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Brief Communications |
Department of Biological Sciences, Mount Holyoke College, 50 College Street, South Hadley, Massachusetts 01075 USA
Received for publication November 9, 2001. Accepted for publication January 29, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODS AND RESULTSDISCUSSIONLITERATURE CITED |
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Key Words: dispersal • establishment • germination • Reid's paradox • Sarracenia purpurea • Sarraceniaceae
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODS AND RESULTSDISCUSSIONLITERATURE CITED |
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). This observation, termed "Reid's paradox" (Clark et al., 1998
), has received renewed attention because global climate change and habitat fragmentation present new challenges for plant dispersal and colonization of new habitats (Cain, Damman, and Muir, 1998
; Higgins and Richardson, 1999
; Nathan and Muller-Landau, 2000
; Cain, Milligan, and Strand, 2000
; Pakeman, 2001
). Although Reid's paradox essentially has been solved for trees (e.g., Clark, 1998
; Higgins and Richardson, 1999
), it remains an open question for herbs.
Plants that occur in naturally isolated habitats present additional challenges for models of seed dispersal (Clark, 1998
; Higgins and Richardson, 1999
; Pakeman, 2001
). These models implicitly assume that long-distance dispersers can cross habitat boundaries unimpeded or that at least all habitat between the source population and the furthest colonizing site is suitable for establishment. For many habitats, this is not the case, as increasing urbanization and other landscape transformations render intervening habitat unsuitable either for establishment or for disperser stopovers.
In northeastern North America, Sphagnum bogs occur as isolated wetlands, usually surrounded by forests and fields. Even large expanses of bogs are being fragmented by peat mining and afforestation (Pellerin and Lavoie, 2000
). Many plants that grow in bogs are habitat specialists, yet they have broad geographic ranges. Colonization of these habitats had to have occurred by crossing large expanses of nonwetlands (forests, grasslands, etc.) in which wetland plants cannot survive. Thus, understanding dispersal and establishment dynamics of bog plants may provide additional insights into models for the migration dynamics of plants as our climate changes rapidly. In this paper, we describe the dispersal and seedling establishment of the northern pitcher plant, Sarracenia purpurea L. subsp. purpurea (Raf.) Wherry, a common plant in bogs throughout Canada and the northeastern United States (Schnell, 1979
). This subspecies ranges from Maryland northward, but the entire species (including the three varieties of subsp. venosa [Raf.] Wherry) ranges from Florida to Newfoundland on the eastern coast of North America and westward across Canada (Schnell, 1979
). The single-flowered inflorescence of S. purpurea blooms in late May to early June. Capsules mature over the summer and dehisce in late fall, releasing 500–1500 seeds each. Its seeds are small (1.8–2.4 mm in length) and unornamented and have nondeep morphophysiological dormancy (Ellison, 2001
). Field work for this study was conducted at Hawley Bog, an approximately 3-ha ombrotrophic stream headwaters bog in northwestern Massachusetts, USA (Moizuk and Livingston, 1966
). Sarracenia purpurea seeds are not found in the persistent seed bank at Hawley Bog (A. M. Ellison and H. R. Steinhoff, Mount Holyoke College, unpublished data).
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METHODS AND RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODS AND RESULTSDISCUSSIONLITERATURE CITED |
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). From 16 October through 3 December we counted the total number of seeds on each plate weekly. Most seeds had dispersed and snow covered our plates by mid-December. In total, only 200 seeds were caught on the traps; this represents only approximately 10% of the estimated seed production of these five plants. Most seeds (78%) recovered were within 5 cm of the parent plant; the median dispersal distance was 5 cm, and the mean was 12.8 cm. The shape of the dispersal curve closely was fit by a relatively steep negative exponential curve (Fig. 1).
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Second, in October 1999 we planted five replicate plots of 100 seeds each on the Sphagnum mat. Seeds were planted just below the Sphagnum surface on relatively flat portions of the bog mat in regular 10 x 10 grids with 2-cm spacing between seeds. The plots' corners were flagged to allow for their relocation. We censussed these plots monthly during the 2000 growing season and again at the end of the 2001 growing season. Very few seedlings successfully established in these plots. Only one seedling was found in 2000, whereas in 2001 we found 20 seedlings (8, 1, 10, 0, 1, respectively, in the five plots). Based on these data, we estimate the probability of successful seedling establishment to be approximately 5%.
Does estimated dispersal and establishment mirror observed plant distributions?
If our estimates of average dispersal distance, shape of dispersal curves, and probability of establishment are realistic, we should see these values reflected in the spatial distribution of adult and juvenile plants. We therefore mapped all S. purpurea individuals in two 5 x 5 m plots at Hawley Bog. Each plant was located (±1 cm) using a Sonin electronic rangefinder (Forestry Suppliers) and the rosette diameter measured. Plants <10 cm in diameter were considered juveniles, and those 
10 cm were considered adults, as we have never seen a plant <10 cm flower. We located 43 plants in plot 1 and 303 in plot 2. The smallest plants found had rosette diameters of 0.5–1 cm. Spatial clumping was assessed visually and by comparing the empirical distribution of plants to that predicted from a random (Poisson) process. Visualization and analysis were done with the SpatialStats module of S-Plus 6 for Windows (Insightful Corporation, Seattle, Washington, USA). In all plots, spatial clustering of plants was clear (Fig. 2). In both plots, significant clustering of plants occurred at spatial scales up to 200 cm (P < 0.05, based on Monte Carlo simulations of Ripley's K function [Ripley, 1976
]). Additionally, in plot 2, we observed significant spatial autocorrelation in rosette diameters; large plants tended to cluster with other large plants, and small plants tended to cluster with other small plants (Moran's statistic = 3.4, exact P = 0.006). For plot 1, in which sample size was comparatively small, no significant spatial autocorrelation was observed (Moran's statistic = –0.09, exact P = 0.47).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODS AND RESULTSDISCUSSIONLITERATURE CITED |
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), Sarracenia purpurea seeds disperse over short distances (Fig. 1) and seedlings cluster together about the parent plant (Fig. 2). Based on controlled plantings, seedling mortality in the field appears to be high (approximately 95%). Although germination in laboratory conditions approaches 100% (Ellison, 2001
), we did not estimate directly germination in the field. Rather, our estimate of establishment probability includes the unmeasured effects of seed predators, herbivory, and disease, among other post-germination risk factors. The observed negative exponential distribution of dispersal, and the observed clumping of plants suggests that seed dispersal is important in generating small-scale patterns of plant distribution. If small seedling mortality were nonrandom, the spatial patterns generated by seed dispersal models based on the negative exponential distribution would not match the field distribution of juveniles nearly as well.
Sarracenia seeds are very small, and they have no obvious ornamentation, eliasomes, or other structures to attract potential long-distance dispersers. Although seeds of S. purpurea are hydroscopic (A. M. Ellison, Mount Holyoke College, personal observation) and long-distance dispersal may be effected by flotation, the maximum dispersal distance recorded for another water-dispersed herb, Mimulus guttatus DC, is only 4 m (Waser, Vickery, and Price, 1982
). If S. purpurea seeds dispersed by water have a similar dispersal distance, or even one that is 1–2 orders of magnitude greater, they are still unlikely to disperse easily between isolated bogs. Thus, the broad range expansion of S. purpurea since the end of the Pleistocene glaciation, from the southern coastal plains up the east coast of North America and across Canada to British Columbia, is explicable only by invoking rare, very long-distance transport events.
The population genetic structure of S. purpurea suggests that such long-distance dispersal events have occurred and have led to diversification by isolation. This taxon is normally divided into two subspecies, subsp. purpurea and subsp. venosa, with the former occurring northward from Maryland, just south of the limit of the Pleistocene glaciation, and the latter occurring from Maryland south (Schnell, 1979
). Three disjunct varieties have been described for the southern subspecies: var. venosa (Raf.) Fernald that is found on the southern Atlantic coastal plain of the United States, var. montana Schnell & Determann that grows in the Appalachian Mountains of Georgia and the Carolinas, and var. burkii Schnell that is found only on the coastal plain of the Gulf of Mexico in Florida and Louisiana (Schnell, 1993
; Schnell and Determann, 1997
). The latter variety has been recently elevated to species status as S. rosea Naczi, Case & Case (Naczi et al., 1999
). These disjunct occurrences of the subspecies and varieties of S. purpurea correspond to genetic differentiation (Godt and Hamrick, 1998
), morphological variation (Naczi et al., 1999
), and differences in seed size and dormancy requirements (Ellison, 2001
).
Throughout its range, however, S. purpurea occurs in isolated bogs. Rare long-distance dispersal events could have resulted in the distributional pattern of subspecies and varieties if diversification occurred in peripheral bogs following dispersal. Seed size and dormancy vary little within a subspecies or variety (Ellison, 2001
), but genetic and morphological differentiation within and among populations of each subspecies and variety have not been investigated. Such studies are needed to assess the prevalence of founder effects in this species, with an eye towards determining the genetic variability among bog plants generally. The occurrence of so many isolated populations of a single taxon presents a unique challenge to any general theory of long-range seed dispersal (e.g., Clark et al., 1998
) and an opportunity to understand better the potential for species persistence in highly fragmented habitats.
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FOOTNOTES |
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2 Author for reprint requests (aellison{at}fas.harvard.edu
), current address (as of 1 July 2002): Harvard University, Harvard Forest, P.O. Box 68, Petersham, MA 01366 USA
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMETHODS AND RESULTSDISCUSSIONLITERATURE CITED |
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Cain M. L. B. G. Milligan A. E. Strand 2000 Long-distance seed dispersal in plant populations. American Journal of Botany 87: 1217-1227
Clark J. S. 1998 Why trees migrate so fast: confronting theory with dispersal biology and the paleorecord. American Naturalist 152: 204-224[CrossRef][ISI]
Clark J. S. et al 1998 Reid's paradox of rapid plant migration. BioScience 48: 13-24[CrossRef][ISI]
Ellison A. M. 2001 Interspecific and intraspecific variation in seed size and germination requirements of Sarracenia (Sarraceniaceae). American Journal of Botany 88: 429-437
Godt M. J. W. J. L. Hamrick 1998 Genetic divergence among infraspecific taxa of Sarracenia purpurea. Systematic Botany 23: 427-438[CrossRef][ISI]
Higgins S. I. D. M. Richardson 1999 Predicting plant migration rates in a changing world: the role of long-distance dispersal. American Naturalist 153: 464-475[CrossRef][ISI]
Moizuk G. A. R. B. Livingston 1966 Ecology of red maple (Acer rubrum L.) in a Massachusetts upland bog. Ecology 47: 942-950[CrossRef][ISI]
Naczi R. F. C. E. M. Soper F. W. Case Jr. R. B. Case 1999 Sarracenia roesa (Sarraceniaceae), a new species of pitcher plant from the southeastern United States. Sida 18: 1183-1206
Nathan R. H. C. Muller-Landau 2000 Spatial patterns of seed dispersal, their determinants and consequences for recruitment. Trends in Ecology and Evolution 15: 278-285
Pakeman R. J. 2001 Plant migration rates and seed dispersal mechanisms. Journal of Biogeography 28: 795-800[CrossRef][ISI]
Pellerin S. C. Lavoie 2000 Peatland fragments of southern Quebec: recent evolution of their vegetation structure. Canadian Journal of Botany 78: 255-265
Rabinowitz D. J. K. Rapp 1980 Seed rain in a North American tall grass prairie. Journal of Applied Ecology 17: 793-802[CrossRef][ISI]
Reid C. 1899 The origin of the British flora. Dulau, London, UK
Ripley B. D. 1976 The second-order analysis of stationary point processes. Journal of Applied Probability 13: 255-266[CrossRef][ISI]
Schnell D. E. 1979 A critical review of published variants of Sarracenia purpurea L. Castanea 44: 47-59[ISI]
Schnell D. E. 1993 Sarracenia purpurea L. ssp. venosa (Raf.) Wherry var. burkii Schnell (Sarraceniacaea): a new variety of the Gulf coastal plain. Rhodora 95: 6-10[ISI]
Schnell D. E. R. O. Determann 1997 Sarracenia purpurea L. ssp. venosa (Raf.) Wherry var. montana Schnell & Determann (Sarraceniaceae): a new variety. Castanea 62: 60-62
Waser N. M. R. K. Vickery M. V. Price 1982 Patterns of seed dispersal and population differentiation in Mimulus guttatus. Evolution 36: 761
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