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Centre d'études nordiques and Département de biologie, Université Laval, Sainte-Foy, Québec, Canada G1K 7P4
Received for publication April 4, 2000. Accepted for publication June 20, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONSTUDY SPECIESMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: autocorrelation • Corema conradii • coastal dunes • dioecy • Îles-de-la-Madeleine • Moran's I • reproductive cost • reproductive investment • sex ratio • spatial pattern analysis • spatial segregation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONSTUDY SPECIESMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Hemborg and Karlsson, 1999
). In contrast, females must not only produce flowers, but they must also invest in the embryos, providing them with reserves and elaborating the accessory structures that help in dispersal. Although females may also reabsorb part of the nutrients from some of the reproductive structures before they senesce, reproductive investment of females is generally much higher than that of males (Wallace and Rundel, 1979
; Allen and Antos, 1988
; Tang, 1990
; Guitián, Medrano, and Rodriguez, 1997
; Nicotra, 1999
). If investment in reproduction is at the expense of investment in other functions such as growth, maintenance, and/or defense (Williams, 1966
; Gadgil and Bossert, 1970
), there may thus be a higher cost of reproduction for females (Putwain and Harper, 1972
; Lloyd and Webb, 1977
; Bawa, 1980
). This cost may be manifested by lower growth, survival, and/or frequency of flowering, and/or by a more variable reproductive effort through time (Oyama and Dirzo, 1988
; Popp and Reinartz, 1988
; Obeso, Alvarez-Santullano, and Retuerto, 1998
; Antos and Allen, 1999
; Nicotra, 1999
). However, the evidence of such differential costs of reproduction in dioecious plant species is still controversial (Sakai and Burris, 1985
; Willson, 1986
; Sakai and Sharik, 1988
; Marion and Houle, 1996
).
Spatial segregation of the sexes (Bierzychudek and Eckhart, 1988
) may alleviate, at least in part, the higher reproductive cost of females if they preferentially occupy resource-rich habitats (Freeman, Klikoff, and Harper, 1976
; Lloyd and Webb, 1977
; Cox, 1981
; but see: Wallace and Rundel, 1979
; Hancock and Bringhurst, 1980
; Sakai, 1990
; Barot, Gignoux, and Menault, 1999
). Spatial segregation of the sexes may arise from several mechanisms, among which are differential survival (true segregation) and differential frequency of flowering (apparent segregation), in relation to habitat quality (Freeman, Klikoff, and Harper, 1976
; Meagher, 1980
; Vitale and Freeman, 1986
; Dawson and Bliss, 1989
; Dawson and Ehleringer, 1993
; Nicotra, 1998
). Spatial segregation of the sexes may be reinforced by a contagious dispersion pattern with more intense competition between individuals of different sexes and site-dependent competitive superiority (Putwain and Harper, 1972
; Cox, 1981
).
Spatial segregation of the sexes implies habitat-specific sex ratio: for instance, female-biased or balanced sex ratios in favorable habitats but male-biased sex ratios in less hospitable environments. The literature on population sex ratio in dioecious plant populations is considerable and shows variable results. Sex ratios have traditionally been studied at the whole-population level and have been frequently found to be male biased (Opler and Bawa, 1978
; Meagher, 1981
; Ornduff, 1985
; Cavigelli et al., 1986
; Allen and Antos, 1988
; Sakai, 1990
; Thomas and LaFrankie, 1993
); however, female-biased sex ratios are also common (Falinski, 1980
; Crawford and Balfour, 1983
; Iestwaart, Offerijns, and van der Waal, 1984; Danell et al., 1985
; Kay and Stevens, 1986
; Alliende and Harper, 1989
; Crawford and Balfour, 1990
; Houle and Duchesne, 1999). Sex ratios have been found to vary with elevation (Grant and Mitton, 1979
; Hoffmann and Alliende, 1984
), water availability (Freeman, Klikoff, and Harper, 1976
; Fox and Harrison, 1981
; Dawson and Ehleringer, 1993
), nutrient availability (Cox, 1981
), light regime (Lovett Doust and Cavers, 1982
), stress (Freeman, Klikoff, and Harper, 1976
; Vitale and Freeman, 1986
), and disturbance level (Vernet and Harper, 1980
; Barradas and Correia, 1999
). Yet, few studies have considered within-population variations in sex ratio (Eppley, Stanton, and Grosberg, 1998
).
We studied populations of Corema conradii Torrey, a rare dioecious shrub of the coastal heathlands of northeastern North America. Our objectives were to determine (1) whether the demographic costs often associated with reproduction differed between sexes and (2) whether males and females were spatially segregated, with females less frequent in resource-poor microsites. To do so, we contrasted reproductive investment between sexes, analyzed the spatial patterns of males and females, determined spatial variations in population sex ratio, and compared male and female age and size frequency distributions.
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STUDY SPECIES |
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TOPABSTRACTINTRODUCTIONSTUDY SPECIESMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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50 cm in height), bearing subverticillate, needle-like leaves that persist for 2 yr (Houle and Rocheleau, 1998
). It is profusely ramified, and branches that lay on the ground often produce adventitious roots. Yet there is no strict clonal growth in this species and all branches remain connected to the central root system. Data on plant longevity are scarce, but individuals are believed to reach 40–50 yr of age (Dunwiddie, 1990
).
Reproduction occurs in April, before leaf buds open. Flowers are borne in a subterminal whorl on the branches. The female flowers are discreet, but the male flowers are more showy with their three (or four) exerted purple stamens. Pollen is transported mostly by wind (Couillard, Pelletier, and Gagnon, 1996
). There are no secondary sexual characters to help identify plant gender in the absence of reproductive structures. The fruit is a small (1.5–2.0 mm diameter), dry drupe containing usually three nutlets. In late July, while the fruits near maturity, an elaiosome-like, ephemeral structure develops at the base of the drupe. This white, fleshy structure attracts ants, which then disperse the fruits. However, most of the drupes appear to fall directly beneath the maternal plant. Seed germination and seedling emergence have been reported to occur in the fall (Dunwiddie, 1990
).
The species is limited to the eastern coast of North America, from New Jersey to Quebec, where it reaches the northern limit of its distribution (Fernald, 1950
). It is considered rare in most of its range, i.e., in Prince Edward Island and Québec, and in Massachusetts, New Jersey and New York (Bouchard et al., 1983
; Lavoie, 1992
; Gagnon et al., 1995a and b
; Couillard, Pelletier, and Gagnon, 1996
). It typically grows on fixed dunes, i.e., in heathlands, with several species of Ericaceae and Empetraceae (Gagnon et al., 1995a, b
). In Quebec, it is known only from Îles-de-la-Madeleine, with four occurrences (Attention FragÎles, 1995
; Dunwiddie, Zaremba, and Harper, 1996
).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONSTUDY SPECIESMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). The region is characterized by a cold temperate climate, with a strong maritime influence. Mean annual temperature is 4.5°C and annual precipitation totals 987 mm, of which 30% falls as snow. There are 1323 degree-days above 5°C (Atmospheric Environment Service, 1993
). The four populations known at the Îles-de-la-Madeleine were studied for their sex ratio: Dune du Havre aux Basques, 47°16' N, 61°56' W; Dune du Sud, 47°28' N, 61°45' W; Dune du Nord, 47°34' N, 61°39' W; and Pointe de l'Est, 47°36' W, 61°28' W. Four to six 20-m long transects were haphazardly positioned in each population. The cumulative transect length intersected by the crown of male and of female C. conradii individuals was used as an index of population sex ratio.
Reproductive investment
In early spring of 1997, eight male and eight female individuals (crown surface of 0.3 m2) were selected from the Dune du Sud population (close to, but outside the quadrat used to study population structure, see below) to estimate reproductive investment in flowers. The branch tips from a 100-cm2 surface of the crown of each individual were collected. From these, reproductive structures were sorted, dried at 75°C for 36 h, weighed, and then milled for nutrient analyses. A similar sampling was done towards the end of July to determine reproductive investment in fruits. Eight female individuals were then selected close to those sampled in the spring. N (micro-Kjeldahl on Kjeltec Auto 1030, Tecator, Sweden) and P, K, Mg, and Ca content (ICP, model P40, Perkin-Elmer, Norwalk, Connecticut, USA) was determined for the sampled structures.
Population structure and abiotic variables
A 24 x 16 m quadrat was positioned across two ridges and a dry slack, in a representative section of the C. conradii population at the Dune du Sud. Great care was taken to avoid human disturbances (walking paths and all-terrain vehicle tracks). In the quadrat, Corema conradii was the dominant shrub species (cover: 37%), with Arctostaphylos uva-ursi (33%), Juniperus communis (21%), Empetrum nigrum (20%), Vaccinium angustifolium (11%), and V. vitis-idaea (10%). Bare ground represented 10% of the quadrat surface.
The quadrat was subdivided into 96 4-m2 subquadrats. Within subquadrats, the rooting position of each individual of C. conradii was determined in relation to the southwest subquadrat corner. Plant gender could be determined by the presence of flowers for a majority of the individuals (>74%). Unsexed individuals were mostly juveniles with a stem diameter 
1.5 mm (at the root collar). For each individual, crown shape was characterized (circular, oval, triangular, or rectangular), and crown size was measured along each of two perpendicular axes. Stem diameter at the root collar was measured in the field, and a section of the stem at the root collar was taken for age determination. In the laboratory, the stem sections were cut with a sharp razor blade and annual rings were counted under a dissecting microscope.
Topography was determined with a laser level (SokkiaTM) at each subquadrat corner (for a total of 117 measures). Measures were taken from the southwest corner of the quadrat (the reference point) and later standardized in relation to the lowest point in the quadrat (0 m elevation, in the center of the slack). Two substrate samples (top 15 cm) were taken from each of 20 randomly chosen subquadrats: in 20 of them, one sample was taken underneath the crown and another one at 20 cm from the crown of a female individual no closer than 50 cm from any other C. conradii plant. The same sampling scheme was repeated for 20 other subquadrats, with male individuals as focal plants. Several physicochemical characteristics were determined: pH (1:1 substrate to water), organic carbon (Heanes, 1984), N (macro-Kjeldahl), C:N ratio, available P (modified Bray II), and exchangeable K (Amacher et al., 1990
), and Na and Cl (McKeague, 1978
).
Statistical tests
Spatial analyses
We used Moran's I, an index of spatial autocorrelation, to determine the dispersion pattern of the individuals within the population (Legendre and Fortin, 1989
). Moran's I can be calculated for different distance classes and a graphical representation of I as a function of distance (d), i.e., a spatial correlogram, can be elaborated. Before individual values of Id can be tested for significance, the correlogram must be globally significant, i.e., at least one Id has to be significant at P = 0.05/n, n representing the number of distance classes (Bonferroni correction). In the absence of autocorrelation, Id is not significantly different from 0 (Cliff and Ord, 1981
). A positive value indicates contagion and a negative value repulsion. The overall shape of the correlogram reflects the spatial pattern of the process considered (at the scale studied), although it does not identify the cause(s) of such patterns.
We studied the spatial patterns of the number of individuals (males and females) and the proportion of females in each 4-m2 subquadrat. We also calculated the spatial autocorrelation for the variable age for male and female individuals using the specific position of each shrub.
To estimate the intensity of the relationship between the variables analyzed above, while removing the spurious effect of spatial autocorrelation, we used partial Mantel tests (Smouse, Long, and Sokal, 1986
; Legendre and Fortin, 1989
). A partial Mantel test accounts for spatial autocorrelation by computing matrices of residuals of the linear regression of two variables (e.g., the number of males and the number of females per subquadrat; the elevation and the number of males per subquadrat) over the values of a third variable, i.e., geographic position (x–y coordinates). Because we used a given data set more than once for the correlations, we applied a Bonferroni correction. All spatial analyses were performed with the R software package of Legendre and Vaudor (1991)
.
Traditional statistics
The population sex ratios were compared to the theoretical value of 1 male:1 female with G tests (Sokal and Rohlf, 1995
). Reproductive investment was compared between males and females with one-way analyses of variance (ANOVAs: male flowers vs. female flowers; male flowers vs. female fruits). To determine whether substrate variables were associated with plant gender and canopy position, two-factor factorial ANOVAs were done. The stem diameter, crown size, and age distributions of males and females were contrasted with Kolmogorov-Smirnov tests. Mean stem diameter, crown size, and age were compared between males and females with t tests. We performed a linear regression of crown size and of stem diameter against age separately for males (N = 581) and for females (N = 531); the slope of such regressions represents an estimate of the crown or stem annual growth rate. The growth rates of males and of females were considered significantly different when they were mutually excluded from their respective 95% confidence interval.
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RESULTS |
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TOPABSTRACTINTRODUCTIONSTUDY SPECIESMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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1.6-m difference in elevation between the highest and the lowest points. The slack and the crests were 12-m wide.
Reproductive investment
At flowering, males invested significantly more than females in terms of biomass (twice as high), N (1.9 times higher), Mg (2.4 times higher), Ca (2.1 times higher), and K (3.2 times higher), but only marginally so in terms of P (1.8 times higher; Table 2). However, female reproductive investment was significantly higher when expressed in biomass (1.7 times higher), Mg (1.6 times higher), and Ca (2.1 times higher) when fruit production was considered. Overall male (flowers) and female (fruits) reproductive investment did not differ significantly in terms of N, P, or K.
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Population structure
A total of 1534 individuals were present in the 16 x 24 m quadrat at Dune du Sud, for an average density of 4 individuals/m2. Among these, there were 537 females (531 of which could be aged), 583 males (581 of which could be aged), and 412 nonreproductive individuals (409 of which could be aged; Table 3). The great majority of these nonreproductive individuals (i.e., 91%) were juveniles <10 yr old. Two individuals had both male and female flowers, but these were not included in our analyses. The sex ratio was consistently balanced among age classes (Table 4; all G tests not significant at P > 0.05), as it was over all age classes (1.1 males : 1 female, G = 2.069, P > 0.05).
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12–14 m (see also Fig. 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONSTUDY SPECIESMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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on Corema album in Spain: in this species, females were found to invest three times as much in terms of biomass into reproduction as males did, essentially because of fruit production. Our results are consistent with a higher cost of reproduction for females reflected by lower growth rates, but not by later sexual maturity, or lower frequency of flowering or lower survival (see also: Oyama, 1990
; Bullock, 1992
). Spatial segregation of the sexes, with females occupying richer patches, was not found in the C. conradii population studied and thus cannot be proposed as a strategy for females to alleviate their higher reproductive cost.
Reproductive investment and sex ratio
At flowering, male C. conradii invested more in reproduction than females, independent of the currency used. Such a pattern appears to be quite general among dioecious plant species (Korpelainen, 1992
; Antos and Allen, 1999
; Nicotra, 1999
). However, when seed and fruit production were included in our estimates, female C. conradii invested more than males in terms of biomass (mostly C), Mg, and Ca, although their reproductive investment was similar to that of males in terms of N, P, or K (assuming that all that was invested into flowering was transferred to fruiting). Differential reproductive investment may lead to significant differences in growth, frequency of flowering, and/or survival between males and females, with potential effects on the sex ratio at both the subpopulation (age-dependent sex ratio and apparent spatial segregation of the sexes; Nicotra 1998
; Houle and Duchesne, 1999) and the whole population levels (Nicotra, 1999
).
Because of their overall lower reproductive investment, males might be expected to have a better growth and a higher survival than females and, as a consequence, population sex ratio might be male biased. These expectations were met, at least in part: C. conradii population sex ratios were all significantly male biased when estimated by the transect intercept method. However, when the number of individuals (i.e., genets) was used to assess the sex ratio of the Dune du Sud population (for which the sex ratio was 1.5 males : 1 female according to the transect method), we found the sex ratio to be balanced (see below). Indeed, crown growth rate was higher for males than for females and such a difference in growth could be responsible for the biased sex ratios found with the transect intercept method. Because N and P, along with water, are typically the most limiting resources in coastal dunes (Kachi and Hirose, 1983
) and because there were no differences between males and females in global reproductive investment in terms of N and P, a balanced sex ratio may, in fact, not be surprising for C. conradii. These results should, however, caution us against the use of variables other than the number of genets to determine the sex ratio of plant populations (Putwain and Harper, 1972
; Obeso, Alvarez-Santullano, and Retuerto, 1998
).
Sexual maturity and frequency of reproduction
Reproductive maturity may be reached at 5 yr of age in C. conradi. Although individuals <10 yr old represented only a small proportion of the reproductive population (8%), >31% of the 5–9-yr-old individuals were reproductive. If sexual maturity is reached at 5 yr of age and approximately only one-third of the 5–9-yr-old class is reproductive in any given year, this may suggest that the attainment of sexual maturity may vary in space in relation with local resource availability or that young individuals have a lower frequency of reproduction than older ones (97% of the 10–29-yr-old individuals were reproductive). Both of these effects would be similar for males and females because the sex ratio of the 5–9-yr-old class was not biased and the population sex ratio was not spatially structured (see below; for contrasting results see: Allen and Antos, 1993
; Garcia and Antor, 1995
; Nicotra, 1998
). Furthermore, because the age frequency distribution of males did not differ from that of females for the whole reproductive population and the sex ratio did not vary with age class (consistently balanced), males do not appear to reach sexual maturity at a younger age, reproduce more frequently, or have a greater longevity than females. Consequently, differential sexual maturity, longevity, and frequency of reproduction according to sex seem unlikely in C. conradii, and differential costs of reproduction between males and females, assuming they exist, are to be found elsewhere.
Growth
Females had lower crown and radial growth rates than males; this is consistent with a higher cost of reproduction. Males of several dioecious plant species have been found to have a higher growth rate than females (Lloyd and Webb, 1977
; Obeso, Alvarez-Santullano, and Retuerto, 1998
; Nicotra, 1999
); however, this pattern is not universal (lower growth rate for males: Grant and Mitton, 1979
; Sakai and Burris, 1985
; Crawford and Balfour, 1990
; similar growth rates between males and females: Willson, 1986
; Marion and Houle, 1996
). Age-dependent differences in growth rate between males and females have also been reported, with males having a higher growth rate than females in older age classes, the reverse being true in younger age classes (Gamache, 1997
).
Spatial segregation of the sexes
Substrate characteristics did not differ significantly according to plant gender in our quadrat at the Dune du Sud, suggesting that there was no apparent microhabitat segregation between males and females. Furthermore, topography, which influences wind speed, snow accumulation, and substrate characteristics (e.g., higher nutrient and water availability in slacks than on ridges), did not have any significant effects on the spatial pattern of mature C. conradii individuals or on that of the sex ratio (but see: Freeman, Klikoff, and Harper, 1976
; Grant and Mitton, 1979
).
Although the proportion of females varied spatially, it was randomly dispersed over the quadrat. This result reinforces our conclusion of the absence of spatial segregation between sexes (see also: Sakai and Oden, 1983
; Marion and Houle, 1996
; Houle and Duchesne, 1999). Guitián, Medrano, and Rodriguez (1997)
also reported a random pattern for the sex ratio of the Corema album populations they studied in Spain.
Male and female individuals showed the same spatial pattern. They were not distributed randomly over the sampled surface: contagion was present, at a scale of 0–8 m, and repulsion at a scale of 14–22 m. This pattern was mostly related to the local topography and a southwest/northeast gradient in density. Age was also contagiously dispersed at a scale of 0–6 m, with positive autocorrelation again at a scale of 20–26 m, but negative autocorrelation at a scale of 8–18 m. These results suggest a patchy recruitment process within the population, with patchiness persisting as individuals age. Such patchiness may result from fine-scale disturbances, such as those related to ant activity that may be significant for C. conradii recruitment in the absence of fire (e.g., Hughes, 1990
; Houle and Rocheleau, 1998
).
Conservation aspects
Dioecious plant species may pose special conservation problems (Kevan, Ambrose, and Kemp, 1991
; Mack, 1997
), because the dynamics of their populations may be influenced by spatial segregation of the sexes and by sex-specific age of reproductive maturity, longevity, and susceptibility to enemies (Lloydd and Webb, 1977
; Dawson and Bliss, 1989
; Allan and Antos, 1993). For instance, a contagious dispersion pattern, coupled with a spatial segregation of the sexes, may lead to low pollination success, a failure of reproduction and of regeneration, and a high risk of population extinction (Bawa and Opler, 1977
). In the rare Corema conradii, sex-related ecological differences are not major and do not appear to create specific conservation problems.
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FOOTNOTES |
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2 Author for correspondence (e-mail: gilles.houle{at}bio.ulaval.ca
).
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONSTUDY SPECIESMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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———, and ———. 1993 Sex ratio variation in the dioecious shrub Oemleria cerasiformis. American Naturalist 141: 537–553
Alliende, M. C., and J. L. Harper. 1989 Demographic studies of a dioecious tree. I. Colonization, sex and age structure of a population of Salix cinerea. Journal of Ecology 77: 1029–1047
Amacher, M. C., R. E. Henderson, M. D. Breithaupt, C. L. Seale, and J. M. LaBauve. 1990 Unbuffered and buffered salt methods for exchangeable cations and effective cation-exchange capacity. Soil Science Society of America Journal 54: 1036–1042
Antos, J. A., and G. A. Allen. 1999 Patterns of reproductive effort in male and female shrubs of Oemleria cerasiformis: a 6-year study. Journal of Ecology 87: 77–84
Ashman, T.-L. 1994 A dynamic perspective on the physiological cost of reproduction in plants. American Naturalist 144: 301–316
Atmospheric Environment Service. 1993 Canadian climate normals. 1961–1990. Environment Canada, Atmospheric Environment Service, Downsview, Ontario, Canada
Attention FragÎles. 1995 La répartition du corème de conrad (Corema conradii) aux Îles-de-la-Madeleine. Ministère de l'Environnement et de la Faune, Direction de la conservation et du patrimoine écologique, Québec, Canada
Barot, S., J. Gignoux, and J.-C. Menaut. 1999 Demography of a savanna palm tree: predictions from comprehensive spatial pattern analyses. Ecology 80: 1987–2005[ISI]
Barradas, M. C. D., and O. Correia. 1999 Sexual dimorphism, sex ratio and spatial distribution of male and female shrubs in the dioecious species Pistacia lentiscus L. Folia Geobotanica 34: 163–174[CrossRef][ISI]
Bawa, K. S. 1980 Evolution of dioecy in flowering plants. Annual Review of Ecology and Systematics 11: 15–39
———, and P. A. Opler. 1977 Spatial relationships between staminate and pistillate plants of dioecious tropical forest trees. Evolution 31: 64–68[CrossRef][ISI]
Bierzychudek, P., and V. Eckhart. 1988 Spatial segregation of the sexes of dioecious plants. American Naturalist 132: 34–43[CrossRef][ISI]
Bouchard, A., D. Barabé, M. Dumais, and S. Hay. 1983 Les plantes vasculaires rares du Québec. Syllogeus no. 48. Musées nationaux du Canada, Musée national des sciences naturelles, Ottawa, Ontario, Canada
Bullock, S. H. 1992 Effects of size and substrate on growth and mortality of trees in tropical wet forest. Oecologia 91: 52–55[ISI]
Cavigelli, M., M. Poulos, E. P. Lacey, and G. Mellon. 1986 Sexual dimorphism in a temperate dioecious tree Ilex montana (Aquifoliaceae). American Midland Naturalist 115: 397–406[CrossRef][ISI]
Cliff, A. D., and J. K. Ord. 1981 Spatial processes: models and applications. Pion Limited, London, UK
Couillard, L., D. Pelletier, and J. Gagnon. 1996 La situation du corème de Conrad (Corema conradii) au Québec. Ministère de l'Environnement et de la Faune, Direction de la conservation et du patrimoine écologique, Québec, Canada
Cox, P. A. 1981 Niche partitioning between sexes of dioecious plants. American Naturalist 117: 295–307[CrossRef][ISI]
Crawford, R. M. M., and J. Balfour. 1983 Female predominant sex ratios and physiological differentiation in arctic willows. Journal of Ecology 71: 149–160
———, and ———. 1990 Female-biased sex ratios and differential growth in arctic willows. Flora 184: 291–302[ISI]
Danell, K., T. Elmqvist, L. Ericson, and A. Salomonson. 1985 Sexuality in willows and preference by bark-eating voles: defense or not? Oikos 44: 82–90[CrossRef][ISI]
Dawson, T. E., and L. C. Bliss. 1989 Patterns of water use and the tissue water relations in the dioecious shrub, Salix arctica: the physiological basis for habitat partitioning between sexes. Oecologia 79: 332–343[CrossRef][ISI]
———, and J. R. Ehleringer. 1993 Gender-specific physiology, carbon isotope discrimination, and habitat distribution in boxelder, Acer negundo. Ecology 74: 798–815
Dunwiddie, P. W. 1990 Rare plants in coastal heathlands: observations on Corema conradii (Empetraceae) and Helianthemum dumosum (Cistaceae). Rhodora 92: 22–26[ISI]
———, R. E. Zaremba, and K. A. Harper. 1996 A classification of coastal heathlands and sandplain grasslands in Massachusetts. Rhodora 98: 117–145[ISI]
Eppley, S. M., M. L. Stanton, and R. K. Grosberg. 1998 Intrapopulation sex ratio variation in the salt grass Distichlis spicata. American Naturalist 152: 659–670
Falinski, J. B. 1980 Changes in the sex- and age-ratio in populations of pioneer dioecious woody species (Juniperus, Populus, Salix) in connection with the course of vegetation succession in abandoned farmlands. Ekologia Polska 28: 327–365
Fernald, M. L. 1950 Gray's manual of botany, 8th ed. American Book, New York, New York, USA
Fox, J. F., and A. T. Harrison. 1981 Habitat assortment of sexes and water balance in a dioecious grass. Oecologia 49: 233–235[CrossRef][ISI]
Freeman, D. C., L. G. Klikoff, and K. T. Harper. 1976 Differential resource utilization by the sexes of dioecious plants. Science 193: 597–599
Gadgil, M., and W. H. Bossert. 1970 Life historical consequences of natural selection. American Naturalist 104: 1–24
Gagnon, J., G. Lavoie, G. Jolicoeur, and F. Boudreau. 1995a Les plantes susceptibles d'être désignées menacées ou vulnérables de l'Île de l'Est, Îles-de-la-Madeleine. Ministère de l'Environnement et de la Faune, Direction de la conservation et du patrimoine écologique, Québec, Canada
———, ———, ———, and ———. 1995b Les plantes susceptibles d'être désignées menacées ou vulnérables de la lagune du Havre aux Basques, Îles-de-la-Madeleine. Ministère de l'Environnement et de la Faune, Direction de la conservation et du patrimoine écologique, Québec, Canada
Gamache, I. 1997 Structure spatiale d'une population d'arbustes dioïques, Salix planifolia, le long d'un gradient successionnel en milieu dunaire subarctique. M. Sc., Université Laval, Québec, Canada
Garcia, M., and R. Antor. 1995 Sex ratio and sexual dimorphism in the dioecious Borderea pyrenaica (Dioscoreaceae). Oecologia 101: 59–67
Grant, M. C., and J. B. Mitton. 1979 Elevational gradients in adult sex ratios and sexual differentiation in vegetative growth rates of Populus tremuloides Michx. Evolution 33: 914–918[CrossRef][ISI]
Guitián, P., M. Medrano, and M. Rodriguez. 1997 Reproductive biology of Corema album (L.). D. Don (Empetraceae) in the northwest Iberian Peninsula. Acta Botanica Gallica 144: 119–128[ISI]
Hancock, J. F., and R. S. Bringhurst. 1980 Sexual dimorphism in the strawberry, Fragaria chiloensis. Evolution 34: 762–768
Hemborg, A. M., and P. S. Karlsson. 1999 Sexual differences in biomass and nutrient allocation of first-year Silene dioica plants. Oecologia 118: 453–460[CrossRef][ISI]
Hoffmann, A. J., and M. C. Alliende. 1984 Interactions in the patterns of vegetative growth and reproduction in woody dioecious plants. Oecologia 61: 109–114[CrossRef][ISI]
Houle, G., and M. Duchesne. 1999 The spatial pattern of a Juniperus communis var. depressa population on a continental dune in subarctic Québec, Canada. Canadian Journal of Forest Research 29: 446–450[CrossRef]
———, and A.-F. Rocheleau. 1998 Écologie du corème de conrad (Corema conradii) aux Îles-de-la-Madeleine, Québec. Québec, Ministère de l'Environnement et de la Faune, direction de la conservation et du patrimoine écologique, Québec, Canada
Hughes, L. 1990 The relocation of ant nest entrances: potential consequences for ant-dispersed seeds. Australian Journal of Ecology 16: 207–214[ISI]
Ietswaart, J. H., F. J. M. Offerijns, and J. H. van der Waal. 1984 Origin and structure of the Salix pentandra population on Schiermonnikoog. Gorteria 12: 5–14
Kachi, N., and T. Hirose. 1983 Limiting nutrient for plant growth in coastal sand dune soils. Journal of Ecology 71: 937–944
Kay, Q. O. N., and D. P. Stevens. 1986 The frequency, distribution and reproductive biology of dioecious species in the native flora of Britain and Ireland. Botanical Journal of the Linnean Society 92: 39–64
Kevan, P. G., J. D. Ambrose, and J. R. Kemp. 1991 Pollination in an understorey vine, Smilax rotundifolia, a threatened plant of the Carolinian forests in Canada. Canadian Journal of Botany 69: 2555–2559
Korpelainen, H. 1992 Patterns of resource allocation in male and female plants of Rumex acetosa and R. acetosella. Oecologia 89: 133–139
Lavoie, G. 1992 Plantes vasculaires susceptibles d'être désignées menacées ou vulnérables au Québec. Ministère de l'Environnement, Direction de la conservation et du patrimoine écologique, Québec, Canada
Legendre, P., and M.-J. Fortin. 1989 Spatial pattern and ecological analysis. Vegetatio 80: 107–138
———, and A. Vaudor. 1991 Le progiciel R: analyse multidimensionnelle, analyse spatiale. Version Macintosh. Université de Montréal, Montréal, Quebec, Canada
Lloyd, D. G., and C. J. Webb. 1977 Secondary sex characters in seed plants. Botanical Review 43: 177–216
Lovett Doust, J., and P. B. Cavers. 1982 Sex and gender dynamics in jack-in-the-pulpit, Arisaema triphyllum (Araceae). Ecology 63: 797–808[CrossRef][ISI]
Mack, A. L. 1997 Spatial distribution, fruit production and seed removal of a rare, dioecious canopy tree species (Aglaia aff. flavida Merr. et Perr.) in Papua New Guinea. Journal of Tropical Ecology 13: 305–316[ISI]
Marion, C., and G. Houle. 1996 No differential consequences of reproduction according to sex in Juniperus communis var. depressa (Cupressaceae). American Journal of Botany 83: 480–488[CrossRef][ISI]
McKeague, J. A. 1978 Manuel de méthodes d'échantillonnage et d'analyse des sols, 2nd ed. Société canadienne de la science du sol, Ottawa, Ontario, Canada
Meagher, T. R. 1980 Population biology of Chamaelirium luteum, a dioecious lily. I. Spatial distributions of males and females. Evolution 34: 1127–1137[CrossRef][ISI]
———. 1981 Population biology of Chamaelirium luteum, a dioecious lily. II. Mechanisms governing sex ratios. Evolution 35: 557–567[CrossRef][ISI]
Nicotra, A. B. 1998 Sex ratio variation and spatial distribution of Siparuna grandiflora, a tropical dioecious shrub. Oecologia 115: 102–113[CrossRef][ISI]
———. 1999 Reproductive allocation and the long-term costs of reproduction in Siparuna grandiflora, a dioecious neotropical shrub. Journal of Ecology 87: 138–149[CrossRef]
Obeso, J. R., M. Alvarez-Santullano, and R. Retuerto. 1998 Sex ratios, size distributions, and sexual dimorphism in the dioecious tree Ilex aquifolium (Aquifoliaceae). American Journal of Botany 85: 1602–1608
Opler, P. A., and K. S. Bawa. 1978 Sex ratios in tropical forest trees. Evolution 32: 812–821[CrossRef][ISI]
Ornduff, R. 1985 Male-biased sex ratios in the cycad Macrozamia riedlei (Zamiaceae). Bulletin of the Torrey Botanical Club 112: 393–397[CrossRef][ISI]
Owens, E. H., and S. B. McCann. 1980 The coastal geomorphology of the Magdalen islands, Quebec. In S. B. McCaan (editor), The coastline of Canada, 51–72. Geological Survey of Canada, Paper 80–10, Ottawa, Ontario, Canada
Oyama, K. 1990 Variation in growth and reproduction in the neotropical dioecious palm Chamaedorea tepejilote. Journal of Ecology 78: 648–663
———, and R. Dirzo. 1988 Biomass allocation in the dioecious tropical palm Chamaedorea tepejilote. Plant Species Biology 3: 27–33
Popp, J. W., and J. A. Reinartz. 1988 Sexual dimorphism in biomass allocation and clonal growth of Xanthoxylum americanum. American Journal of Botany 75: 1732–1741
Putwain, P. D., and J. L. Harper. 1972 Studies in the dynamics of plant populations. V. Mechanisms governing the sex ratio in Rumex acetosa and R. acetosella. Jounal of Ecology 60: 113–129
Sakai, A. K. 1990 Sex ratios of red maple (Acer rubrum) populations in northern lower Michigan. Ecology 71: 571–580[CrossRef][ISI]
———, and T. A. Burris. 1985 Growth in male and female aspen clones: a twenty-five-year longitudinal study. Ecology 66: 1921–1927[CrossRef][ISI]
———, and N. L. Oden. 1983 Spatial pattern of sex expression in silver maple (Acer saccharinum L.): Morisita's index and spatial autocorrelation. American Naturalist 122: 489–508[CrossRef][ISI]
———, and T. L. Sharik. 1988 Clonal growth of male and female bigtooth aspen (Populus grandidentata). Ecology 69: 2031–2033[CrossRef][ISI]
Smouse, P. E., J. C. Long, and R. R. Sokal. 1986 Multiple regression and correlation extensions of the Mantel test of matrix correspondence. Systematic Zoology 35: 627–632[CrossRef]
Sokal, R. R., and F. J. Rohlf. 1995 Biometry, 3rd ed. Freeman, New York, New York, USA
Tang, W. 1990 Reproduction in the cycad Zamia pumila in a fire-climax habitat: an eight-year study. Bulletin of the Torrey Botanical Club 117: 368–374[CrossRef][ISI]
Thomas, S. C., and J. V. LaFrankie. 1993 Sex, size, and interyear variation in flowering among dioecious trees of the Malayan rain forest. Ecology 74: 1529–1537[CrossRef][ISI]
Vernet, P., and J. L. Harper. 1980 The costs of sex in seaweeds. Biological Journal of the Linnean Society 13: 129–138
Vitale, J. J., and D. C. Freeman. 1986 Partial niche separation in Spinacia oleracea L.: an examination of reproductive allocation. Evolution 40: 426–430[CrossRef][ISI]
Wallace, C. S., and P. W. Rundel. 1979 Sexual dimorphism and resource allocation in male and female shrubs of Simmondsia chinensis. Oecologia 44: 34–39
Williams, G. C. 1966 Natural selection, the costs of reproduction, and a refinement of Lack's principle. American Naturalist 100: 687–690[CrossRef][ISI]
Willson, M. F. 1986 On the costs of reproduction in plants: Acer negundo. American Midland Naturalist 115: 204–207
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