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Department of Biology, Queen's University Kingston, Ontario, Canada K7L 3N6
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INTRODUCTION |
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 TOP INTRODUCTION CONCLUSIONLITERATURE CITED |
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; Harper, 1978
). During the decade that followed, other biologists joined Harper & co. in exploring the consequences of modularity for the ecology, and evolution of clonal plants, and animals (reviewed in Jackson, Buss, and Cook, 1985
; Harper, Rosen, and White, 1986
). From this activity, emerged a set of questions concerning morphology, physiology, ecology and evolution that continues to dominate current research on clonal plants, and has been pursued by a cosmopolitan but close-knit group of researchers. Since the late 1980s, clonal plant enthusiasts have met every two years or so at small workshops, the proceedings of which have been duly recorded in several volumes appearing as books (van Groenendael and de Kroon, 1990
), special issues of journals (Callaghan et al., 1992
) or both (Soukupová et al., 1994
; Oborny and Podani, 1996
). The Ecology and Evolution of Clonal Plants is the latest installment from the clonal plant cognoscenti. Like the four previous volumes, it is a compendium of review articles that, according to the editors (Hans de Kroon and Jan van Groenendael), "demonstrates how elementary processes at the level of the individual, the population and the community are influenced by clonal growth." My overall impression is that this is a fairly comprehensive and nicely put-together summary of contemporary research on clonal plants. Even though the book covers a lot of ground, from physiology and development to community ecology and global change, most of the chapters provide the general reader with sufficient background to appreciate why the questions at hand are important and to evaluate the progress made in addressing them. Each chapter ends with an extensive list of references so that the book, as a whole, provides a wide window on the diverse literature concerning clonal plants. This book would make an excellent starting point for a graduate-level course on clonal plants. In this way, it may ultimately fulfill one of the editors' major objectives: to make the interesting things that clonal plants do "more widely appreciated in future studies" at the various levels of biological organization.
It is somewhat unusual that The Ecology and Evolution of Clonal Plants was published on the heels of four other edited volumes that have appeared at regular intervals over the last ten years. In addition, all these volumes have involved many of the same authors. At least one author on 15 of the 18 chapters in this recent book has also authored a chapter in one of the previous volumes, usually on the same general topic. What does this proliferation of review articles mean? How distinct is this most recent volume from previous efforts? In reviewing this book, I have used these four previous volumes as a kind of fossil record to gauge how quickly this field is evolving. Below, I discuss a sample of chapters from this new book in terms of what questions are being asked, what approaches are being used to answer them, and how the field of clonal plant research has progressed over the last decade.
Development, integration, and environmental heterogeneity
Much of the interest in clonal plants focuses on how their pattern of development influences the way they grow, how they capture resources, and how they respond to environmental variation in space and time. Six of the 18 chapters in this book address these general questions from a variety of angles. Watson, Hay, and Newton
discuss how the developmental program of clonal plants can be understood in terms of meristem demography (i.e., meristem birth, death, dormancy, and allocation to growth vs. reproduction). The demographic properties of meristems may, in turn, influence how clonal genotypes (genets) respond to environmental heterogeneity. The meristem perspective is closely related to the large body of work on module demography by Harper and his students (reviewed by Harper, 1978
), and has been promoted for some time by Watson and others
(e.g., Watson, 1984
; White, 1984
; Fagerström, 1992
; Bonser and Aarssen, 1996
). This chapter summarizes a fair bit of previous work on water hyacinth (Eichhornia crassipes) and describes some recently published data on mayapple (Podophyllum peltatum).
Watson et al.
argue that meristem demography will strongly influence the persistence and proliferation of ramets and genets in natural populations. Accordingly, they urge the reader to "move away from the notion that sexual reproduction is the primary means by which clonal plants achieve fitness" and embrace the view that "ramet persistence and the success of clones at generating new ramets need to be considered as equally important elements in estimations of clonal fitness". This statement puts clonal propagation on the same level as sexual reproduction in terms of the transmission of genes over evolutionary time: a rather controversial position (see, for example, Mogie, 1992, pp. 4–15
). I agree that the rate and mode of clonal spread will influence fitness, but I am not fully convinced by Watson et al.'s
argument that clonal growth is a means of achieving fitness. Much depends on the evolutionary dynamics of the species in question. For example, rapid clonal spread may strongly enhance the representation of genotypes within individual populations, but may be of less importance in the long term if populations undergo frequent extinction and recolonization via seed. At this point we do not know enough to say whether a ramet- or meristem-based definition of clonal fitness will really improve our understanding of microevolution in clonal plants.
One of the most significant and widely investigated characteristics of clonal plants is that the component ramets that make up a genet may be physiologically integrated and share resources. Marshall and Price review how experiments involving radioactive tracers and selective defoliation, the mainstay techniques in this field, are used to define the extent of "integrated physiological units" and to help understand how morphology influences physiological integration. This chapter is a good review, but does not present much in the way of new ideas and data. Most of the chapter goes over the same ground reviewed by Price and Hutchings (1992)
, and features data previously presented by Price and Hutchings (1992)
and Kemball and Marshall (1995)
.
Jónsdóttir and Watson approach clonal integration by defining the potential for physiological integration in terms of three variables: ramet longevity, ramet generation time, and the longevity of physical connections between ramets. A factorial combination of these variable results in five potential categories of plants that differ in the size of connected ramet networks and the extent to which the ramet network is physiologically integrated. A new comparative analysis that places 51 species in these five categories suggests that the maintenance of large, integrated ramet networks is most common in plants inhabiting stable but resource-poor habitats, although this result may be weakened by potentially confounding correlations between clonal mode, habitat type, and phylogenetic relatedness.
The adaptive significance of physiological integration in resource-poor environments is further explored by comparing Jónsdóttir's previously published data on 14C translocation in Carex bigelowii with new data from Podophyllum peltatum. This is a difficult problem to tackle experimentally. Reduced growth of individual ramets and whole genets has been observed after connections between ramets are experimentally severed. However, this experimental approach focuses on the advantages of clonal integration but does not assess the risks and costs of maintaining physiological connections between ramets. These costs would seem much more difficult to quantify.
It is clear from these chapters that the extent to which ramets are physiologically integrated varies markedly between species, yet very few species have been studied in much detail. Later on in the book, the reader is told that many important consequences of clonal growth for population and community dynamics depend on the degree of clonal integration. The difficulty of making generalizations about the nature of integration within genets is likely to hinder the interpretation of patterns and processes in clonal populations and communities.
Consider a clonal plant that maintains physiological connections between ramets in a patchy environment. Some ramets wind up in patches with an abundance of one resource, whereas others are in spots where a complementary resource is abundant. If the resource-extraction activities of ramets could be co-ordinated, the genet as a whole could benefit from different ramets acquiring different locally abundant resources and then sharing them with each other. Alpert and Stuefer develop the idea of division of labor in clonal plants by analogy with division of labor in human economic systems. The analogy results in a particularly clear definition of the problem. Division of labor in clonal plants requires that: (1) Ramets share resources. (2) Individual ramets allocate most effort to extracting resources that are most abundant to them rather than scarce resources that would limit their growth if they were not connected to other ramets. (3) Genets benefit more from resource sharing when individual ramets specialize on locally abundant resources. In other words, there should be a positive interaction between resource sharing and specialization. They go on to present fairly convincing published and unpublished evidence for (1) and (2) and discuss the need for future work on (3). They also outline the various risks and costs of maintaining division of labor, most of which depend on ecological factors such as disturbance and pathogens. Confirming that division of labor is adaptive will require some highly controlled experiments, the results of which will ultimately have to be validated in the highly uncontrolled world of real habitat.
How clonal plants spread in space and whether their meandering through heterogeneous environments can be viewed as "foraging" have received considerable attention. Oborny and Cain review the various theoretical approaches used to study the spread of clonal genets in relation to environmental heterogeneity. Although this seems to be an active area of research, these authors dwell extensively on previously reviewed material. In fact, the four figures they present are recycled directly from reviews published in previous books on clonal plants (Sutherland and Stillman, 1990
; Oborny, 1994
; Cowie, Watkinson, and Sutherland 1996
).
Ramet- and genet-level interactions in populations and communities
Populations and communities of clonal plants may often consist of complex networks of ramets that are physiologically integrated to some extent. Four chapters explore how these aspects of clonal growth affect the nature of intraspecific and interspecific interactions. Van der Hoeven and During review a substantial body of experimental work with mosses that investigates the interactions between shoots and the mechanisms that regulate shoot densities (see also During, 1990
). While there seems to be good evidence for positive interactions between shoots leading to facilitation in moss populations, most of this chapter has relatively little to do with clonal growth per se. Because clonal propagation in mosses occurs primarily through ramet fragmentation and the production of specialized asexual propagules, there is little opportunity for physiological integration among ramets and, thus, higher order effects of clonal growth.
The development of size hierarchies in plant populations and self-thinning are central issues in plant population ecology. Suzuki and Hutchings explore how clonal growth affects these processes. Clonal plants may differ in terms of shoot dynamics from widely studied annual plants because large quantities of resources are often stored in perennating organs, and clonal integration may allow internal control of shoot proliferation. The authors review empirical and theoretical evidence that resource storage reduces self-thinning in clonal populations. However, as the authors point out, resource storage also occurs in non-clonal perennials. Whether there are specific consequences of clonal growth (i.e., internal control) remains unclear.
Clonal growth adds an extra dimension to competition in plant communities because clonal plants not only compete for resources via overtopping and root competition, but also compete for space through clonal expansion. Space invasion may, in turn, be facilitated by physiological integration among ramets: connected ramets can support one another while battling neighbors. Herben and Hara explore the consequences of clonal growth for interspecific competition. They begin by discussing the relative importance of founder control vs. dominance control in determining the outcome of experimental two-species interactions (see also Herben, 1996
), move through how modeling might link ramet-level processes to community dynamics, and end by considering the factors that affect species coexistence in communities where clonal plants are common. Given the limited data on mechanisms of clonal integration in individual species (see above), it is no surprise that hypotheses far outnumber conclusions at the community level.
In the last chapter dealing with the consequences of clonal growth for community-level interactions, de Kroon and Bobbink review both published and unpublished case studies to fuel some informed speculation on why atmospheric deposition of nitrogen in many areas of western Europe has been associated with increasing dominance by clonal species. They hypothesize that clonal plants compete effectively under high nutrient conditions by sequestering nitrogen in well-developed storage organs, and rapidly preempting space via clonal spread. However, this chapter is reminiscent of many other efforts to identify general characteristics of invasive or aggressive species; no hard and fast rules emerge.
Population genetics and evolution of clonal plants
Because clonal spread results in the proliferation of genetically identical ramets and is often associated with a low frequency of sexual recruitment, clonal growth may directly affect the distribution of phenotypic and genetic variation within and among populations, and, ultimately, how populations evolve. The four chapters that explore the evolutionary implications of clonal growth were of particular interest to me. However, I found them largely lacking in terms of new ideas or critical analysis of old ideas. McLellan, Prati, Kaltz, and Schmid provide a wide-ranging review of the population-genetic consequences of clonal growth. They begin by discussing some of the technical and statistical issues that complicate the analysis of allelic vs. genotypic diversity in clonal populations, go on to review the limited work on how clonal growth affects the amount and structuring of genetic variation within and among populations, and end by discussing what little is known about the extent to which genetic diversity in clonal populations is maintained by various factors, including sporadic sexual recruitment, frequency-dependent selection involving pathogens, environmental heterogeneity, somatic mutation, and gene flow.
There are gaping holes in our knowledge of the evolutionary genetics of clonal plants. Although McLellan et al. do an excellent job of identifying the questions that warrant serious attention, their review touches on too many things to critically dissect any particular issue. For instance, they reiterate one of the most widely cited conclusions from population-genetic studies of clonal plants: "on average, clonal plants do not seem to be genetically less variable than non-clonal plants" (see also Ellstrand and Roose, 1987
; Hamrick and Godt, 1990
; Widén, Cronberg, and Widén, 1994
). While I agree that the old notion of clonal monomorphism has rightly fallen by the wayside, no one has attempted a rigorous comparison of genetic structure in obligately sexual perennials vs. perennials where sexual recruitment is extremely limited (i.e., what the authors of this volume typically view as clonal plants). Moreover, there have been relatively few studies that have exploited intraspecific variation in reproductive mode (i.e., clonal vs. sexual) to more directly investigate how clonal growth affects genetic structure.
It seems that our efforts to understand how clonal growth affects genetic diversity has focussed too closely on average levels of diversity and ignored the repeated observation that clonal species exhibit particularly wide variation in genetic structure among populations. For instance, data on genotypic diversity within populations of 32 predominantly clonal species summarized by Widén, Cronberg, and Widén (1994)
indicate that single-genotype populations occur along with genotypically diverse populations in more than a dozen species. What are the ecological and evolutionary causes and consequences of this variation? In many species, the relative importance of clonal vs. sexual recruitment may be influenced strongly by environmental factors. As a result, populations at the periphery of a species' geographical range may contain relatively little genotypic diversity, thereby limiting the potential for evolutionary expansion of the species' distribution.
McLellan et al. emphasize the lack of data on how clonal growth affects the spatial distribution of genotypes and alleles within populations. However, they do not clearly indicate why this is an interesting question. Yet, the spatial dispersion of clones within a population may have several significant consequences. For example, localized clonal spread may interfere with sexual reproduction by reducing pollen transfer and mating between genets (Handel, 1985
). High levels of between-ramet self-pollination (geitonogamy) may have a major impact on the evolution of the breeding system (e.g., Harder and Barrett, 1996
). The breeding system may, in turn, impose selective pressures on clonal growth form (Silander, 1985
). However, there has been almost no effort to quantify the effect of different modes of clonal growth on the mating system, or to investigate whether plants with different breeding systems possess different modes of clonal growth.
Extensive clonal growth may affect the evolution of life history traits in a variety of ways. Ove Eriksson explores the evolution of seed dispersal and recruitment in clonal populations. Experimental and comparative evidence suggests that the evolution of seed size and dispersal behavior is influenced by when seed recruitment occurs during the history of a population. If recruitment is restricted to the time when a population is founded, small dispersible seeds should be favored. If recruitment occurs infrequently but repeatedly throughout the life span of a population, large competitive seeds should be favored. Eriksson has been developing this idea for about a decade. This chapter presents a bit of new empirical data and minor refinements to the general hypothesis, but no new conclusions or predictions. The evolution of seed recruitment and dispersal in clonal populations has recently been examined in the context of metapopulation dynamics (Olivieri, Michalakis, and Gouyon, 1995
; Piquot et al., 1998
). Cycles of colonization and extinction are strongly implied in Eriksson's model, and it would have been nice to see a link developed between these two theoretical approaches.
Eriksson's hypothesis assumes that there is a trade-off between sexual reproduction and clonal growth. This notion is discussed in passing several times throughout the book but is never dealt with directly. The cost of sexual reproduction in terms of reduced survival and growth is a major issue in life-history theory. However, determining the nature of the cost is often complicated because the expected genetic trade-off between sexual reproduction and growth may be obscured by positive phenotypic correlations: vigorous individuals do everything well (Roff, 1992
). Clonal plants offer excellent opportunities for investigating life-history trade-offs because individual genotypes may be replicated and then subjected to various treatments in an array of experimental environments.
Clonal growth may influence the long-term evolution of life history by affecting the accumulation of mutations. Klekowski discusses this idea with respect to his "somatic mutation theory of clonality." He argues that, in the absence of sexual recombination, genetic diversity in clonal populations is generated through somatic mutation. As clones age and spread they are likely to accumulate deleterious mutations; a process similar to the mutational meltdown in asexual populations envisioned by Michael Lynch and co-workers (e.g., Lynch et al., 1993
). However, mutation-selection balance in highly clonal populations is expected to allow the build-up of mutational load that specifically impairs sexual reproduction, because the processes involved in sex contribute little to the fitness of genets. This is analogous to the loss of eyes and pigments in cave-dwelling organisms (Fong, Kane, and Culver, 1995
), a case of "use it or lose it." This hypothesis might explain the many observations of infrequent sexual reproduction in highly clonal populations (e.g., Sculthorpe, 1967
). Yet, the somatic mutation theory of clonality has received little empirical investigation (Eckert, Dorken, and Mitchell, 1999
). In fact, few studies have even determined the extent to which low seed production in clonal populations is due to genetic vs. environmental factors. The lack of attention to this hypothesis has always seemed strange to me, as Klekowski has published a lot of interesting work on this subject (e.g., Klekowski, 1988
). Because this chapter only scratches the surface of his previous work and does not present any substantially new thinking on the topic, it may not generate new interest in the long-term evolutionary interactions among clonal reproduction, mutation, and sexual reproduction.
Somatic mutation is repeatedly invoked as a source of genetic variation in clonal populations, yet there has been little or no effort to directly quantify the rate at which somatic mutation generates variation. McLellan et al. suggest that "it would be interesting to produce genetically homogeneous populations of clonal plants by vegetative reproduction or by selection and then follow the reappearance of heritable variation in the subsequent generations of sexually and vegetatively produced offspring." Unfortunately, they do not indicate how many generations it might take until the variation created by somatic mutation might be experimentally detectable, the sample sizes required, or how one might compare the amount of heritable variation between sexually- and clonally-produced offspring.
Because clonal genotypes are represented by multiple ramets, phenotypic variation and genetic variation are organized hierarchically at the population level. There may be heritable differences in fitness at the ramet and genet level that arise from variation in the performance of individual ramets as well as higher order differences in fitness among genets that arise from genet-level characteristics (e.g., ramet number and density). Vuorisalo, Tuomi, Pedersen, and Käär present a statistical model for quantifying phenotypic selection in hierarchically structured populations. This is an important problem that challenges some fundamental notions about the operation of natural selection in clonal populations. However, it is a complex problem that is unlikely to attract much attention unless rendered in an accessible way. This chapter succeeds to some extent in this regard, because it boils down their previously published model into a more accessible format and illustrates how the model might be used with data on the factors affecting fruit dispersal in Podophyllum peltatum.
Determining the spatial boundaries of individual genets has long been one of the most vexing problems for workers interested in clonal plants. In the last five years, ecologists and evolutionary biologists have gained access to a wide variety of new, highly variable PCR-based genetic markers, such as RAPDs, ISSRs, AFLPs, SSCPs, and microsatellites, which greatly increase our ability to unambiguously identify individual genets. McLellan et al. briefly mention these new markers, but do not delve into how we might use them in novel ways. For example, a combination of chloroplast and nuclear markers could be used to quantify the relative importance of sexual reproduction and recruitment vs. gene flow in maintaining genotypic diversity in clonal populations. Sex would tend to break down disequilibrium between markers, whereas gene flow (without subsequent sex) would tend to increase it. A chapter exploring the new uses and potential pitfalls of PCR markers would have made a nice addition to this book, especially considering the profound implications of this technology for research on clonal plants.
Probably the most overarching evolutionary question involving clonal plants concerns the adaptive significance of clonal growth itself. Various chapters in this book briefly touch on the fitness benefits of clonal growth, especially with respect to spatial heterogeneity of resource abundance and risk-spreading. However, no one chapter presents a comprehensive evolutionary analysis. This is a difficult question because, among species, clonal growth happens in many different ways and is, therefore, likely to be associated with diverse costs and benefits (e.g., multiplication of ramets, dispersal, resource acquisition, anchorage, storage; Grace, 1993
). Given this diversity, a combination of experimental and comparative studies would seem most effective. Judging from the chapters in this book, there seem to be little experimental investigation of the costs and benefits of clonal growth. Experimental work would be greatly facilitated by intraspecific variation in one or more of the parameters that define clonal growth (e.g., Jónsdóttir and Watson's three variables relating to clonal integration, see above). For example, Watson, Hay, and Newton
briefly mention variation in the rate of ramet production among genotypes of Eichhornia crassipes. However, this kind of variation is clearly underexploited.
Klime
, Klime;t1ová, Hendriks, and van Groenendael use a comparative analysis to ask whether clonal growth is more common in some types of habitats than others or is associated with particular life history traits. This chapter is an expanded version of a chapter that appeared in another recent edited volume (van Groenendael et al., 1997
; see Mazer, 1998, for a critical review). In an attempt to organize the diversity of ways in which plants clone themselves, the authors come up with a rather cumbersome taxonomy of clonal growth modes that consists of 21 categories, each typified by a particular species (some of which may not be familiar to the North American readers). They go on to categorize the mode of clonal growth as well as ecological and life-history characteristics for 2760 plant species in central Europe.
Using species as independent data points, they show that clonal plants are less likely to self-fertilize, or be pollinated by insects than non-clonal plants, and are disproportionately common in cool, moist or low-nutrient environments. They explore the extent to which phylogenetic history influences these associations between ecology, life history and clonal growth by repeating the analyses at the family level. Families were mapped on to a DNA-based phylogeny of the seed plants, and correlations between family means for ecological and life history characters and the proportion of species in each family exhibiting clonal growth were tested using phylogenetically-independent contrasts. These analyses confirmed some of the species-level results but not others. Yet, it is not clear how we should reconcile the differences between the species-level and "phylogenetically-correct" analyses. Family-level averages for ecological and life history traits may be relatively meaningless if there is broad variation within families. The species- and family-level patterns detected also do not lend themselves to any clear evolutionary interpretation because the authors only report univariate analyses. As with most univariate analyses of multiple trait-environment and trait-trait correlations, it is often impossible to determine which associations represent adaptations and which arise indirectly from intercorrelations between various environmental and life history characteristics (i.e., multiple colinearity).
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CONCLUSION |
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TOPINTRODUCTIONCONCLUSIONLITERATURE CITED |
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However, if you have been closely following the literature on clonal plants and are looking for new ideas, this book may be less useful. It presents some new data plus refinements to existing hypotheses and approaches but does not contain much in the way of new breakthroughs. For the most part, this book represents the regular crowd asking the same questions in more or less the same way.
After reading this book in the context of the four previous edited volumes, I began to see clonal plants as a metaphor for the field of research that seeks to understand them. There has been extensive proliferation of journal papers and review articles exploring various aspects of clonal growth. There has also been considerable integration between the workers in this field, even though they are scattered around the globe. Ideas have been extensively shared, and there seems to have been a lot of collaboration between workers interested in similar questions. However, the ideas and approaches in clonal plant research have not evolved much in the past decade. Hopefully, the solid summary of past progress provided by this book will serve as a launching pad for future innovation.
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FOOTNOTES |
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Phone (613-533-6158), FAX (613-533-6617), e-mail (eckertc{at}biology.queensu.ca)
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LITERATURE CITED |
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TOPINTRODUCTIONCONCLUSIONLITERATURE CITED |
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Callaghan, T. V., B. M. Svensson, I. S. Jónsdóttir, and B. A. Carlsson [eds.]. 1992 Clonal plants and environmental change. Oikos 63: 339–453.
Cowie, N. R., A. R. Watkinson, and W. J. Sutherland. 1996 Modeling the growth dynamics of the clonal herb Anemone nemorosa in an ancient coppice wood. In B. Oborny and J. Podani [eds.], Clonality in plant communities, 35–50. Opulus Press, Uppsala, Sweden.
During, H. J. 1990 Growth patterns among bryophytes. In J. van Groenendael and H. de Kroon [eds.], Clonal growth in plants: regulation and function, 153–176. SPB Academic Publishing, The Hague, The Netherlands.
Eckert, C. G., M. E. Dorken, and S. A. Mitchell. 1999 Loss of sex in clonal populations of a flowering plant, Decodon verticillatus (Lythraceae). Evolution, in press.
Ellstrand, N. C., and K. L. Roose. 1987 Patterns of genotypic diversity in clonal plant species. American Journal of Botany 74: 123–131.[CrossRef][ISI]
Fagerström, T. 1992 The meristem-meristem cycle as a basis for defining fitness in clonal plants. Oikos 63: 449–453.[CrossRef][ISI]
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Hamrick, J. L., and M. J. Godt. 1990 Allozyme diversity in plant species. In A. H. D. Brown, M. T. Clegg, A. L. Kahler, and B. S. Weir [eds.], Plant population genetics, breeding, and genetic resources, 43–63. Sinauer, Sunderland, MA.
Handel, S. N. 1985 The intrusion of clonal growth patterns on plant breeding systems. American Naturalist 125: 367–384.[CrossRef][ISI]
Harder, L. D., and S. C. H. Barrett. 1996 Pollen dispersal and mating patterns in animal pollinated plants. In D. G. Lloyd and S. C. H. Barrett [eds.], Floral biology: studies on floral evolution in animal-pollinated plants, 140–190. Chapman & Hall, New York, NY.
Harper, J. L. 1978 The demography of plants with clonal growth. In A. H. J. Freysen and J. W. Woldendorp [eds.], Structure and functioning of plant populations, 27–48. North-Holland, Amsterdam, The Netherlands.
————, B. R. Rosen, and J. White [eds.]. 1986 The growth and form of modular organisms. The Royal Society, London, U.K. (also appeared in Philosophical Transactions of the Royal Society, Series B, 313: 1–250, 1986)
————, and J. White. 1974 The demography of plants. Annual Review of Ecology and Systematics 5: 419–463.
Herben, T. 1996 Founder and dominance control: neglected concepts in the community dynamics of clonal plants. In B. Oborny and J. Podani [eds.], Clonality in plant communities, 3–10. Opulus Press, Uppsala, Sweden.
Jackson, J. B. C., L. W. Buss, and R. E. Cook [eds.]. 1985 Population biology and evolution of clonal organisms. Yale University Press, New Haven, CT.
Kemball, W. D., and C. Marshall. 1995 Clonal integration between parent and branch stolons in white clover: a developmental study. New Phytologist 129: 513–521.[CrossRef][ISI]
Klekowski, E. J., Jr. 1988 Mutation, developmental selection and plant evolution. Columbia University Press, New York, NY.
Lynch, M., R. Bürger, D. Butcher, and W. Gabriel. 1993 The mutational meltdown in asexual populations. Journal of Heredity 84: 339–344.
Mazer, S. J. 1998 Alternative approaches to the analysis of comparative data: compare and contrast. American Journal of Botany 85: 1194–1199.[Abstract]
Mogie, M. 1992 The evolution of asexual reproduction in plants. Chapman & Hall, New York, NY.
Oborny, B. 1994 Spacer length in clonal plants and the efficiency of resource capture in heterogeneous environments: a Monte Carlo simulation. In L. Soukupová, C. Marshall, T. Hara and T. Herben [eds.], Plant clonality: biology and diversity, 33–52. Opulus Press, Uppsala, Sweden.
————, and J. Podani [eds.]. 1996 Clonality in plant communities. Opulus Press, Uppsala, Sweden (also appeared in Abstracta Botanica 19:1–127, 1995)
Olivieri, I., Y. Michalakis, and P. H. Gouyon. 1995 Metapopulation genetics and the evolution of dispersal. American Naturalist 146: 202–228.[CrossRef][ISI]
Piquot, Y., D. Petit, M. Valero, J. Cuguen, P. de Laguerie, and P. Vernet. 1998 Variation in sexual and asexual reproduction among young and old populations of the perennial macrophyte Sparganium erectum. Oikos 82: 139–148.
Price, E. A. C., and M. J. Hutchings. 1992 The causes and developmental effects of integration and independence between different parts of Glechoma hederacea clones. Oikos 63: 376–386.[CrossRef][ISI]
Roff, D. A. 1992 The evolution of life histories. Chapman and Hall, New York, NY.
Sculthorpe, C. D. 1967 The biology of aquatic vascular plants. Edward Arnold, London.
Silander, J. A., Jr. 1985 Microevolution in clonal plants. In J. B. C. Jackson, L. W. Buss, and R. E. Cook [eds.], Population biology and evolution of clonal organisms, 107–152. Yale University Press, London.
Soukupová, L., C. Marshall, T. Hara, and T. Herben [eds.]. 1994 Plant clonality: biology and diversity. Opulus Press, Uppsala, Sweden. (also appeared in Folia Geobotanica et Phytotaxonomica 29: 113–320, 1994)
Sutherland, W. J., and R. A. Stillman. 1990 Clonal growth: insights from models. In J. van Groenendael and H. de Kroon [eds.], Clonal growth in plants: regulation and function, 95–112. SPB Academic Publishing, The Hague, The Netherlands.
van Groenendael, J. M., L. Klime, J. Klime;t1ová, and R. J. J. Hendriks. 1997. Comparative ecology of clonal plants. In J. Silvertown, M. Franco, and J. L. Harper [eds.], Plant life histories. Ecology, phylogeny and evolution, 191–209. Cambridge University Press, Cambridge.
————, and H. de Kroon [eds.]. 1990. Clonal growth in plants: regulation and function. SPB Academic Publishing, The Hague, The Netherlands.
Watson, M. A. 1984 Developmental constraints: effect on population growth and patterns of resource allocation in a clonal plant. American Naturalist 123: 411–426.[CrossRef][ISI]
White, J. 1984 Plant metamerism. In R. Dirzo and J. Sarukhán [eds.], Perspectives on plant population ecology, 15–47. Sinauer, Sunderland, MA.
Widén, B., N. Cronberg, and M. Widén. 1994 Genotypic diversity, molecular markers and spatial distribution of genets in clonal plants. In L. Soukupová, C. Marshall, T. Hara, and T. Herben [eds.], Plant clonality; biology and diversity, 139–157. Opulus Press, Uppsala, Sweden.
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E. LHUILLIER, J.-F. BUTAUD, and J.-M. BOUVET Extensive Clonality and Strong Differentiation in the Insular Pacific Tree Santalum insulare: Implications for its Conservation Ann. Bot., November 1, 2006; 98(5): 1061 - 1072. [Abstract] [Full Text] [PDF]
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