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. 2002 Aug 1;90(2):245–251. doi: 10.1093/aob/mcf178

Influence of Litter and Weather on Seedling Recruitment in a Mixed Oak–Pine Woodland

ZBIGNIEW DZWONKO 1,*, STEFAN GAWROŃSKI 1
PMCID: PMC4240421  PMID: 12197522

Abstract

The effects of regular litter removal and annual variation in temperature and precipitation on seedling recruitment of species differing in their seed size and mode of dispersal were studied in a 16‐year (1984–1999) experiment in a mixed oak–pine wood in southern Poland. Litter was the most important factor in determining spatial variability in seedling recruitment, whereas differences in climatic conditions among years, especially temperature fluctuations in late winter and early spring, determined the temporal variability in seedling recruitment. Compared with control plots, significantly more new individuals of bryophytes and seedlings as well as a number of new species of vascular plants were noted in the litter‐removal plots over the 16‐year study. Litter strongly impeded seedling emergence of small‐seeded species. The negative effect of litter on seedling recruitment of large‐seeded species and the recruitment of new shoots in species growing clonally was much weaker. There was a significant positive correlation between the numbers of seedlings in the litter‐removal and control plots and temperatures in January to March. In the litter‐removal plots this mainly affected small‐seeded species. Seedling recruitment was less consistently related to variation in precipitation. Positive relationships were found only between the number of seedlings of large‐seeded species in the litter‐removal plots and precipitation in July of the current year and in September of the previous year, and between the number of seedlings in the control plots and precipitation in September and November of the previous year.

Key words: Air temperature, climate, litter, permanent plots, precipitation, seedling recruitment, southern Poland, woodland species

INTRODUCTION

The recruitment of seedlings of woodland species is an important stage in colonization of new sites bordering on ancient woodlands and has a marked effect on the local dynamics of vegetation within such woodlands. Woodland species differ considerably in rates of seedling recruitment. In the majority of clonal plant species, which reproduce chiefly by vegetative propagation, seedling recruitment is infrequent (Cook, 1985; Eriksson, 1989). However, in many non‐clonal woodland species the recruitment of seedlings can be fairly common (Eriksson, 1995). Various studies suggest that the rate of seedling recruitment in temperate woodlands may be strongly dependent on the availability of diaspores and suitable micro‐sites as well as on climatic conditions.

Many woodland species are unable to colonize spatially isolated recent woods because of their poor dispersal ability (Peterken and Game, 1984; Dzwonko and Loster, 1992; Grashof‐Bokdam, 1997). For this reason the natural regeneration of woodland communities is very slow, and in the present‐day agricultural landscape it is possible only in sites immediately adjacent to ancient woodlands—the sources of diaspores of woodland species. Detailed studies have shown that even recent woods adjacent to ancient deciduous woodlands are only slowly colonized by woodland species. Migration rates of many species to such woods are usually slower than 1 m year–1 (Brunet and von Oheimb, 1998a; Bossuyt et al., 1999; Dzwonko, 2001a). In addition, species may migrate at different rates which depend considerably on the modes of diaspore dispersal and the availability of microsites suitable for seed germination and seedling development (Dzwonko, 1993, 2001a, b; Brunet and von Oheimb 1998a, b). Similar factors also affect the spatial and temporal dynamics of the field layer vegetation within ancient woods. Fröborg and Eriksson (1997) found that the colonization rate of permanent plots in a non‐grazed deciduous wood was positively related to the possession of features that may enhance seed dispersal and to seed mass, whereas the local extinction rate was negatively correlated only with seed mass.

Seed germination depends strongly on the quality and thickness of litter and on the quality of light. Major periodic fluctuations of these two factors in temperate deciduous woodlands result in the predominance of species with a seasonal type of regeneration strategy (Grime and Hillier, 1992). In deciduous and mixed woodlands, the best light conditions occur in early spring before the leaf canopy closes. It is in this period that seeds of most woodland species germinate (Grime, 1979; Grime and Hillier, 1992). It has been noted that seedlings of trees that emerged early in spring often survived better and attained a greater biomass in the first year than those that emerged later, even within the same species (Seiwa and Kikuzawa, 1991; Jones et al., 1997; Seiwa, 2000). Comparing the germination ecophysiology of over 300 herbaceous species from the mesic temperate region, Baskin and Baskin (1988) have shown that temperature is the primary environmental factor determining the season in which seeds of most species germinate, whereas light and soil moisture alone are less important in this zone. Many woodland herb species are capable of germinating at lower temperatures than species of other habitats and, moreover, their germination is often restricted to a narrow range of temperatures (Grime et al., 1981; Baskin and Baskin, 1988; Schütz, 1997). It is therefore likely that the temperature requirements of these species are the result of natural selection for germination, restricting the emergence of seedlings to the period before the leaf canopy closes in late spring.

Litter has a negative effect on recruitment of the majority, if not all, woodland species (Sydes and Grime, 1981a, b; Facelli and Pickett, 1991; Eriksson, 1995; Brunet and von Oheimb, 1998b; Dzwonko, 2001a, b). Nevertheless, this effect may be diverse. Various experiments have shown that seedling emergence of large‐seeded tree species is inhibited by litter to a much lesser extent than that of small‐seeded species (Peterson and Facelli, 1992; Myster, 1994; Seiwa and Kikuzawa, 1996). It is generally believed that seedlings of large‐seeded species are better able to survive environmental hazards, including burial under soil or litter, deep shade during the cotyledon stage and drought (Westoby et al., 1996; Kidson and Westoby, 2000; Walters and Reich, 2000).

Interannual variation in temperature and/or precipitation has a significant impact on the timing and abundance of flowering and the cover of many woodland herb species (Fitter et al., 1995; Diekmann, 1996; Brunet and Tyler, 2000; Tyler, 2001). The results of the these studies also imply that seedling recruitment and, in effect, the temporal and spatial dynamics of the field layer vegetation are greatly affected by climatic conditions and litter, but there are no long‐term studies on permanent plots available to confirm the effect of these factors on the emergence of seedlings within woodlands. The purpose of this study was to examine whether interannual variation in seedling recruitment in a mixed oak–pine wood is related to fluctuations in temperature and precipitation, and the degree to which the regular removal of litter affects seedling recruitment of species with seeds of different sizes.

MATERIALS AND METHODS

Study site

The study was carried out in a mixed oak–pine woodland with Quercus robur L., Pinus sylvestris L. and Fagus sylvatica L. (Pino‐Quercetum), situated in the Wierzbanówka valley in the northern part of the Carpathian foothills, at 270 m a.s.l., 25 km south‐west of Kraków (southern Poland; 49°54′N, 19°42′E). The wood studied is growing on soil lessivés transitional to podzolized soil, in the central part of a wooded area containing ancient and recent deciduous and mixed woods covering 27·5 ha in total. A perusal of historical documents shows that, although the study wood was managed, it may be recognized as ancient. More details of the floristic composition and species richness of these woods are given by Dzwonko and Gawroński (1994, 2002). Litter was regularly removed from the study wood by local farmers until the end of the 1950s. In the past few decades the frequency of acidophilous species in this woodland has decreased considerably and eutrophication of vegetation has been observed. A 16‐year experiment in the study wood showed that litter removal resulted in substantial impoverishment of the soil. After 16 years, soil in the plots subjected to litter removal contained significantly less P, Mg and Ca, and had a lower cation exchange capacity (CEC) in the epihumus subhorizon, and less Ca and a lower CEC in the humus and lessivage horizons than soil in the control plots. Within 16 years, species richness increased significantly in the field layer of the litter‐removal plots, but the abundance of dominant species and the character of the vegetation remained unchanged, while vegetation in the control plots changed from acidophilous to neutrophilous (Dzwonko and Gawroński, 2002). These results suggest that acidophilous vegetation in the field layer of the study wood largely resulted from regular litter removal by man over a long period, and its eutrophication resulted first of all from the cessation of traditional methods of management.

Data collection and analysis

In 1983, three pairs of permanent plots, each 5 × 5 m, were established in the study wood in sites with homogeneous vegetation. Plots of each pair were spaced 3 m apart. Each year from 1983 to 1998, litter was raked and removed from one plot of each pair by the end of October. Every August from 1984 to 1999 we recorded all new bryophytes and seedlings as well as shoots of vascular plant species that had spread vegetatively from plants growing outside of the plots. The same plots were also used to assess the effects of litter removal on species richness and acidification of mixed woodland (Dzwonko and Gawroński, 2002). In the study period the most abundant species in the field layer of the plots were Carex brizoides L., Vaccinium myrtillus L., Majanthemum bifolium (L.) F.W. Schmidt, Luzula pilosa (L.) Willd., Milium effusum L. and Rubus idaeus L.

The rate of colonization of different plots and the rate of appearance of new species in the plots were characterized by the mean cumulative numbers of all new seedlings and new species, calculated by adding new seedlings and species found in successive years to the numbers of seedlings and species recorded in the first observation year. Vascular plant species were divided into five dispersal groups: light anemochores (diaspores with pappus and winged diaspores <1·5 mg or unwinged ones <0·5 mg); heavy anemochores (winged diaspores <3 mg or unwinged ones <1·5 mg); endozoochores (diaspores with fleshy fruits); dyszoochores; and myrmecochores (cf. Kornaś, 1972; van der Pijl, 1982). Information on seed mass and seed banks were taken from Müller‐Schneider (1986), Erikson and Ehrlén (1992), Hodgson et al. (1995), Fröborg and Eriksson (1997) and Thompson et al. (1997).

Climatic data were obtained from the meteorological station in Polanka‐Haller, situated approx. 1 km from the study woodland. Precipitation was measured at this station for the duration of our study but air temperature was only measured until 1995. For this reason, analyses include temperature and precipitation data for 12 and 16 years, respectively. During this period, mean annual temperatures varied from 6·8 °C (in 1985) to 9·4 °C (in 1994), the mean temperature of the coldest month from –10·7 °C (January 1987) to 0·7 °C (January 1989), and the mean temperature of the warmest month from 17·4 °C (August 1986) to 22·2 °C (August 1992). Total annual precipitation varied from 458 mm (in 1984) to 939 mm (in 1996).

Relationships between temperature and precipitation data and numbers of seedlings were analysed by correlation and multiple regression. The data considered included mean temperatures for each month from January to July, and mean temperatures for two and three consecutive months beginning from January. Precipitation sums were analysed for the same months and periods. Mean temperatures and total precipitation for months from August to December of the previous year were also used in the analyses.

RESULTS

Effect of litter

The litter‐removal plots were much more frequently colonized by vascular plants than control plots, and in 1988, 1989 and 1992, differences in the number of seedlings were particularly marked (Fig. 1A). Already from the fifth year of observation, the mean cumulative numbers of seedlings and new species of vascular plants in the raked plots were substantially higher than those in control plots (Fig. 1C). Consequently, over the 16‐year study, significantly more seedlings and new species of vascular plants and bryophytes appeared in the litter‐removal plots (Fig. 1C and D; Table 1). In this period, seedlings of 27 vascular plant species and new individuals of ten species of bryophytes emerged in the litter‐removal plots, whereas all bryophytes disappeared and seedlings of only ten vascular plant species were found in the control plots. The most frequent seedlings and new individuals of mosses in the litter‐removal plots were those of Majanthemum bifolium (4·3), Rubus idaeus (3·0), Frangula alnus Miller (2·7), Sorbus aucuparia L. (2·3), Quercus robur (2·3), Milium effusum (1·7), Picea abies (L.) Karsten (1·3), Carex pallescens L. (1·3), Atrichum undulatum (Hedw.) P. Beauv. (2·3), Polytrichum formosum Hedw. (2·0) and Dicranella heteromalla (Hedw.) Schimp. (1·3) (mean numbers of seedlings that emerged during the 16 years given in parentheses). Seedlings of Quercus robur (1·7) and Milium effusum (1·3) emerged most often in the control plots. A complete list of species that emerged in the study plots is given by Dzwonko and Gawroński (2002).

graphic file with name mcf178f1.jpg

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Fig. 1. Mean numbers of seedlings (A) and bryophyte individuals (B), and mean cumulative numbers of seedlings (circles) and vascular plant species (squares) (C) and individuals and species of bryophytes (D) that emerged in litter‐removal (open symbols) and control (closed symbols) plots during 1984–1999.

Table 1.

Mean numbers of seedlings, shoots (in the case of vegetation propagation) and bryophytes, and mean numbers of species among seedlings, shoots and bryophytes that emerged over the 16‐year study in the litter‐removal and control plots

Seedlings or shoots Species
Litter‐removal plots Control plots P Litter‐removal plots Control plots P
Vascular plants 28·3 (23–33) 7·3 (5–12) <0·05 12·7 (10–16) 3·7 (2–5) <0·05
 Seed mass (mg)
  ≤ 1·00 6·0 (4–8) 0·7 (0–1) <0·05 4·3 (2–8) 0·7 (0–1) <0·05
  1·01–2·00 6·0 (3–9) 1·7 (1–2) <0·05 1·7 (1–3) 0·3 (0–1) n.s.
  2·01–10·00 8·7 (5–11) 1·7 (0–4) <0·05 2·3 (2–3) 0·7 (0–1) <0·05
  > 10·00 7·0 (6–8) 3·3 (2–6) n.s. 3·7 (3–4) 2·0 (1–3) n.s.
 Seed bank
  Transient 16·3 (15–18) 5·3 (3–10) <0·05 7·0 (5–10) 3·0 (2–4) <0·05
  Persistent 10·3 (6–15) 2·0 (1–3) <0·05 4·0 (3–5) 0·7 (0–1) <0·05
 Type of dispersal
  Light anemochores 4·7 (2–8) 0·3 (0–1) <0·05 4·0 (2–8) 0·3 (0–1) <0·05
  Heavy anemochores 3·3 (2–4) 1·7 (1–2) n.s. 1·3 (1–2) 0·7 (0–1) n.s.
  Endozoochores 13·3 (8–17) 3·0 (1–6) <0·05 3·0 (3–3) 1·3 (0–2) <0·05
  Dyszoochores 3·7 (3–4) 2·3 (1–4) n.s. 2·0 (2–2) 1·3 (1–2) n.s.
  Myrmecochores 3·0 (2–4) 0·3 (0–1) <0·05 2·0 (2–2) 0·3 (0–1) <0·05
  Vegetative propagation 9·7 (5–16) 4·0 (2–6) n.s. 2·0 (2–2) 2·0 (1–3) n.s.
Bryophytes 10·3 (10–11) 0·0 (0–0) <0·05 5·7 (5–7) 0·0 (0–0) <0·05
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Ranges of seedling, shoot and species numbers are given in parentheses.

P denotes the probability of no difference (Mann–Whitney U‐test).

The comparison of the numbers of seedlings in various groups revealed that, in the raked plots, substantially more species which have persistent as well as transient soil seed banks appeared (Table 1). These plots were colonized significantly more often by bryophytes and vascular plants with lighter diaspores, dispersed by the wind (light anemochores) or by animals (endozoochores and myrmecochores). No significant differences between the litter‐removal and control plots were found in the number of seedlings that developed from the heaviest seeds (mass >10 mg), dispersed by animals (dyszoochores) or by the wind (some heavy anemochores), or in the number of seedlings that developed from seeds with a mass of 1·01–2·00 mg, although in the litter‐removal plots seedling recruitment of species in both these groups was notably higher. There were also no significant differences between plots in the number of new shoots of vascular plants that spread into the plots vegetatively from plants growing outside the plots.

Influence of temperature and precipitation

The number of seedlings that emerged in the litter‐removal and control plots varied considerably over the 16‐year period. More seedlings emerged in the litter‐removal plots between 1988 and 1992 than in the remaining years of observation (Fig. 1A). The number of newly recorded bryophytes in these plots also varied (Fig. 1B). Examination of correlations between the number of seedlings and climate variables showed that seedling recruitment was most related to temperatures in late winter and early spring (Table 2). Higher temperatures in the period from January to March favoured seedling recruitment in both the litter‐removal and control plots. In the litter‐removal plots this affected mainly small‐seeded species. The number of seedlings of large‐seeded species in these plots was significantly negatively correlated with the temperature in April and positively related to the temperature in November of the previous year. In the control plots, the number of seedlings of large‐seeded species, like the number of all seedlings, was significantly positively correlated with the temperature in March. The numbers of seedlings from most of the species groups were only very weakly correlated with the temperature in April (r < 0·16). Therefore, it appears that the significant positive relationships found between seedling recruitment and mean temperatures in the periods from February to April and from March to April result above all from the high correlations between the number of seedlings and temperatures in February and March.

Table 2.

Correlation coefficients (Pearson’s r) between climate variables and mean numbers of seedlings and bryophytes per year in the litter‐removal and control plots

Temperature Precipitation
Litter‐removal plots Control plots Litter‐removal plots Control plots
Vascular plants
 All seedlings January–February +0·64* January–March +0·60* January –0·56* September +0·54*
January–March +0·67* March +0·68* November +0·56*
February–March +0·67* March–April +0·59*
February–April +0·68*
 Small‐seeded species (seed mass ≤ 10·0 mg) January–March +0·60* January +0·59*
February–March +0·62* January–February +0·58*
February–April +0·66* January–March +0·63*
 Large‐seeded species (seed mass > 10·0 mg) April –0·66* March +0·69* July +0·57* November +0·54*
November +0·66* March–April +0·62* September +0·87***
Bryophytes January –0·54*
January–February –0·73***
January–March –0·63**
February–March –0·52*
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Only significant r values (P ≤ 0·05) are shown.

Climatic variables for the previous year are given in italics.

* P ≤ 0·05; ** P ≤ 0·01; *** P ≤ 0·001.

Seedling recruitment was less connected with precipitation. Positive relationships were found only between the number of seedlings of large‐seeded species in the litter‐removal plots and precipitation in July of the current year and in September of the previous year, and between the number of seedlings in the control plots and precipitation in September and November of the previous year. Perhaps low precipitation in autumn caused an excessive drying of fruits and seeds during ripening, resulting in lower viable seed production. The recruitment of new bryophytes in the litter‐removal plots was negatively influenced by precipitation in the period from January to March. The number of all seedlings in these plots was also negatively correlated with precipitation in January. However, the results of multiple regression using precipitation in January and mean temperature in the period from January to March as independent variables showed that in the litter‐removal plots only temperature had a significant effect on the number of all seedlings (Table 3). Similar results were obtained in a multiple regression for the number of all seedlings in the control plots when precipitation in September of the previous year and the temperature in March of the current year were used as independent variables. Of these two variables, only temperature significantly affected the number of seedlings. In multiple regressions with other combinations of temperatures in the current year and precipitation in the previous year, none of the independent variables was significantly related to the number of all seedlings in the control plots.

Table 3.

Results of the multiple regression used to predict number of all seedlings that emerged in the litter‐removal and control plots

Temperature Precipitation R 2
Litter‐removal plots January–March +0·59* January –0·33 0·56
Control plots March +0·65* September +0·36 0·60
March +0·51 November +0·32 0·54
January–March +0·53 September +0·30 0·45
January–March +0·46 November +0·45 0·54
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Values are standardized partial regression coefficients.

Climatic variables for the previous year are in italics.

* P < 0·05.

DISCUSSION

This study has shown that within ancient temperate woodlands, litter is the most important general factor determining the spatial variation in seedling recruitment, whereas inter‐annual fluctuations in climatic conditions, and, above all, temperature variability in late winter and early spring, affect the temporal variation in seedling recruitment. The impact of both these factors on seedling recruitment was variable and depended on seed mass. Litter strongly hindered the emergence of bryophytes and seedlings of small‐seeded species. This resulted in significantly lower bryophyte and vascular plant species richness in the control plots after 5 and 11 years, respectively, compared with plots where litter was removed (Dzwonko and Gawroński, 2002). The effect of litter on seedling emergence of large‐seeded species was much smaller and non‐significant, as it was in the case of shoots spreading vegetatively from plants growing outside the plots. These results are consistent with earlier observations that showed that accumulation of litter strongly reduced seedling emergence of small‐seeded tree species (Peterson and Facelli, 1992; Myster, 1994; Seiwa and Kikuzawa, 1996). Large seeds have a greater ability to germinate under litter and their seedlings can penetrate the litter as a result of rapid and extensive initial growth of the shoot and root which uses a comparatively large amount of seed reserves. Thus, seedlings of large‐seeded tree species such as Fagus sylvatica and Quercus robur emerged with a similar frequency in the litter‐removal and control plots.

Experiments have shown that for many tree species which occur in deciduous woodlands of the temperate region, early germinating seedlings have a higher rate of survival and attain a greater biomass than seedlings that germinate later, because the former can utilize more light prior to leaf canopy closure (Seiwa and Kikuzawa, 1991; Jones et al., 1997; Seiwa, 2000). A higher temperature in early spring can thus stimulate earlier germination and enhance the survival rate of seedlings. This could explain the positive relationship between seedling emergence in the study plots and temperature in the period January–March. It is also known that young seedlings are extremely vulnerable to low temperatures. Spring ground frosts can result in high seedling mortality (Le Tacon and Malphettes, 1974). Presumably, higher mean temperatures in late winter and early spring also indicate the lack or rarer occurrence of spring ground frosts and are therefore positively correlated with increased seedling emergence. In the case of litter‐removal plots, the number of seedlings of large‐seeded species was positively but not significantly correlated with temperatures from January to March. This suggests that recruitment of these seedlings is less dependent on the length of the period with a large amount of light in early spring. Seed size is associated with many other plant traits, including the mode of dispersal, growth form and specific leaf area (Westoby et al., 1996). Different observations imply that under shaded conditions large‐seeded species have a reproductive advantage over small‐seeded species. Large seeds, owing to greater initial energy reserves, enable seedlings to tolerate shade for longer and provide them with a better chance of survival in the shade (Leishman and Westoby, 1994; Walters and Reich, 2000). Seiwa and Kikuzawa (1996) noted that seedlings of large‐seeded tree species can also complete all their annual leaf production within a shorter period after seedling emergence than seedlings of small‐seeded tree species, in which the period of leaf emergence is longer and leaf longevity is shorter irrespective of the light conditions. It is thus possible that seedling recruitment of large‐seeded species in the litter‐removal plots was less dependent on early germination since the seedlings of these species were able to use reserves for faster development of shoots and leaves.

Apart from suitable microsites and weather, the availability of seeds is also of major importance for seedling recruitment (Eriksson and Ehrlén, 1992; Ehrlén and Eriksson, 2000). Seed availability may, in turn, be determined by such factors as seed production, dispersal and seed predation (Jensen, 1985; Eriksson, 1997). Seeds of some long‐lived plants which are characterized by intermittent production of large seed crops may be available in higher numbers only in mast years (Kelly, 1994; Selås, 2000). It has been reported in many studies that flowering and fruiting in trees, dwarf shrubs and woodland herbs may be controlled or influenced by air temperature and humidity in the preceding years (Holmsgaard and Olsen, 1966; Inghe and Tamm, 1985, 1988; Sork and Bramble, 1993; Selås, 2000; Tyler, 2001). Our study took into account climatic conditions in the preceding year, but only for the period August–December. The weather in this period could affect only the development and ripening of seeds because flower‐bud formation and flowering were affected by the climatic conditions that prevailed 2 years earlier and in the first half of the preceding year, respectively. It may thus be presumed that the positive correlations with precipitation in September and November of the previous year and seedling emergence of large‐seeded species in the study plots indicate the effect of precipitation during the period of seed ripening and fruiting on the number of available seeds. This conclusion is supported by results of studies on seed production in some tree and dwarf shrub species that demonstrated the positive effect of precipitation during seed ripening on the seed crop (Sork and Bromble, 1993; Selås, 2000).

Eriksson and Ehrlén (1992) and Ehrlén and Eriksson (2000) showed that the recruitment of various woodland species can be limited either by insufficient availability of diaspores, or by a combined lack of diaspores and suitable microhabitats. The results of our study suggest that climatic conditions during the period of seed germination and seedling development also have a significant impact on the recruitment of woodland species and the temporal dynamics of the field layer vegetation in temperate woodlands. Higher air temperatures in late winter and early spring favour seedling recruitment, particularly in places where litter has been removed. In the periods with such thermal conditions, the rate of local colonization can be much higher than that in years with late and cool springs.

ACKNOWLEDGEMENTS

We thank two anonymous referees for valuable comments and suggestions.

Supplementary Material

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Received: 4 February 2002; Returned for revision: 18 March 2002; Accepted: 7 May 2002

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