Costs of reproduction in Nyssa sylvatica: sexual dimorphism in reproductive frequency and nutrient flux

Summary We examined the influence of differential reproductive frequency between the sexes on tertiary (phenotypic) sex ratios in the the dioecious tree Nyssa sylvatica (Nyssaceae). Reproduction was evaluated in relation to sex, size and canopy exposure using flowering data collected from 1229 marked trees over a four year period. For subsets of each population we used data on flower number, fruit crop size, fruit/flower ratios, and individual flower and fruit mass to compare biomass invested in reproductive structures of males and females. We also examined seasonal changes in stem nitrogen and soluble carbohydrate content in relation to flower and fruit production for trees of each sex. Our results indicate that: 1) Male-biased tertiary sex ratios could be explained by more frequent reproduction by male trees; 2) Estimated secondary sex ratios based on sums of all known males and females were not significantly different from 1:1; 3) Flowering frequency of males and females was significantly related to plant size (DBH) and exposure of the canopy to light; 4) Estimtes of reproductive biomass allocation ranged from 1.36 to 10.8 times greater for females relative to males; 5) Flower production was related to stem nutrient status for both sexes, but nutrient depletion and its effect on subsequent flowering was much more pronounced for female trees. We conclude that less frequent flowering by female trees may result from depletion of stored reserves, and that differential flowering frequency in N. sylvatica may ultimately reduce apparent sexual differences in the costs of reproduction.     

Sexual dimorphism in clonal growth forms and ramet distribution patterns in <Emphasis Type="Italic">Rumex acetosella</Emphasis> (Polygonaceae)

We investigated clonal traits in the dioecious herb Rumex acetosella to characterize sexual dimorphism in clonal forms and to correlate below-ground clonal patterns and above-ground ramet distributions. We recorded creeping root length, branching patterns, ramet and clump (caespitose ramets from the same position on the root) sprouting patterns, and biomass allocations in three females and males. We also estimated the patch size of flowering ramets within a quadrat. No sexual dimorphism was detected in the frequencies of branches and flowering ramets per root length. Male plants allocated proportionally more biomass to below-ground organs. Total root length did not differ between the sexes. Females sprouted more clumps with fewer flowering ramets per root length than males, which sprouted fewer clumps with more flowering ramets, which meant that clump sprouting patterns were phalanx-like in females and guerrilla-like in males. Flowering ramets were aggregately distributed in both females and males and patch sizes were similar between sexes, indicating that the spreader propagations were not found in the guerrilla-like males. We assumed that sexual dimorphism occurred in response to physiological integration for higher reproductive effort in females.     

Differences in response to defoliation between males and females of Silene dioica

Summary The response by male and female plants to herbivory was studied by experimental defoliation of the dioecious perennial herb Silene dioica in a green-house. Male and female plants were defoliated prior to and during the early flowering phase at two intensities (50% and 100% of leaf-area removed) in two consecutive years. Defoliation resulted in a decrease in the number of flowers initiated in both sexes, while a larger delay of peak flowering and a higher mortality was observed in males compared to females. In female plants, severe defoliation resulted in a reduction in seed number per capsule and in seed size compared to control. Females showed a negative correlation between the production of flowers in the first and second season in all treatments, while flowering in males the first season was not correlated with flowering in the second season. Females also showed a lower frequency of flowering than males during the two seasons studied. However, during the flowering period, males allocated significantly more biomass to flowers than did females. This outcome supports the idea that females may have a higher total reproductive expenditure than males, but males have a higher reproductive effort during flowering. Male rosette leaves were significantly preferred by the generalist herbivore Arianta arbustorum in experiments. This preference was most pronounced in trials with leaves from fertilized plants compared to nonfertilized plants. A greater storage of resources in aboveground leaves during winter by males compared to females may explain the higher preference for male leaves and the higher male mortality following early defoliation. Furthermore, males are smaller than females and may have a lower ability than females to replace lost resources needed for reproduction when defoliated early in the season.     

Male-biased sex ratio and promiscuous pollination in the dioecious island treeLaurus azorica (Lauraceae)

Pollination ofLaurus azorica (Lauraceae), a dioecious Macaronesian tree, was studied. Male and female trees had the same size distribution. The population had 2.5 times as many male trees as females. In addition, males produced more flowers, and their inflorescences lasted longer. Individual flower lifetime and length of flowering season were the same in both sexes. Between the years of observation, one tree changed sex. Pollinators wereHalictinae bees and the flyTachina canariensis. The bees collected pollen and nectar and the fly collected nectar from both sexes. Both species visited other plants as well. The evolution of breeding systems inLauraceae is discussed.     

Masting uncoupling: mast seeding does not follow all mast flowering episodes in a dioecious juniper tree

Evolutionary selective forces, like predator satiation and pollination efficiency, are acknowledged to be major causes of masting (the variable, periodic and synchronic production of seeds in a population). However, a number of recent studies indicate that resources might also play an important role on shaping masting patterns. Dioecious masting species offer a privileged framework to study the role of resources on masting variation, since male and female plants often experience different reproductive costs and selective pressures. We followed masting and reproductive investment (RI) of the dioecious tree Juniperus thurifera in two populations along 10 years and studied the different response of males and females to experimentally increased water and nutrient availability in a third population. Juniperus thurifera females invested in reproduction three times more resources than males. Such disparity generated different resource‐use strategies in male and female trees. Tree‐ring growth and water use efficiency (WUE) confirmed that sexes differed in their resource investment temporal pattern, with males using current resources for reproduction and females using resources accumulated during longer periods. Watered and fertilized female trees presented significantly higher flowering reproductive investments than males and experienced an extraordinary mast‐flowering event. However, seeding RI and mast seeding were not affected by the treatment. This suggests that although resource availability affects the reproductive output of this species, there are other major forces regulating masting on J. thurifera. During 10 years, J. thurifera male and female trees presented high and low flowering years more or less synchronously. However, not all mast flowering episodes resulted in mast seeding, leading to masting uncoupling between flowering and seeding. Since flowering costs represent only 1% of females’ total reproductive investments, masting uncoupling could be a beneficial bet‐hedging strategy to maximize reproductive output in spite of unpredictable catastrophic events.     

Sex ratio, reproductive investment and flowering phenology in dioecious Rhamnus alaternus (Rhamnaceae)

Guitián, J. 1995. Sex ratio, reproductive investment and flowering phenology in dioecious Rhamnus alaternus (Rhamnaceae). - Nord. J. Bot. 15: 139–143. Copenhagen. ISSN 0107–055X.
During 1992 and 1993 I investigated the reproductive biology of the dioecious Mediterranean shrub Rhamnus alaternus in a population in the northwest Iberian Peninsula. Reproductive investment was estimated as the mean total dry weight of reproductive organs per branch tip. I also estimated number of flowers produced per plant and population sex ratio, and investigated the spatial distribution of the sexes and flowering phenology. The sex ratio was 1:1, and the spatial distribution of the sexes was random. Male plants produced 2.6 times more flowers than female plants, but the overall reproductive investment by females was 5- to 9-fold higher. In both years of study, male plants commenced flowering first. The male and female flowering peaks coincided closely (in late March) in both years. The results of this study suggest that male and female R. alaternus differ most notably in the amount of resources allocated to reproduction.     

ECOLOGICAL ASPECTS OF SEX EXPRESSION IN SUBDIOECIOUS SCHIEDEA GLOBOSA (CARYOPHYLLACEAE)

The importance of ecological factors such as sex lability, spatial segregation, and resource allocation in the evolution of dioecy were examined in Schiedea globosa. S. globosa is a subdioecious species with equal numbers of plants possessing strictly male or female function and a small proportion of hermaphrodites. The propensity for labile sex expression was under both environmental and genetic control; some plants with male function became hermaphroditic (by producing female flowers) under better growing conditions in the field and in the greenhouse. There was some spatial segregation of the sexes. Because of sex lability, more hermaphrodites than males occurred on moister slopes. Although there were not measurable sex-related differences in mortality within or between two flowering seasons, more females than males and hermaphrodites occurred at the bottom of slopes. Males and females produced the same number of ramets and inflorescences, but females had a greater number of flowers per inflorescence. Males and females had the same number of ovules (vestigial in males), but females had larger ovules and longer stigmas. Hermaphrodites and males had the same amount of pollen per flower despite the production of fruit by the hermaphrodites. In hermaphrodites, there was no apparent tradeoff within flowers between pollen production and ovule production. These results indicate that spatial segregation, sex lability, and environmental conditions influence allocation patterns of S. globosa, and in combination with high inbreeding depression and selling rates, may promote the further evolution of dioecy in S. globosa.     

Promotion of flowering in the Pinaceae by gibberellins

Significant male and female flowering (cone bud production) by girdled branches of 6-year-old Douglas fir (Pseudotsuga menziesii (Mirb. Franco) seedlings was promoted by applications (mid-April to June) of 1.6 or 3.2 mg per branch (in total) of certain non-polar gibberellins (GA's). Girdling alone was ineffective. When tested alone, a mixture of GA_4/7 was most effective. GA₉ less so, while GA₅ and the more polar GA₃ were essentially ineffective. For female cone buds GA_4/7+ GA₉ were synergistically effective, but for male cone buds GA_4/7 alone was best. The auxin naphthaleneacetic acid (NAA) was not tested alone, but at low dosage (0.175 mg/branch in total) NAA enhanced the flowering efficacy of GA's for both sexes; at a high dosage (0.875 mg/branch in total) male cone bud production was further enhanced, but only at the expense of females. For female flowering the best treatment (90% frequency of flowering 6.8 cone buds/branch), was GA_4/7+ GA₉+ low NAA; for male flowering, it was GA_4/7+ high NAA (30% frequency and 4.2 cone buds/branch. Frequency of flowering for controls was 18% and 0%, average number of cone buds/branch was 0.9 and 0, for females and males, respectively. The successful treatments did not affect promordia initiation, rather they caused the differentiation of previously initiated, but undetermined, lateral primordia into cone and latent buds at the expense of vegetative bud differentiation. The lack of success reported by earlier workers in promoting flowering in Pinaceae species by GA's appears to be the unfortunate result of selecting GA₃ for initial testing. The practical implications of this early and enhanced flowering by non-polar GA's seedlings of a commercially important conifer are discussed in relation to accelerating the processes of tree improvement.     

Flowering phenology,floral display and reproductive success in dioecious,Aralia nudicaulis L. (Araliaceae)

Summary Aralia nudicaulis L. is a dioecious, perennial, herbaceous plant that is commonly found in the understory vegetation throughout the boreal forest of North America. Female remets have fewer flowers per inflorescence, initiate flowering earlier, and reach peak flowering before male ramets. The consequences of the asynchrony in flowering between the sexes on pollination and seed set were examined during a two-year study. In both years there was significant variation in seed set associated with the flowering times of individual female ramets. In 1983, seed production was highest in the middle of the flowering season. In 1984, seed production was greatest in the later stages of flowering. Variation in seed set was not attributed to lack of pollination in 1983. In 1984, pollination limited seed set per flower during peak flowering. However, seed production never reached the potential five seeds per flower, suggesting that resource limitation was the most important factor affecting fecundity in both years. The asynchronous pattern of flowering is suggested to be the result of the different inflorescence sizes between the sexes.     

Sex-specific physiology and source-sink relations in the dioecious plant Silene latifolia

Differences in reproductive demands between the sexes of dioecious plants could cause divergence in physiology between the sexes. We found that the reproductive effort of female Silene latifolia plants increased to more than twice that of male plants or female plants that were prevented from setting fruit by lack of pollination after 4 weeks of flowering. Whole-plant source/sink ratios of pollinated females were significantly lower than those of males or unpollinated females because of investment in fruit. We hypothesized that these differences in source/sink ratio between the sexes and within females, depending on pollination, would lead to differences in leaf photosynthetic rates. Within females, we found that photosynthetic capacity was consistent with measurement of whole-plant source/sink ratio. Females that were setting fruit had 30% higher light-saturated photosynthetic rates by 28 days after flowering than females that were not setting fruit. Males, however, had consistently higher photosynthetic rates than females from 10 days after flowering onwards. Males also had approximately twice the dark respiration rates of fruiting females. We found that female reproductive structures are longer-lived and contribute more carbon to their own support than male reproductive structures. Despite the higher rates of leaf dark respiration and lower calyx photosynthetic rates, males fix more carbon than do females. We conclude that females have a sink-regulated mechanism of photosynthesis that allows them to respond to variations in fruit set. This mechanism is not, however, sufficient to explain why male S. latifolia plants have higher rates of photosynthesis, higher source/sink ratios, and lower reproductive allocation, but fail to grow larger than female plants.     

Impact of prairie fragment size on proportion of females and reproductive success of Lobelia spicata Lam., a gynodioecious species

• Due to ongoing human impacts, plant species increasingly occur in landscapes that are highly fragmented, with remaining natural habitats occupying small areas, resulting in populations that are smaller and more isolated than in previous time periods. This changed metapopulation structure is expected to have negative impacts on seed production. For example, the proportion of female plants within gynodioecious populations may be more volatile due to genetic drift in small populations associated with small habitat fragments, with concomitant impacts on seed production. My aims were to determine: (i) if variation in proportion of females is larger in smaller fragments; and (ii) if such changes in female frequency in small fragments result in reduced seed production.
• Thirty‐two populations of Lobelia spicata Lam., a gynodioecious species, were surveyed in 2000, 2001 and 2009 in the tallgrass prairie region of Midwestern North America (Illinois and Indiana, USA). Data were collected for: proportion of female plants, total number of flowering plants (measure of population size), seed set per plant and prairie fragment size (another measure of population size).
• The proportion of females is more variable in smaller prairie fragments. Seed number per fruit decreases as the proportion of females increases in a population, but only significantly for female plants. The number of flowering plants is positively associated with fruit production for both genders. Populations within larger prairie fragments have higher seed production.
• The reproductive consequences of habitat fragmentation depend on the plant breeding system. While both sexes were negatively impacted, females were more adversely affected.

     

Studies on ecology and behavior of Japanese black swallowtail butterflies. 6. Nectar feeding ofPapilio helenus nicconicolens Butler andP. protenor demetrius Cramer as main pollinators of glory bower,Clerodendron trichotomum,Thunb

The butterfliesPapilio helenus andP. protenor were shown to feed mainly on the nectar of the glory bower,Clerodendron trichotomum, which was the most abundant nectar plant in summer in the study area. Both the species were found to have a proboscis longer than 24 mm corresponding to the length of the corolla tube ofC. trichotomum. Visits to the flowers by these butterflies were observed more frequently than visits by sphingid moths which had previously been believed to be the major pollinators ofC. trichotomum. The male butterflies visited trees ofC. trichotomum frequently, while visits by the females were less frequent. However, once females had visited the tree ofC. trichotomum, they remained there longer than the males. Since the flower ofC. trichotomum has long protruding sexual organs, its pollen grains were found to adhere efficiently to the bodies of butterflies, mainly the thorax, during nectar feeding. Most of the butterflies became loaded withC. trichotomum pollen, and the mean number of pollen grains per butterfly was 1,776 forP. helenus and 2,817 forP. protenor. The flowers opened at any time of day but more frequently in the morning. The nectar was secreted throughout the day. In the maturation of the protandrous flower ofC. trichotomum, the duration of the pistillate phase was about twice as long as the staminate phase. The long flowering period and the short duration of the staminate phase resulted in asynchrony of the flowering stages even within a single cyme on a tree. Such asynchrony and the abundance of attractive flowers on a tree facilitates efficient pollination by the butterflies.     

Flower bud abortion influences clonal growth and sexual dimorphism in the understorey dioecious shrub Aucuba japonica (Cornaceae)

Sexual differences were investigated to determine the significance of flower bud abortion in the dioecious shrub Aucuba japonica Thunb. The mean number of flowers per inflorescence and the mean number of flowering inflorescences (as opposed to aborted inflorescences) per individual were greater in males than in females in 1997 and 1998. Reproductive investment by males was 0.4-times (1997) and 1.4-times (1998) that by females. In addition, females aborted 30.9% (1997) and 42.7% (1998) of their total flower buds without blooming, whereas no male flower buds aborted. One of the architectural traits of this shrub is that in the year that a flower bud is produced at the shoot apex, the shoot will branch into two or more shoots. Thus, there was less sexual difference in the number of current shoots per individual than there was in the number of flowering inflorescences. The relationship between annual growth and reproduction, and the probability of reproduction in the following year, suggested that the higher investment in female reproduction was manifested as a cost for reproductive frequency rather than as a cost for annual growth. The spatial distribution of both males and females was clumped, which may be the result of clonal growth. In addition, overall sex ratios were not skewed and the number of sprouts did not differ significantly between sexes. These results suggested that flower bud abortion by females might reduce sexual dimorphism in terms of clonal growth.     

Pollination of Androdioecious Xerospermum intermedium Radlk. (Sapindaceae) in a rain forest

Xerospermum intermedium, a fairly common understorey species in the West Malesian lowland dipterocarp forest is androdioecious, with delayed self-compatibility. It flowers annually, and has an extended flowering period, with individuals flowering somewhat asynchronously, presenting small flowers with minimal visual lures. The flowers are visited by an unrestricted array of apparently imprecise opportunistic feeders, predominated by trigonid bees and butterflies. Most of these visitors exhibit low fidelity and forage opportunistically on numerous competing tree species. Presentation of nectar in alternating rhythms between the male and hermaphrodite trees appears to induce pollinator movement between them. Out of such alternating visits between the sexes, a low level of short-range inter-tree pollen transfer by trigonids and butterflies seems to occur. Despite the elaborate system which promotes pollinator movement between the sexes, this species has retained a low level of self-compatibility. The physical barriers to pollinator movement in the understorey, and high interspecific competition for pollinators by a large number of tree species, appear therefore to make it difficult to maintain an obligate outbreeding system. Hence, androdioecism may be one of the explanations for the survival of X. intermedium.     

武夷山不同龄级香果树开花物候及其生殖特性

该研究采用固定样地观测研究了不同龄级香果树的开花物候和生殖特性,分析了物候指数与生殖构件之间的相关性。结果表明：香果树单花花期一般为5~9 d,平均为6.79 d,随着树龄增大,其单花花期呈幂函数增长趋势;不同树龄香果树的始花期不同,但终花期基本一致;随着树龄的增大,始花期逐渐提前,花期持续时间延长。20~50 a树龄的香果树始花期约为8月9日,花期持续时间最短,仅为36 d。110~140 a树龄的香果树始花期约为7月15日,花期持续时间为61 d;对于单株母树来讲,香果树的花枝数,花朵总数以及果实总数随着树龄的增大均显著增加;单花枝产花量随着树龄的增加呈上升趋势,单花枝产果实数量呈先上升后下降的趋势;香果树花枝和单花枝产花量在树冠方位上的分布由大到小依次为南>东/西>北、上>中>下,但单花枝果实数量则表现出不同规律,即东>南>西>北、中>下>上。香果树具有花多果少现象,开花物候指数与生殖构件数量间的相关性分析表明,其花期持续时间与花枝数、花数和果实数均存在极显著正相关关系,始花期与生殖构件数量呈极显著负相关关系,而物候指数与香果树树龄亦存在显著相关关系。     

Annual variation in floral phenology and pollen production in a 25‐year‐old plantation of Tectona grandis

Teak is a timber tree that is widely distributed in the tropics. Several studies on pollination and reproductive biology have been conducted, but generally information on flowering phenology and annual variation in total pollen production per tree is lacking. The reproductive phenology as well as flower‐ and pollen grain production of individuals in a population is important to theoreticians, field biologists and plant breeders, as they determine the distribution of genotypes within populations and influences the degree of differentiation among populations. This study reports flowering phenology and variation in total flower, fruit‐ and pollen production per tree in teak in a 25‐year‐old plantation across three consecutive years (2006–2008). The results show that the date of onset and end of flowering was highly variable across years. The longest flowering period of 93 days was observed in 2006. There was an asynchrony in the number of open flowers due to differences in time of anthesis among individuals (± 2 days) and inflorescences within individuals (± 6 h). The production of pollen grains per tree in 2007 was 33%, i.e. 16% more compared to 2006 and 2008. The fruit production per tree was 42% and 27% higher in 2007 compared to 2006 and 2008. Concentration of pollen grains (both on jelly‐coated microscopic slides and stigmas) were highest between noon and 2 pm. At this time, the stigmatic pollen load ranged between 4–8 pollen grains per stigma, which is sufficient for fruit development. The study concludes that the asynchronization of the flower opening might give rise to a high amount of self‐pollination in the stand, ultimately leading to poor fruit setting. Also, the large production of flowers and pollen per tree induced geitonogamy and decreased female fitness, as T. grandis is preferentially an out‐crossing species.     

Variable mycorrhizal benefits on the reproductive output of Geranium sylvaticum,with special emphasis on the intermediate phenotype

In several gynodioecious species, intermediate sex between female and hermaphrodite has been reported, but few studies have investigated fitness parameters of this intermediate phenotype. Here, we examined the interactions between plant sex and arbuscular mycorrhizal (AM) fungal species affecting the reproductive output of Geranium sylvaticum, a sexually polymorphic plant species with frequent intermediate sexes between females and hermaphrodites, using a common garden experiment. Flowering phenology, AM colonisation levels and several plant vegetative and reproductive parameters, including seed and pollen production, were measured. Differences among sexes were detected in flowering, fruit set, pollen production and floral size. The two AM species used in the present work had different effects on plant fitness parameters. One AM species increased female fitness through increasing seed number and seed mass, while the other species reduced seed mass in all sexes investigated. AM fungi did not affect intermediate and hermaphrodite pollen content in anthers. The three sexes in G. sylvaticum did not differ in their reproductive output in terms of total seed production, but hermaphrodites had potentially larger fathering ability than intermediates due to higher anther number. The ultimate female function – seed production – did not differ among the sexes, but one of the AM fungi used potentially decreased host plant fitness. In addition, in the intermediate sex, mycorrhizal symbiosis functioned similarly in females as in hermaphrodites.     

Fruit set in a deceptive orchid: The effect of flowering phenology,display size,and local floral abundance

We investigated the effect of flowering time, display size, and local floral density on fruit set in Tolumnia variegata, a pollination-limited orchid that offers no reward to its pollinator(s). During 1990, natural variation in flowering time, display size, and fruit set were monitored in 508 plants at one locality in Puerto Rico. The following season, orchid floral abundance per host tree (Randia aculeata) was manipulated to investigate its effect on fruit set. Four floral abundance treatments were established (700, 500, 300, and 100), each replicated four times. Flowering time was the most important trait affecting fruit set. The proportion of plants setting at least one fruit was significantly high early and late in the season, but low during the flowering peak. Thus, strong disruptive selection differential on flowering phenology was found. Display size had little effect on fruit set. A weak, but significant disruptive selection differential on display size was found. Orchid floral abundance per host tree had a significant effect on fruit set. Early in the season, T. variegata flowers with intermediate number of conspecific flowers exhibited a greater probability of setting fruit than those in host trees with fewer or more flowers. Our results show that flowering phenology may be evolutionarily unstable, possibly a consequence of the deception pollination system. Furthermore, a deception strategy would be relatively unsuccessful in populations where plants are found in either very dense or sparse patches.     

Pollination success in a population of dioecious rain forest trees

Pollination success in female trees was determined for a population of Neolitsea dealbata (R. Br.) Merr., a locally abundant dioecious tree pollinated by small, unspecialized insects in northern Queensland rain forest, Australia. The population consisted of a clustered group of trees with a mean male-to-female distance of 4.5 m and more isolated individuals, including females more than 90 m away from the nearest pollen source. A map of all reproductive trees was produced to determine accurate male-to-female distances. The size of the pollen source available to females was defined as a function of the distance to the nearest ten male trees and their sizes (male neighbourhood index). The rate of pollen movement to females was measured by counting pollen tubes (and the number of tubes per style) in female trees 6 days after the commencement of population flowering. The pollination rate decreased steeply to less than half when the nearest male was only 6.5 m away. Although pollen reached a female 330 m away from the nearest pollen source, only 10% of receptive flowers had been pollinated. The short flowering period (2–3 weeks) combined with the the slow rate of pollen movement means that a large proportion of flowers in isolated trees are unpollinated, confirming an earlier finding that isolated females set fewer fruits than gregarious females. The reliability of pollen transfer to females was determined by quantifying insects and their pollen loads trapped at female trees with a range of male neighbourhood indices. Quantities of insects and pollen were significantly correlated with the size of the male neighbourhood index, indicating a strong density-dependent response by vectors to flowering. Pollen was also collected from insect visitors to non-flowering trees. Females with large male neighbourhood indices received more pollen than non-flowering trees with equivalent male neighbourhood indices. However, when the male neighbourhood indices were small for both female and non-flowering trees, the changces of pollinators encountering female and non-flowering trees were similar, suggesting random movements of pollinators in sparse-flowering sub-populations. The dioecious breeding system, brief, synchronous flowering period, clustered population structure and random, opportunistic foraging behaviour of vectors interacted in a way that reduced reproduction in relatively isolated trees. These results demonstrate a mechanism for differential breeding success between trees in natural populations and emphasize the possible impact of logging regimes on pollen flow between trees. Large interconspecific distances in species-rich environments may have been a factor in the selection for synchronous flowering between trees in outcrossing tree species with generalist insect pollinators.     

Floral display, pollinator visitation and reproductive success in the dioecious perennial herb Wurmbea dioica (Liliaceae)

Floral traits that increase attractiveness to pollinators are predicted to evolve through selection on male function rather than on female function. To determine the importance of male-biased selection in dioecious Wurmbea dioica, we examined sexual dimorphism in flower size and number and the effects of these traits on pollinator visitation and reproductive success of male and female plants. Males produced more and larger flowers than did females. Bees and butterflies responded to this dimorphism and visited males more frequently than females, although flies did not differentiate between the sexes. Within sexes, insect pollinators made more visits to and visited more flowers on plants with many flowers. However, visits per flower did not vary with flower number, indicating that visitation was proportional to the number of flowers per plant. When flower number was experimentally held constant, visitation increased with flower size under sunny but not overcast conditions. Flower size but not number affected pollen removal per flower in males and deposition in females. In males, pollen removal increased with flower size 3 days after flowers opened, but not after 6 days when 98% of pollen was removed. Males with larger flowers therefore, may have higher fitness not because pollen removal is more complete, but because pollen is removed more rapidly providing opportunities to pre-empt ovules. In females, pollen deposition increased with flower size 3 days but not 6 days after flowers opened. At both times, deposition exceeded ovule production by four-fold or more, and for 2 years seed production was not limited by pollen. Flower size had no effect on seed production per plant and was negatively related to percent seed set, implying a tradeoff between allocation to attraction and reproductive success. This indicates that larger flower size in females is unlikely to increase fitness. In both sexes, gamete production was positively correlated with flower size. In males, greater pollen production would increase the advantage of large flowers, but in females more ovules may represent a resource cost. Selection to increase flower size and number in W. dioica has probably occurred through male rather than female function. Received: 15 June 1997 / Accepted: 12 February 1998