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Ecology |
ek2,6
ina Bímová3
ch Jaroík2,4
tpánek2
2Institute of Botany, Academy of Sciences of the Czech Republic, CZ-252 43 Pr
honice, Czech Republic;
3Institute of Applied Ecology, Czech Agricultural University Prague, CZ-281 63 Kostelec nad 
ern
mi lesy, Czech Republic;
4Department of Zoology, Charles University, Prague, Vini
ná 7, CZ-128 00 Praha 2, Czech Republic
Received for publication January 30, 2003. Accepted for publication May 9, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Czech Republic • emergence time • Fallopia • genotype • isozyme analysis • plant invasion • Polygonaceae • regeneration • Reynoutria • shoot growth
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, 2000
; Lonsdale, 1999
; Richardson et al., 2000
; Grotkopp et al., 2002
) and practical connotations (McNeely et al., 2001
; Rejmánek and Reichard, 2001
). Vast numbers of species have been translocated over the globe, and these alien taxa contribute remarkably to species numbers of local floras (e.g., Weber, 1997
). By competing with native species, invaders may reduce native species diversity (Pyek and Pyek, 1995
; Higgins et al., 1999
), although only a small number of alien species appear to negatively affect native communities (Williamson, 2001
; Cronk and Fuller, 1995
). For example, only 90 species of the 1378 Czech alien species are classified as invasive (Pyek et al., 2002
), and only a fraction of these can be considered as transformer species that change the character, condition, form, or nature of ecosystems (Wells et al., 1986
; Richardson et al., 2000
). The low percentage of alien species that interfere with human objectives appears to be typical, described as the "tens rule" (Williamson, 1996
; Williamson and Fitter, 1996
). Species of the genus Reynoutria represent a remarkable example of transformer species not only in central Europe but in a number of regions of the world (Sukopp and Sukopp, 1988
; Cronk and Fuller, 1995
; Sukopp and Starfinger, 1995
).
The mode of reproduction is a crucial determinant of any successful invasion (Daehler and Strong, 1994
, 1996
). High fecundity and capability of sexual reproduction, which generates genetic variation (Crawley, 1997
), has been a common attribute in most lists of characteristics that promote invasion since early attempts to define an ideal invader (e.g., Baker, 1965
; Noble, 1989
; Roy, 1990
; Saxena, 1991
; Richardson and Cowling, 1992
; Rejmánek, 1995
; Crawley et al., 1996
). However, vigorous vegetative reproduction can be a similarly effective means of spread (Baker, 1965
, 1986
; Ashton and Mitchell, 1989
; Roy, 1990
; Mitchell and Gopal, 1991
; Saxena, 1991
). Plants that reproduce clonally are, in terms of whole floras, as successful invaders as those relying solely on sexual reproduction (Pyek, 1997
). Clonal plants are common in the central European flora (Klime et al., 1997
), and their successful invasion can be related to various aspects of plasticity in clonal growth (van Groenendael and de Kroon, 1990
; Beerling et al., 1994
). Indeed, some highly successful invaders reproduce mainly vegetatively in their introduced geographical range (Room, 1990
; Williamson, 1996
), including Reynoutria japonica (Hollingsworth et al., 1998
; Hollingsworth and Bailey, 2000
). In such cases, we face an appealing natural experiment because when production of new individuals is limited to vegetative means, genetically identical progeny are produced and vegetative regeneration becomes the necessary condition of successful invasion (Bímová et al., 2003
).
In central Europe, the genus Reynoutria is represented by R. japonica Houtt var. japonica, R. japonica Houtt. var. compacta Moldenke, R. sachalinensis (F. Schmidt) Nakai, and a hybrid between R. sachalinensis and R. japonica var. japonica, i.e., R. xbohemica Chrtek and Chrtková. The situation of two parental species and their hybrid, which are all invasive, creates a challenging opportunity to study the effect of hybridization on plant invasions (Abbott, 1992
; Ellstrand and Schierenbeck, 2000
; Vila et al., 2000
; Daehler and Carino, 2001
). The fitness of hybrid progeny affects genetic structure of populations in particular taxa, and such studies contribute to assessing the role of natural hybridization in adaptive ecology and post-invasion evolution (Arnold, 1997
; Ellstrand and Schierenbeck, 2000
; Lee, 2002
).
Previous studies on invasive Reynoutria congeners in the Czech Republic focused on the history of introduction and spread (Pyek and Prach, 1993
), the effect of meadow management on their establishment in the wild (Brabec and Pyek, 2000
), long-term persistence of clones in the landscape (Pyek et al., 2001
), possibilities for control (Bímová et al., 2001
), and vegetative regenerative capability (Bímová et al., 2003
). Because the latter work studied differences in vegetative regeneration from rhizome and stem fragments between the taxa, in the present paper we focus on the genotype level and raise the following questions: Do particular genotypes of the Reynoutria taxa that exhibit genetic diversity in the Czech Republic differ in their regenerative potential? If so, is this difference related to the early growth immediately after the establishment (do more successfully regenerating genotypes also grow faster?), and are these differences manifested at the regional level (are easily regenerating genotypes more widely distributed and more abundant than those with low regenerative ability?)? The taxon R. japonica var. compacta was not considered in the present study because it is rare in the Czech Republic (Pyek et al., 2002
).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Sukopp and Sukopp, 1988
; Pyek and Prach, 1993
; Sukopp and Starfinger, 1995
). They are stout, vigorously growing, clonal perennials that produce a large amount of biomass (Brock, 1995
) in vast stands. They are most invasive in riparian and various humanmade habitats and often spread into seminatural vegetation (Pyek and Prach, 1993
). The spread of Reynoutria taxa in central Europe is mainly vegetative, through regeneration from rhizome and stem segments. A new plant can originate from a rhizome segment as small as 0.7 g (Brock and Wade, 1992
; Brock et al., 1995
), and the regeneration rate is generally very high (Child, 1999
; Bímová et al., 2003
). New shoots have been reported to arise from both rhizome and stem nodes (Brock and Wade, 1992
; Beerling et al., 1994
; Brock et al., 1995
), where lateral buds are located (Adachi et al., 1996
).
Members of the genus Reynoutria were originally reported not to reproduce sexually within the secondary geographical range because of their almost complete incapability to produce viable seed and/or its inefficient seedling establishment (Bailey et al., 1995
). However, genetic variability found in R. xbohemica, the hybrid between R. japonica var. japonica and R. sachalinensis, indicates that occasional hybridization might occur (Hollingsworth et al., 1998
; Hollingsworth and Bailey, 2000
). The species differ in their ecology, establishment success (Brabec and Pyek, 2000
; Bímová et al., 2003
), response to control measures (Bímová et al., 2001
), and genetic variation (Hollingsworth et al., 1998
).
Regeneration experiment
The identity of particular genotypes was confirmed by isozyme analysis performed on clones sampled within the territory of the Czech Republic in 1998–2001 and brought to cultivation in the experimental garden of the Institute of Botany, Academy of Sciences of the Czech Republic, in Prhonice, near Prague (49°59'41'' N, 14°33'56'' E). This research provided an overall picture of the genetic variation within the studied complex of taxa and yielded information about the number of genotypes present in the region and their frequency (Fig. 1). Genotypes used in the present study were chosen on the basis of this knowledge.
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) had shown that regeneration from rhizomes is the major mode of regeneration in the complex of taxa studied, other parts of the plant body were not considered in this study. Rhizomes were collected, brought to Prhonice, and kept at 4°C until 30 May when the experiment was established.
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), rhizome segment size was taken into account only when comparing the growth of genotypes within taxa. A single treatment with the rhizome segment placed on the soil surface was adopted, based on the results of a previous study in which this treatment provided good regeneration (Bímová et al., 2003
). It also most reasonably simulates the real situation, when rhizome fragments are spread by water and laid on the soil surface along stream banks. In the present study, regeneration was understood as asexual (vegetative) production of ramets from rhizome segments. Growth was monitored every 2–3 d starting on 30 May (= day 1); the germinated buds were recorded, and the bud was considered germinated if it was swollen and bright red. For each shoot, the day of emergence was recorded and the interval between day 1 and the day of emergence was termed "emergence time." On day 30, regenerated shoots were counted and the percentage of regenerated rhizome segments was recorded and used as a measure of "regeneration rate" of each genotype. Shoots were harvested, separated to roots, stems, leaves, and rhizomes, then dried and weighed. No new rhizome growth was realized throughout the duration of the experiment. Mass at harvest was termed "final shoot mass" and included dry mass of roots, stems, and leaves produced during experiment.
To estimate the relatedness of hybrid genotypes within R. xbohemica to the parental species, the genetic marker data were used to calculate the hybrid index (HI) for each genotype:
; Carney et al., 2000
) was used for these data in combination with the knowledge of ploidy level (all hybrids were 2n = 66). Since the chromosome number of hybrids was known, nonspecies-specific alleles were also taken into account. The values of HI obtained by this method are applicable for comparative purposes in the present paper and valid for the introduced geographical range considered here. They cannot be interpreted in terms of the overall genetic variation within the whole complex, however, because the range of the parental genetic characteristics from the native geographical range is unknown.
Field data
To assess whether the clonal growth recorded at the genotype level influences the performance of R. xbohemica genotypes in the landscape, the following data were obtained for each genotype: (1) number of localities in the Czech Republic recorded by field sampling and subsequent isozyme analysis of material transferred to the common garden, (2) total area covered by the genotype in the field, and (3) mean clone size (total clone area divided by the number of localities). Variation in these characteristics was related to the regeneration parameters obtained in the experiment described. Note that this analysis was performed only for R. xbohemica, because there is only a single genotype of the parent R. japonica in the country, and the number of genotypes available for the second parent, R. sachalinensis, are too few for an analysis.
Statistical analysis
The regeneration rate of a genotype within taxa is a binomial variable and was therefore generalized using linear models following logistic regression (Jongman et al., 1987
). Genotype was used as a fixed effect, and the t test followed by Bonferroni correction (Rice, 1989
) was used for comparison between particular genotypes within each species. The difference between regeneration rate of taxa, R. xbohemica and R. sachalinensis, was tested using angular-transformed average proportions of regenerated rhizomes for individual genotypes of R. xbohemica and R. sachalinensis.
To evaluate whether the final shoot mass was an intrinsic feature of a taxon or genotype, the data were analyzed by ANCOVA with the natural log of shoot mass as the response variable. Rhizomes that failed to regenerate were not included in the analysis. The difference between R. xbohemica and R. sachalinensis was tested using average values for individual genotypes of R. xbohemica and R. sachalinensis. Taxon was a fixed effect, and the regeneration rate for each genotype was a covariate. The differences between species-specific genotypes were tested separately for R. xbohemica and R. sachalinensis, using emergence time and initial rhizome mass for each replicate of each genotype as a covariate. Because the regeneration rate was a specific trait of genotype (Fig. 2), each replicate of a given genotype possessed the same value of regeneration rate. Therefore, the regeneration rate was used as a surrogate for the genotype and treated as a fixed effect. The exceptions were two pairs of genotypes in R. xbohemica, which had the same regeneration rate but had different genotypes. These pairs, analyzed separately, were genotypes B9 and B7 with 40% regeneration rate and B2 and B5 with 70% regeneration rate (Fig. 4). The ANCOVAs started by the maximal model, in which each covariate was regressed on a factor with a different intercept and slope. The parameters of this model were inspected, and the least significant term was removed in a deletion test. If the deletion test did not cause a significant increase in deviance, the term was removed. The deletion tests were repeated until only significant terms, creating minimal adequate models, remained (Crawley, 1993
, pp. 199–204). The minimal adequate models are presented in Tables 2–4.
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To evaluate whether the characteristics of genotype regeneration can be used to predict the distribution of R. xbohemica in the landscape, the number of localities, total clone area, and mean clone size of each genotype (response variables) were compared with its regeneration rate, final shoot mass, and emergence time (explanatory variables). To achieve a comparable influence in absolute values, the explanatory variables were standardized to have a zero mean and variance of one and analyzed for the response variables using the analysis of deviance table for multiple regression, in which all deviances were assessed by removal from the maximal model (Crawley, 1993
, pp. 192–195). Numbers of localities were square-root transformed to obtain an appropriate transformation for count data (e.g., Sokal and Rohlf, 1981
, pp. 421–423).
The adequacy of the fitted models was checked by plotting standardized residuals against fitted values and by the normal probability plots of the fitted values (Crawley, 1993
). Calculations were made using general linear modeling (McCullagh and Nelder, 1989
) in the commercial statistical package GLIM version 4 (Francis et al., 1994
).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Within taxa, there was a significant effect of genotype on regeneration rate in R. xbohemica (P = 0.038, df = 8, Wald statistics = 16.34), but not in R. sachalinensis (P = 0.220, df = 4, Wald statistics = 5.67). Between taxa, the effect of taxon on average regeneration rate was not significant (F = 0.0008; df = 1, 12; NS). However, there was greater variation in regeneration rate of the hybrid (coefficient of variation 50.3%, N = 9) than of R. sachalinensis (30.5%, N = 5).
Regeneration rates had a distinct pattern with respect to the genetic relatedness of genotypes to parental taxa. Genotypes with genetic characteristics intermediate between the parents tended to have a high regeneration rate, which symmetrically decreased when the hybrid index was very low or very high, indicating closer similarity of genetic markers to one of the parents (Fig. 3).
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Factors affecting shoot growth
Final shoot mass after 30 d of growth varied between taxa (Fig. 5). Regeneration growth, measured as final shoot mass, was faster in R. sachalinensis than R. xbohemica, as indicated by a significant increase in log shoot mass between taxa (Table 2). However, the genotype affected final shoot mass only in R. xbohemica, as indicated by a positive and significant regression slope of log shoot mass on regeneration rate (Table 2). The genotype did not affect regeneration rate in R. sachalinensis, for which the regression slope of log shoot mass on regeneration rate was nonsignificant (F = 0.19; df = 1, 11; NS). The fact that genotype significantly affected shoot growth only in R. xbohemica is consistent with the previous finding that the regeneration rate was significantly affected by genotype in R. xbohemica but not in R. sachalinensis.
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Effect of genotype regeneration characteristics on distribution in the landscape
Mean clone size recorded in the field for individual genotypes of R. xbohemica was not significantly related to final shoot mass and genotype regeneration rate, but there was a marginally significant effect of emergence time (Table 5, Fig. 6). All predictors were nonsignificant (P > 0.3) when predicting the number of localities and total area of clones recorded in the field. Clone area and the number of localities were significantly correlated (r = 0.96), but the mean clone size was not correlated with either clone area (r = 0.08) or number of localities (r = 0.009).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Niche breadth of invaders along environmental gradients seems to be more often the result of phenotypic plasticity than correlated with genetic variation within populations (Sultan, 1987
). However, genetic change in ecologically marginal populations allows the exploitation of habitats that otherwise would be off limits, thus promoting invasion (Rejmánek et al., 2003
). The role of adaptive evolution following the colonization of new environment was recently highlighted for both plants and animals (Ellstrand and Schierenbeck, 2000
; Daehler and Carino, 2001
; Lee, 2002
). The complex of polyploid Reynoutria taxa invading Europe provides a unique opportunity to study the effect of genetic variation on invasion success. Although ecological consequences of polyploidy are still not clear, polyploids tend to have more genetic diversity and occur across a large range of habitats with respect to stress, competition, and disturbance (Rejmánek et al., 2003
). In the case of Reynoutria, a species genetically uniform in the introduced geographical range (Hollingsworth et al., 1998
) hybridizes with its alien congener, which exhibits limited genetic variation, and genetically more variable progeny is produced (Bailey and Stace, 1992
; Child, 1999
). The present paper shows that this process has ecological consequences detectable at the regional level. We found significant differences in the regeneration rate of individual genotypes of R. xbohemica. Easily regenerating genotypes also grew faster, indicating that measures of regeneration success are closely associated. Emergence time affected the regeneration rate but only in genotypes with low regeneration rates. It also appears that poorly regenerating genotypes grow better if they regenerate from a larger rhizome fragment, a characteristic that is irrelevant in genotypes capable of high regeneration. A genotype with poor regeneration capability is therefore dependent also on other characteristics, such as early germination of rhizome buds and amount of reserves provided by the rhizome fragments, while those with superior regeneration do well regardless of these characteristics. These results suggest that major characteristics contributing to the fitness of a plant spread almost exclusively by vegetative means (i.e., early and effective regeneration and subsequent fast growth) act in concert and are an intrinsic feature of each genotype. It should be also noted that a potential "maternal" effect of the local environment could be involved in explaining variation observed among genotypes. However, the vast majority of genotypes were sampled from comparable nutrient rich habitats, where these species typically grow, and there was a little variation in climatic conditions among regions. We therefore believe that the results reflect intrinsic features of the genotypes and that the bias resulting from a "maternal" local environment is minor.
In a previous study showing differences in regeneration between taxa, the hybrid R. xbohemica performed better than the parents (Bímová et al., 2003
). Here, we focused on the genotype level, and differences between taxa were taken only marginally into account. No significant differences in regeneration rate were found either between taxa or between genotypes within R. sachalinensis. It must be borne in mind, however, that the lack of significant results at the taxa level may be, to some extent, explained by the low number of R. sachalinensis genotypes available for statistical analysis. The present paper also elucidates rather inconsistent results of previous studies comparing particular taxa in the UK (Child, 1999
) and the Czech Republic (Bímová et al., 2003
). The results may vary with respect to the regeneration hierarchy of particular taxa within the complex because of the genetic distance of the hybrid from the parents. Hence, the regeneration potential may depend on which genotype of R. xbohemica was used in the particular experiment.
Our results indicate that regeneration characteristics, and hence the genetic identity, may be related to the extent of R. xbohemica invasion at a regional scale. The size of a clone established in the field seems to depend on its ability to emerge soon after the rhizome was deposited on the soil surface. This relationship was only marginally significant since there were only a limited number of genotypes available for statistical analysis and the success of a rhizome fragment in the wild also depends on other circumstances that were not addressed by the present study (e.g., resistence of rhizome fragments to drought, their buoyancy, and response to microsite conditions). In addition, other factors bias the observed pattern, namely, the duration of a clone in a site. There is evidence that in Reynoutria species, some clones have persisted for more than a century in the same site while the introduction of others might have been quite recent (Pyek et al., 2001
). The time over which clones were allowed to expand is an important determinant of their size. Another factor impossible to take into account in this study is the effect of local disturbances and occasional control measures, which also increase the variation in clone size. Knowledge of exact dates of introduction of a particular clone to the given site and of past events is beyond the reach of most such studies, reflecting the limitation in using the post-hoc character of studies on invasive plants. Nevertheless, our results justify speculation about the role regeneration plays in shaping invasion potential.
It is not surprising that, of the measures used to characterize the invasion at the landscape scale—number of localities, total area occupied, and mean clone size—the latter characteristic provided the best results because it integrates the capability for both long-distance dispersal and local spread, which are both important determinants of invasion success.
The high genotype variability found within R. xbohemica taxa suggests that under favorable conditions, hybridization between R. japonica and R. sachalinensis does occur in the introduced geographical range (B. Mandák et al., Institute of Botany Prhonice, unpublished data). Until recently, it was believed that although seed was produced by R. japonica in the Czech Republic, hybridization was mostly due to pollination by a related species, Fallopia aubertii. Even when R. japonica is pollinated by R. sachalinensis, progeny does not survive as a result of low temperatures at the end of the growing period (Beerling et al., 1994
). A rather low number of genotypes within R. xbohemica suggests selection by environmental conditions against progeny originating by generative reproduction. The origin of novel genotypes has been unknown until now; in both the native and introduced geographical ranges they can originate by (1) hybridization between R. japonica var. japonica and some of the R. sachalinensis genotypes or (2) crossing within the range of R. xbohemica taxa. Consequently, novel genotypes can originate in the introduced geographical range or be introduced from the native geographical range; the relative importance of both events requires further study.
Successful sexual reproduction is rare, but the evidence that it does occur in Reynoutria taxa has important ecological consequences. Novel genotypes resulting from this process differ in their regeneration characteristics; regeneration is a crucial stage in species spread by water, with rhizome fragments laid on the shores of streams, and this mode of spread becomes even more efficient during the floods that occur regularly in central Europe. Our results suggest that some hybrids consist of novel genotypes with higher fitness than that of their parental taxa (as measured by regeneration features) and supports the hypothesis that hybridization increases the invasion potential of Reynoutria taxa in their introduced geographical range. Positive effects of hybridization on invasion potential have been reported (Ellstrand and Schierenbeck, 2000
; Perry et al., 2001
), possibly as a result of increased genetic variation, new gene interactions or the transfer of favorable genes (Lee, 2002
). Examples of adaptation through hybridization are also known, e.g., higher hybrid fitness in habitats where parental taxa do not occur (Artemisia, Iris) or adaptations to changing environment conditions (Geospiza; Arnold, 1997
).
That successful regeneration is associated with fast growth, providing the plant with an effective means to occupy space, and that these species do not retreat from sites once occupied during succession (Pyek et al., 2001
) suggest that early regeneration might determine the success of invasive Reynoutria taxa not only locally but also at a regional scale. In taxa relying almost exclusively on vegetative reproduction, new hybrid combinations are fixed by clonal growth and may become widespread (Room, 1990
; Lee, 2002
). Compared to other mechanisms capable of fixing heterotic genotypes originating through hybridization (agamospermy, allopolyploidy, permanent translocation heterozygosity), clonal spread is the most frequent, being involved in 32% of 28 documented cases (Ellstrand and Schierenbeck, 2000
). A rare hybridization event may thus represent a permanent addition of highly invasive genotypes to the genetic makeup of the complex, and, as pointed out by Ellstrand and Schierenbeck (2000)
, the fitness boost afforded by fixed heterozygosity may be all that is necessary to make a hybrid lineage invasive. Hybridization therefore generates new genotypes with invasion potential that can exceed that of their parents and represent an additional threat to the native vegetation.
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FOOTNOTES |
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and árka Jahodová for logistic support. The work was funded by grant no. A6005805/1998 from the Grant Agency of the Academy of Sciences of the Czech Republic, and by grant no. AV0Z6005908 from the Academy of Sciences of the Czech Republic. V. J. was supported by the MMT grant No. J13/98113100004. 
5 Present address: Environmental Resources Program, 7001 E. Williams Field Road, Arizona State University East, Mesa, Arizona 85212 USA. john.brock{at}asu.edu
.
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