1. Distribution maps showing the presence of Cirsium species in 10 km squares in Britain and Ireland. Reproduced from Preston et al. (2002) with permission.

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This work will build on previous studies that report geographical patterns in single traits within single species. It will allow both interspecific and intraspecific comparison of trait patterns by reporting intraspecific geographical variation in key areas of population distribution, community composition, morphology and seed production within three congeneric species. A companion paper (Jump et al., 2003) will presents geographical patterns in the population genetic structure of these species.

Given the predicted decline in environmental favourability, this study aims to determine whether gradients in morphological characters, seed production, population density and abundance occur from the core to the periphery of C. acaule and C. heterophyllum in the UK and whether geographically peripheral populations of these species occur in atypical plant communities.

Materials and Methods

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

As this work will investigate potential north–south clines in characters in these species, latitude will be used as a proxy for distance from the core of the species distribution. Latitude is a good proxy for the action of many factors such as radiation balance, length of growing season and the frequency of frost events. Indeed, Valentini et al. (2000) reported latitude to be a better correlate of ecosystem respiration than any single factor they tested (including mean annual temperature, precipitation and elevation) when investigating forest carbon balance. Furthermore, investigating core–periphery patterns in species has the inherent problem that range edges are extremely difficult to define reliably (Blackburn et al., 1999). Thus, when investigating north–south clines, latitude is a surrogate measure that provides both an indication of the position of populations and the distance between them in the absence of a reliable and repeatable method of defining a species range edge.

Surveys were carried out during July and August 2000 within the populations listed in Table 1. Cirsium acaule populations were surveyed only on slopes facing south to south-east since Pigott (1968) reported great sensitivity of seed production to aspect in this species. In this species, comparing data from sites of different aspect could potentially obscure any latitudinal pattern that may occur within similar populations. Populations of C. heterophyllum and C. arvense were surveyed on level ground.

Table 1.  Location of study populations
Cirsium acauleCirsium heterophyllumCirsium arvense
Site codeSite locationAltitude (m)Site codeSite locationAltitude (m)Site codeSite locationAltitude (m)
1150.587°N 2.032°W 705153.214°N 1.765°W2801150.677°N 2.644°W130
1250.672°N 2.587°W1505253.231°N 1.844°W3201250.684°N 2.655°W140
1350.630°N 1.969°W 805353.241°N 1.780°W2903151.439°N 2.401°W180
2151.209°N 2.092°W1305453.166°N 1.879°W2703251.430°N 2.404°W230
2251.262°N 2.034°W1606154.408°N 2.337°W2204151.839°N 2.107°W170
2351.269°N 2.023°W1306254.439°N 2.587°W1704251.862°N 2.071°W260
3151.447°N 2.401°W1806354.862°N 2.508°W2005153.214°N 1.765°W280
3251.430°N 2.404°W2306454.447°N 2.387°W2705253.145°N 1.728°W260
3351.327°N 2.791°W1507156.490°N 4.748°W2006154.521°N 2.497°W200
4151.842°N 2.107°W1907256.400°N 5.213°W 806254.527°N 2.329°W160
4251.865°N 2.073°W2607356.321°N 3.685°W1007157.321°N 4.363°W240
4351.842°N 1.996°W2108157.101°N 3.987°W2607257.101°N 3.987°W260
5153.262°N 1.733°W3208257.015°N 4.162°W290   
5253.138°N 1.714°W2508357.327°N 3.021°W320   
   8457.420°N 2.627°W 230   
   9157.990°N 4.814°W 150   
   9258.243°N 5.177°W  50   
   9357.753°N 5.011°W 200   

Population density

Population density was determined by comparing the 2 × 2 km (tetrad) Botanical Society of the British Isles (BSBI) Monitoring Scheme survey data with the 10 × 10 km (hectad) survey data of the Atlas of Flowering Plants and Ferns of Britain and Ireland (Preston et al., 2002). The BSBI Monitoring Scheme survey is a systematic survey that takes up to three tetrad subsamples from one in every nine (11%) of the hectads of the BSBI grid (see Rich & Woodruff, 1990, 1996). Since, from the Atlas survey, we know whether the species of interest occurs in the hectad being sampled, the density of populations of that species within that hectad is indicated by the number of tetrad samples of the monitoring scheme survey that record it. Thus, a decline in population frequency will be represented by a decline in the number of tetrads per hectad that include the species of interest.

A frequency score was calculated by dividing the number of tetrads per hectad containing the species of interest by the number of tetrad samples taken within that hectad. As tetrad samples have been taken only within hectads where the species of interest is known to occur, a score of 0 indicates lowest frequency within this hectad rather than absence. To reflect this, frequency scores were expressed on a scale ranging from 1 (least frequent) to 5.

Population abundance

Abundance was measured as the maximum density of individuals within a population, based on samples of randomly placed quadrats in the densest area of the population. For C. arvense and C. heterophyllum, abundance was recorded as the number of shoots within two 1 × 1 m quadrats at each site; for C. acaule abundance was based on the number of clumps rather than individual shoots and recorded from two 5 × 5 m quadrats. Quadrat size was larger for C. acaule because this species occurs as scattered individuals rather than the dense stands that are typical of C. arvense and C. heterophyllum.

Plant communities

Two community surveys were conducted in each population. Per cent shoot frequency of each species present was calculated based on presence in each of 25 cells of a 50 cm × 50 cm strung quadrat (Goldsmith et al., 1986). Quadrats were placed randomly within the densest area of the population.

Morphological traits and seed production

The traits measured in each species are listed in Table 2. Owing to access agreements made with conservation authorities in the Peak District National Park, UK, it was not possible to perform any destructive analysis (e.g. biomass) on C. acaule or C. heterophyllum in this area. Consequently, with the exception of the collection of seed heads, all measures were nondestructive. Collection of seed heads was restricted to less than 30% of those produced by each population studied.

Table 2.  Morphological and seed traits measured in the Cirsium species investigated
SpeciesCharacters measured (n = 30)
All speciesMaximum length and width of youngest fully expanded leaf (excluding petiole)
Total filled seed mass per capitulum
Percentage of population failing to set seed
Cirsium acaule onlyMaximum clump diameter
Cirsium arvense and C. heterophyllum onlyHeight at flowering
Number of flowers per plant

Within each population, 30 plants were chosen at random and the traits measured as listed in Table 2. Cirsium acaule rarely produces a flowering stem, flowering instead within the rosette. Clump diameter was used as an additional measure of plant growth (as opposed to height) in this species. The total number of capitula produced per plant was not recorded for C. acaule because of the indeterminate flowering of populations of this species within a season. The ratio leaf length to leaf width was used as an indication of leaf shape, while leaf length indicated leaf size (Mooney & Billings, 1961; Rochow, 1970).

To assess seed production, 30 ripe but not dehiscent capitula were collected at random within each population. In C. arvense these were chosen only from female flower heads (Moore, 1975; Heimann & Cussans, 1996), in C. heterophyllum the terminal capitulum was selected from the flowering stem of each plant. Capitula were air dried in paper envelopes for 1 month before deseeding. Seeds were extracted from each capitulum by removing the pappus material and seeds and rubbing this across a soil sieve (4 mm mesh). The seeds collected were then cleaned of debris and sorted by hand using a hand-lens to separate entire, filled seed from unfilled seeds and those damaged by insect predation. Damage due to seed predation was scored between 0 and 4 for each capitulum. Total filled seed mass per capitulum was recorded.

Data analysis

Community data were analysed using a combination of classification and ordination methods. Such methods are usually used to identify particular phytosociological groups, however, our aim in the analysis of these data was to identify outlying samples (rather than the main clusters) to determine whether the peripheral populations of C. acaule and C. heterophyllum occur in atypical communities. A potential problem with this approach is that different methods for the analysis of community data are likely to deal with outlying samples in different ways (Gauch & Whittaker, 1981). The most informative approach is thus to use several different methods of analysis on the same data set and compare the results to get a more comprehensive view of the data structure (Kent & Ballard, 1988).

Community data were classified using the agglomerative increase sum of squares method, followed by calculation of sample proximities based on squared Euclidean distance, using the program clustangraphics (Wishart, 1999). An additional divisive classification was produced using the default parameters of the programme twinspan (Hill, 1979a; Malloch, 1999). Community ordination (detrended correspondence analysis, DCA) was performed using the default parameters of the programme decorana (Hill, 1979b; Malloch, 1999). See Kent & Coker (1992) for a detailed description of these techniques. The authors of these programmes (M. Hill and A. Wishart) were contacted to confirm the validity of this approach.

Population density and abundance, seed production and morphological characters were regressed against latitude using sigmaplot 2001 for Windows v7 (SPSS Inc. Chicago, IL, USA).

Results

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

Population density

A significant relationship was found between maximum population frequency and latitude in C. acaule (Fig. 2a, R2 = 0.82, P < 0.0001) and C. heterophyllum (Fig. 2b, R2 = 0.42, P < 0.05). Both species occur with highest population frequency in core areas of their range and decrease toward the periphery. Owing to the reduced maximum frequency of C. heterophyllum in the far north of Scotland, a quadratic relationship better describes the relationship between frequency and latitude than a straight line (linear regression: R2 = 0.20, P = 0.064). The widespread C. arvense shows no variation of maximum population frequency with latitude (Fig. 2c). Maximum population frequency in this species is at the maximum value of 5 throughout almost the entire length of the UK, only one latitude in the far north of Scotland has a maximum population frequency less than this. At this latitude only a single survey was possible, this was conducted in a predominantly coastal region.

image

Figure 2. Maximum population frequency as a function of latitude. (a) Cirsium acaule (y = 68.0 − 1.59x, R2 = 0.82, P < 0.0001). (b) Cirsium heterophyllum (y = −813 + 28.8x − 0.255x2, R2 = 0.42, P < 0.05). (c) Cirsium arvense. Dotted lines show 95% confidence limits of regression.

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Population abundance

There was a significant relationship between latitude and within-population abundance only in C. heterophyllum (Fig. 3, R2 = 0.36, P < 0.01). Abundance in C. heterophyllum is highest in the core area of its UK distribution in central Scotland and declines approaching its southern range edge. No relationship was found between latitude and abundance in either of the other species.

image

Figure 3. Mean abundance of Cirsium heterophyllum as a function of latitude (y = −213 + 4.66x, R = 0.36, P < 0.01). Dotted lines show 95% confidence limits of regression.

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Community composition

The decorana community ordination is presented in Fig. 4. Most of the separation of the community types in the ordination is accounted for by axis 1 (eigenvalue 0.72), with axis 2 (eigenvalue 0.38) and axis three (eigenvalue 0.25) accounting for much less. The communities of geographically peripheral populations of C. acaule and C. heterophyllum occur in the centre of the ordination clusters rather than as outliers. This result is supported by the community classifications, which also classify peripheral populations in the centres of the main community clusters (data not shown). The community ordination and classification analyses used here suggest that peripheral populations of C. acaule and C. heterophyllum do not occur in atypical communities.

image

Figure 4. Community ordination as produced by decorana. Squares, communities surveyed for Cirsium acaule; triangles, communities surveyed for Cirsium heterophyllum; circles, communities surveyed for Cirsium arvense. Black symbols represent communities in peripheral regions of the geographical range of C. acaule and C. heterophyllum.

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Morphological traits

In C. acaule there is a significant relationship between clump diameter and latitude (R2 = 0.53, P < 0.01, data not shown). Clump size is smallest (25.5 cm) in the core populations of this species and increases toward the periphery (48.5 cm). There is no relationship between any other vegetative character measured and latitude in any of the species investigated.

Seed production

A significant relationship between seed production and latitude was found in both C. acaule (Fig. 5a, R2 = 0.35, P < 0.05) and C. heterophyllum (Fig. 5b, R2 = 0.51, P < 0.005). A preliminary study of seed production in C. heterophyllum conducted in 1999 yielded a similar result (R2 = 0.73, P < 0.01; data not shown). In both species, seed mass per capitulum declined approaching the periphery. This decline is most dramatic in C. heterophyllum, which at its southern periphery produced only 1.2% of the maximum seed mass recorded in the core area of its UK distribution. Maximum seed production in C. acaule at its northern periphery was 37% of the maximum recorded in the core area of its distribution in southern England.

image

Figure 5. Seed production in Cirsium species as a function of latitude: Mean total filled seed mass per capitulum: (a) C. acaule (y = 0.951 − 0.0173x, R2 = 0.35, P < 0.05); (b) C. heterophyllum (y = −1.56 + 0.0293x, R2 = 0.51, P < 0.005). Percentage of population failing to set seed: (c) C. acaule (y = −733 + 14.6x, R2 = 0.63, P < 0.001); (d) C. heterophyllum (y = 1066 − 18.4x, R2 = 0.90, P < 0.0001). Dotted lines show 95% confidence limits of regression.

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In both C. acaule and C. heterophyllum there is a parallel pattern in the percentage of the population that fails to set any seed. Both show a significant rise in failure to set seed toward their peripheral latitudes (C. acaule: Fig. 5c, R2 = 0.63, P < 0.001; C. heterophyllum: Fig. 5d, R2 = 0.90, P < 0.0001). In C. heterophyllum the maximum percentage of individuals bearing seed within any peripheral population is 37% compared with 100% in the core area of its distribution. Cirsium acaule shows a similar decrease in seed-bearing individuals toward its northern periphery. In the peripheral populations, a maximum of 47% of individuals were found to set seed as opposed to a maximum of 100% in core populations. Subsequent investigation of the viability of any filled seed produced suggested this did not vary with latitude (data not shown).

There is no relationship with latitude in either mean seed mass per capitulum or the percentage of the population failing to set seed in C. arvense, neither is there a significant decline in flower production in C. arvense or C. heterophyllum across their geographical range in the UK. Seed predation did not appear to vary with latitude in these species despite a report by Tofts (1999) that such a pattern occurs in the related species Cirsium eriophorum.

Discussion

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

Hengeveld & Haeck (1982) proposed the general biogeographical rule that species become rarer toward their range margins, independent of the spatial scale of observation. They predicted that declining environmental favourability with increasing distance from the core of a species range would lead not only to a decrease in favourable habitat patches, but also a decline in the favourability of any patches that occur; thereby leading to a decline in both density and abundance (Brown, 1984). Although population density declines approaching the periphery of both C. acaule and C. heterophyllum, only in C. heterophyllum is this accompanied by a decline in abundance within populations; C. acaule does not fit the pattern of parallel decline in abundance predicted by both Hengeveld & Haeck (1982) and Brown (1984). Sagarin & Gaines (2002) found that only 39% of the studies that they reviewed supported the predicted decline in abundance. Cirsium acaule may favour an alternative pattern discounted by Hengeveld & Haeck (1982), that the number of favourable habitat patches may decline approaching the range boundary but that abundance within them remains the same – a pattern also reported by Pérez-Tris et al. (2000). This may, in some part, be explained by the great sensitivity of C. acaule to summer irradiance (Pigott, 1968) acting more in terms of a presence/absence (threshold) effect in this species rather than the gradual decline predicted to result from the processes described by Brown (1984).

There is no support from community surveys of C. acaule or C. heterophyllum for the prediction that peripheral populations will occur in atypical habitat. There is only limited support for this prediction in the literature. For example, Fernald (1925) described a mixing of species typical of northern North America with those typical of southern states of the USA, in rare communities at the edge of their geographical ranges. Barden (2000) described southern peripheral populations of the common shrub Quercus ilicifolia, which occur in a rare community type not seen in other areas of the species range. However, since species are often found in a greater variety of habitat types in core rather than peripheral areas of their distribution (Hall et al., 1992; Pérez-Tris et al., 2000) it may be more likely that, at the periphery, they will be restricted to only the most favourable of these, rather than occurring in novel habitat unoccupied elsewhere in the range.

Despite the general absence of latitudinal patterns of morphological traits in these species, there is a significant increase in clump size in C. acaule approaching the periphery of this species. This trait is not comparable with plant size traits measured in the other Cirsium species, since, in C. acaule, clump size refers to the size of individual genets rather than the size of ramets measured as morphological traits for the other species. It may be more accurate to view clump size as an indicator of vegetative reproduction rather than plant size. If so, this raises questions as to whether this may occur as a result of reproductive assurance (Pannell & Barrett, 1998) in this species, given that vegetative offspring may have a greater chance of survival in peripheral populations compared with plants produced by seed (Eckert & Barrett, 1993).

When considered alongside the absence of patterns in morphological traits, the decline in seed production approaching the range edge of C. acaule and C. heterophyllum suggests that reduced reproductive success rather than vegetative survival may limit the geographical distribution of these species in the UK. This pattern is in agreement with that described for many plant species (Pigott & Huntley, 1981; Reinartz, 1984b; Eckert & Barrett, 1993; García et al., 2000; Dorken & Eckert, 2001). Climate is implicated in determining the distribution of C. acaule and C. heterophyllum (Pigott, 1968; Conolly & Dahl, 1970), Pigott (1968) noted a reduction in seed production in C. acaule at its northern limit linked to the reduced heat sum gained by the capitula of northern populations. Despite this, some viable seed is produced (indicated by the presence of seedlings in both peripheral populations in 2000). Although a decline in seed production approaching the range edge is reported frequently, recruitment by seed is likely in peripheral populations since pronounced interannual variation in both climate and seed production occurs (Pigott, 1968; Pigott & Huntley, 1981; Houle & Filion, 1993; Despland & Houle, 1997; Woodward, 1997). However, production of viable seed does not necessarily result in the expansion of a species’ geographical range, since recruitment depends on seed production, seedling establishment and seedling survival (Pigott, 1992; Dorken & Eckert, 2001).

In species that combine seed production with vegetative spread, clonal reproduction allows populations to persist in the absence of recruitment by seed (Ellstrand & Roose, 1987; McLellan et al., 1997; Dorken & Eckert, 2001). Since each of the species investigated here employs both these means of reproduction, the marked reduction in seed production does not imply that populations should be unable to persist at the range edge. However, the founding of new populations of these species will be largely dependent on the dispersal of viable seed and subsequent plant establishment. If the decline in the density of populations reported here is interpreted as indicating a decline in density of favourable habitat patches (Brown, 1984), range expansion may be limited by the combined effects of reduced seed production and decreased probability of successful dispersal to (increasingly distant) favourable sites. Levels of seed production (and hence seed dispersal) at the periphery may be below those required for the successful establishment of new populations (Primack & Miao, 1992) despite the production of some viable seed by existing populations.

Environmental stress levels are expected to rise toward a species periphery as a result of declining environmental favourability (Parsons, 1991; Hall et al., 1992). A predicted consequence of this is a decrease in energy available for growth and reproduction approaching the range edge and hence a decline in plant size and the production of seed. The geographical limit for vegetative survival and that for the production of viable seed may, however, be determined by very different environmental conditions (Woodward, 1997) and sometimes widely separated in space (Woodward, 1990). At geographical limits where species can grow and complete a normal life cycle, successful regeneration is not guaranteed even if viable seed is produced; thus, the realized range of a species in the natural environment may differ from that which it might theoretically occupy based on vegetative survival (Pigott, 1992). The proposed effect of stress might not be seen on plant growth if a species’ distribution is limited by poor reproduction. The range limits of C. acaule and C. heterophyllum may result from reduced seed production acting in combination with a decrease in the availability of favourable habitat patches; the combined effect of these factors may make the establishment of new populations unlikely. These species are therefore likely to be absent from the region that is expected to be highest in stress (due to poor reproduction), despite the fact that they might survive vegetatively in this area. Consequently, since the effects of declining environmental favourability are more likely to be detected through reduced reproduction, the proposed effects of stress may not be detectable as a reduction in growth in the natural environment.

Conclusion

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

In C. acaule and C. heterophyllum there is no support for the prediction that peripheral populations will occur in atypical habitat, or that a reduction in plant size will occur approaching the range edge. In both these species, however, there is a marked reduction in seed production approaching the range edge, demonstrating that reduced reproductive output of peripheral populations may be of critical importance in restricting plant geographical ranges. The decline in population density also observed approaching the range edge of these species may interact with reduced reproductive output to reduce the probability that new populations will establish in peripheral areas of the species distribution. Both decreased density and reduced reproduction of peripheral populations may have consequences for the genetic structure of populations of these species at the range edge. This is investigated in a companion paper.

Acknowledgements

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

This work was supported by a Hossein Farmy scholarship from the University of Sheffield. We are grateful to the Department for the Environment, Transport and Regions (DETR), the BSBI and the Centre for Ecology and Hydrology (CEH) for permission to use botanical survey data and distribution maps. We thank Chris Preston for providing distributional data and Mark Lomas for his assistance with data analysis. We thank the Peak District National Park Authority (Ecology Service), English Nature (Over Haddon), Defence Estates Agency (Salisbury Plain) and vice-county recorders of the BSBI for their assistance in locating study populations. David Beerling and Peter Mitchell are thanked for their comments on the manuscript.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  • Aizen MA, Woodcock H. 1992. Latitudinal trends in acorn size in eastern North-American species of Quercus. Canadian Journal of Botany-Revue Canadienne de Botanique 70: 12181222.
  • Barden LS. 2000. A common species at the edge of its range: conservation of bear oak (Quercus ilicifolia) and its low elevation rocky summit community, in North Carolina (USA). Natural Areas Journal 20: 8589.
  • Benowicz A, El-Kassaby YA, Guy RD, Ying CC. 2000. Sitka alder (Alnus sinuata RYDB.) genetic diversity in germination, frost hardiness and growth attributes. Silvae Genetica 49: 206212.
  • Blackburn TM, Gaston KJ, Quinn RM, Gregory RD. 1999. Do local abundances of British birds change with proximity to range edge? Journal of Biogeography 26: 493505.
  • Brown JH. 1984. On the relationship between abundance and distribution of species. American Naturalist 124: 255279.
  • Clapham AR, Tutin TG, Warburg EF. 1981. Excursion flora of the British Isles. Cambridge, UK: Cambridge University Press.
  • Clausen J, Keck DD, Hiesey WM. 1940. Experimental studies on the nature of species. Washington, DC, USA: Carnegie Institution of Washington.
  • Clevering OA, Brix H, Lukavska J. 2001. Geographic variation in growth responses in Phragmites australis. Aquatic Botany 69: 89108.
  • Conolly AP, Dahl E. 1970. Maximum summer temperature in relation to the modern and quaternary distributions of certain arctic–montane species in the British Isles. In: WalkerD, WestRG, eds. Studies in the vegetational history of the British Isles. Cambridge, UK: Cambridge University Press.
  • Crozier CR, Boerner REJ. 1984. Correlations of understory herb distribution patterns with microhabitats under different tree species in a mixed mesophytic forest. Oecologia 62: 337343.
  • Despland E, Houle G. 1997. Climate influences on growth and reproduction of Pinus banksiana (Pinaceae) at the limit of the species distribution in eastern North America. American Journal of Botany 84: 928937.
  • Dibble AC, Brissette JC, Hunter ML. 1999. Putting community data to work: some understory plants indicate red spruce regeneration habitat. Forest Ecology and Management 114: 275291.
  • Dorken ME, Eckert CG. 2001. Severely reduced sexual reproduction in northern populations of a clonal plant, Decodon verticillatus (Lythraceae). Journal of Ecology 89: 339350.
  • Eckert CG, Barrett SCH. 1993. Clonal reproduction and patterns of genotypic diversity in Decodon verticillatus (Lythraceae). American Journal of Botany 80: 11751182.
  • Ellstrand NC, Roose ML. 1987. Patterns of genotypic diversity in clonal plant species. American Journal of Botany 74: 123131.
  • Endler JA. 1977. Geographic variation, speciation, and clines. Princeton, NY, USA: Princeton University Press.
  • Fernald ML. 1925. Persistence of plants in unglaciated areas of boreal America. American Academy of Arts and Sciences Memoirs 15: 241341.
  • García D, Zamora R, Gomez JM, Jordano P, Hodar JA. 2000. Geographical variation in seed production, predation and abortion in Juniperus communis throughout its range in Europe. Journal of Ecology 88: 436446.
  • Gauch HG, Whittaker RH. 1981. Hierarchical classification of community data. Journal of Ecology 69: 537557.
  • Goldsmith FB, Harrison CM, Morton AJ. 1986. Description and analysis of vegetation. In: MoorePD, ChapmanSB, eds. Methods in plant ecology. Oxford, UK: Blackwell Scientific.
  • Grime JP, Hodgson JG, Hunt R. 1989. Comparative plant ecology. London, UK: Unwin Hyman.
  • Hall CAS, Stanford JA, Hauer FR. 1992. The distribution and abundance of organisms as a consequence of energy balances along multiple environmental gradients. Oikos 65: 377390.
  • Heimann B, Cussans GW. 1996. The importance of seeds and sexual reproduction in the population biology of Cirsium arvense– a literature review. Weed Research 36: 493503.
  • Hengeveld R, Haeck J. 1982. The distribution of abundance. 1. Measurements. Journal of Biogeography 9: 303316.
  • Hill MO. 1979a. twinspan – a fortran program for arranging multivariate data in an ordered two-way table by classification of the individuals and attributes. Ithaca, NY, USA: Section of Ecology and Systematics, Cornell University.
  • Hill MO. 1979b. decorana – a fortran program for detrended correspondence analysis and reciprocal averaging. Ithaca, NY, USA: Section of Ecology and Systematics, Cornell University.
  • Houle G, Filion L. 1993. Interannual variations in the seed production of Pinus banksiana at the limit of the species distribution in northern Quebec, Canada. American Journal of Botany 80: 12421250.
  • Jonas CS, Geber MA. 1999. Variation among populations of Clarkia unguiculata (Onagraceae) along altitudinal and latitudinal gradients. American Journal of Botany 86: 333343.
  • Jump AS, Woodward FI, Burke T. 2003. Cirsium species show disparity in patterns of genetic variation at their range-edge despite similar patterns of reproduction and isolation. New Phytologist 160: 000000.
  • Kent M, Ballard J. 1988. Trends and problems in the application of classification and ordination methods in plant ecology. Vegetatio 78: 109124.
  • Kent M, Coker P. 1992. Vegetation description and analysis: a practical approach. London, UK: Belhaven.
  • Lacey EP. 1984. Seed mortality in Daucus carota populations – latitudinal effects. American Journal of Botany 71: 11751182.
  • Lacey EP. 1988. Latitudinal variation in reproductive timing of a short-lived monocarp, Daucus carota (Apiaceae). Ecology 69: 220232.
  • Lawton J. 1993. Range, population abundance and conservation. Trends in Ecology and Evolution 8: 409413.
  • Lesica P, Allendorf FW. 1995. When are peripheral populations valuable for conservation. Conservation Biology 9: 753760.
  • Malloch AJC. 1999. vespan iii a computer package to handle and analyse multivariate and species distribution data for Windows NT and Windows 95. Lancaster, UK: University of Lancaster.
  • Marshall JK. 1968. Factors limiting the survival of Corynephorus canences (L.) Beauv. in Great Britain at the northern edge of its distribution. Oikos 19: 206216.
  • McLellan A, Prati D, Kaltz O, Schmid B. 1997. Structure and analysis of phenotypic and genetic variation in clonal plants. In: De KroonH, Van GroenendaelJ, eds. The ecology and evolution of clonal plants. Leiden, The Netherlands: Backhuys Publishers.
  • Mooney HA, Billings WD. 1961. Comparative physiological ecology of arctic and alpine populations of Oxyria digyna. Ecological Monographs 31: 129.
  • Moore RJ. 1975. The biology of Canadian weeds. 13. Cirsium arvense (L.) Scop. Canadian Journal of Plant Science 55: 10331048.
  • Pannell JR, Barrett SCH. 1998. Baker's law revisited: Reproductive assurance in a metapopulation. Evolution 52: 657668.
  • Parsons P. 1991. Evolutionary rates: stress and species boundaries. Annual Review of Ecology and Systematics 22: 118.
  • Pérez-Tris J, Carbonell R, Telleria JL. 2000. Abundance distribution, morphological variation and juvenile condition of robins, Erithacus rubecula (L.), in their Mediterranean range boundary. Journal of Biogeography 27: 879888.
  • Pigott CD. 1968. Biological flora of the British Isles: Cirsium acaulon (L.) Scop. Journal of Ecology 56: 597612.
  • Pigott CD. 1992. Are the distributions of species determined by failure to set seed?. In: MarshallC, GraceJ, eds. Fruit and seed production. Cambridge, UK: Cambridge University Press.
  • Pigott CD, Huntley JP. 1981. Factors controlling the distribution of Tillia cordata at the northern limits of its geographical range III. Nature and causes of seed sterility. New Phytologist 87: 817839.
  • Pigott CD, Pigott S. 1993. Water as a determinant of the distribution of trees at the boundary of the Mediterranean zone. Journal of Ecology 81: 557566.
  • Preston CD, Pearman DA, Dines TD. 2002. New atlas of the British and Irish flora. Oxford, UK: Oxford University Press.
  • Primack RB. 1987. Relationships among flowers, fruits, and seeds. Annual Review of Ecology and Systematics 18: 409430.
  • Primack RB, Miao SL. 1992. Dispersal can limit local plant distribution. Conservation Biology 6: 513519.
  • Reinartz JA. 1984a. Life-history variation of common mullein (Verbascum thapsus). 1. Latitudinal differences in population-dynamics and timing of reproduction. Journal of Ecology 72: 897912.
  • Reinartz JA. 1984b. Life-history variation of common mullein (Verbascum thapsus). 2. Plant size, biomass partitioning and morphology. Journal of Ecology 72: 913925.
  • Rich TCG, Woodruff ER. 1990. BSBI Monitoring Scheme 1987–88, NCC CSD, report no. 1265. Peterborough, UK: Nature conservancy Council.
  • Rich TCG, Woodruff ER. 1996. Changes in the vascular plant floras of England and Scotland between 1930 and 1960 and 1987–88: The BSBI monitoring scheme. Biological Conservation 75: 217229.
  • Rochow TF. 1970. Ecological investigations of Thlaspi alpestre L. along an elevational gradient in the central Rocky Mountains. Ecology 51: 649656.
  • Sagarin RD, Gaines SD. 2002. The ‘abundant centre’ distribution: To what extent is it a biogeographical rule? Ecology Letters 5: 137147.
  • Salisbury EJ. 1926. The geographical distribution of plants in relation to climatic factors. Geographical Journal 57: 312335.
  • Tofts R. 1999. Cirsium eriophorum (L.) Scop. (Carduus eriophorus L., Cnicus eriophorus (L.) Roth). Journal of Ecology 87: 529542.
  • Tutin TG, Heywood VH, Burges NA, Moore DM, Valentine DH, Walters SM, Webb DA. 1976. Flora Europaea, Vol. 4. – Plantaginaceae to Compositae (and Rubiaceae). Cambridge, UK: Cambridge University Press.
  • Valentini R, Matteucci G, Dolman AJ, Schulze ED, Rebmann C, Moors EJ, Granier A, Gross P, Jensen NO, Pilegaard K, Lindroth A, Grelle A, Bernhofer C, Grunwald T, Aubinet M, Ceulemans R, Kowalski AS, Vesala T, Rannik U, Berbigier P, Loustau D, Guomundsson J, Thorgeirsson H, Ibrom A, Morgenstern K, Clement R, Moncrieff J, Montagnani L, Minerbi S, Jarvis PG. 2000. Respiration as the main determinant of carbon balance in European forests. Nature 404: 861865.
  • Wesselingh RA, Klinkhamer PGL, De Jong TJ, Boorman LA. 1997. Threshold size for flowering in different habitats: Effects of size-dependent growth and survival. Ecology 78: 21182132.
  • Wesselingh RA, Klinkhamer PGL, De Jong TJ, Schlatmann EGM. 1994. A latitudinal cline in vernalization requirement in Cirsium vulgare. Ecography 17: 272277.
  • Winn AA, Gross KL. 1993. Latitudinal variation in seed weight and flower number in Prunella vulgaris. Oecologia 93: 5562.
  • Wishart D. 1999. Clustan graphics primer. Edinburgh, UK: Clustan Limited.
  • Woodward FI. 1990. The impact of low temperatures in controlling the geographical distribution of plants. Philosophical Transactions of the Royal Society of London Series B 326: 585593.
  • Woodward FI. 1996. Climate and plant distribution. Cambridge, UK: Cambridge University Press.
  • Woodward FI. 1997. Life at the edge: a 14-year study of a Verbena officinalis population's interactions with climate. Journal of Ecology 85: 899906.