Ecophysiological and floristic implications of sex expression in the dioecious neotropical CAM tree Clusia hilariana Schltdl.

Clusia hilariana Schlechtendal is a dioecious tree performing crassulacean acid metabolism (CAM). It grows in the sandy coastal restinga of Brazil. As pioneers and 'nurse plants' individual trees become surrounded by other vegetation forming isolated vegetation islands on the sand plain. The female plants produce large and heavy fleshy fruits suggesting a particular demand for resources. Frugivores may introduce nutrients (faeces) and diaspores into the stands of female plants. Thus, the hypothesis was advanced that sex expression in C. hilariana has important general consequences for the physiology of the trees themselves and also determines aspects of the vegetation around them. Field work in the restinga of Macaé (Rio de Janeiro State, Brazil) showed that morphological leaf characteristics, the expression of CAM and the species diversity and species composition of their respective vegetation islands were similar for both sexes. The female plants had somewhat higher chlorophyll levels. Differences in effective quantum yield of photosystem II (PS II) and non-photochemical quenching of chlorophyll a fluorescence between female and male plants were very small. Female plants were a little less subject to acute photoinhibition at noon. Maximum fluorescence (Fm), variable fluorescence (Fv) and minimum fluorescence (F₀) were significantly lower in female plants. However, maximum quantum yield ratio of male to female plants as well as fluorescence yield ratio and minimum fluorescence ratio of male to female plants related to chlorophyll content were all close to unity. Therefore, although there may be some fine-tuned differences in the light reactions of photosynthesis, PS II of male and female plants are not different. Hence, the hypothesis of possible important consequences of sex determination for expression of morphological and physiological traits of the plants and for the composition of the 'nursed' vegetation underneath them must be rejected.     

Population structure, physiology and ecohydrological impacts of dioecious riparian tree species of western North America

The global water cycle is intimately linked to vegetation structure and function. Nowhere is this more apparent than in the arid west where riparian forests serve as ribbons of productivity in otherwise mostly unproductive landscapes. Dioecy is common among tree species that make up western North American riparian forests. There are intrinsic physiological differences between male and female dioecious riparian trees that may influence population structure (i.e., the ratio of male to female trees) and impact ecohydrology at large scales. In this paper, we review the current literature on sex ratio patterns and physiology of dioecious riparian tree species. Then develop a conceptual framework of the mechanisms that underlie population structure of dominant riparian tree species. Finally, we identify linkages between population structure and ecohydrological processes such as evapotranspiration and streamflow. A more thorough understanding of the mechanisms that underlie population structure of dominant riparian tree species will enable us to better predict global change impacts on vegetation and water cycling at multiple scales.     

Population structure and spatial pattern in the dioecious shrub Ceratiola ericoides

Abstract. The dioecious shrub Ceratiola ericoides (Florida rosemary) dominates xeric, infrequently burned Florida scrub vegetation, often to the near-exclusion of other woody species. We studied the spatial pattern, age, sex and size structure of four populations in Florida, USA: two coastal scrub populations subject to recurrent local disturbances due to sand movement, and two inland scrub populations in sites periodically burned by stand-replacing fires. The age structure of individual genets was estimated from node counts and used to describe the age structure of the populations. The sex ratio of males to females was not significantly different from 1:1, except within a female-biased coastal population subject to frequent sand movement. Node counts indicated that the mean age for reproductive individuals was 15 - 16 yr for the inland populations and 13 - 16 yr for the coastal populations. In all sites, there was no difference in mean age between males and females. Vegetative reproduction was uncommon except for the least-disturbed coastal population where 72 % of the reproductive individuals originated through layering. Individuals were generally randomly dispersed at the coastal sites, whereas significant aggregation of males and females occurred in the inland sites where the populations were initiated following fire. Seedling recruitment was continuous in the disturbed coastal scrub site, where 35% of the individuals were juveniles. Most juveniles were dispersed from 0.5 to 0.75 m around females. At one of the inland sites, where juveniles comprised 11% of the population, juveniles were clustered at 0.25 to 5.75 m around females. Coastal populations were all-aged, while inland populations were uneven-aged. Recruitment appears to follow periods of disturbance; infrequent fire in the inland populations and continuous sand movement on the coast are factors initiating recruitment.     

Biomass and nutrient allocation in a neotropical dioecious palm

Summary The sexes of Chamaedorea ernesti-augusti are largely undifferentiated in the distribution of biomass, nitrogen, phosphorus, potassium, and total non-structural carbohydrates, among leaves and stems. Males bear more inflorescences that are cheaper except in nitrogen, but most females bear greater annual energetic and nutritional burdens due to seed production. The ratio of vegetative to reproductive biomass is 3.5 for males but only 1.2 for females on a per module basis.     

雌雄异株树种黄连木种群性比及空间分布

研究了100m×140m固定样地内黄连木种群的性比格局和空间分布。结果表明,调查样地中dbh≥4cm的黄连木植株共有2116株,其中包括526株雌树,1200株雄树,性别未确定植株390株。黄连木性比(雄/雌=2.28)显著偏雄(P0.001)。雌树和雄树平均胸径分别为7.34和7.81cm,雄树胸径显著大于雌树胸径(P0.05)。黄连木幼树、雌树和雄树均呈显著聚集性分布。黄连木幼树与雌树、幼树与雄树均在较小尺度上表现为相互吸引;雌树与雄树则在空间上相互排斥,即雌树与雄树存在空间分离现象;黄连木不同大小以及不同性别植株之间主要表现为相互排斥。     

Density-dependent shoot-borer herbivory increases the age of first reproduction and mortality of neotropical tree saplings

Shoot herbivory by the sapling specialist shoot-borer Cromarcha stroudagnesia (Lepidoptera, Pyralidae, Chrysauginae) is shown to have large direct and indirect effects on the rates of height increment and mortality of saplings of its host tree, Tabebuia ochracea (Bignoniaceae), in the secondary successional tropical dry forests of the Area de Conservación Guanacaste, Costa Rica. Experiments and field observations over 3-4 years show a substantial reduction in sapling height increments due to C. stroudagnesia herbivory, of equivalent magnitude to the difference in height increments between undamaged saplings in canopy gaps and full understorey shade. Extrapolating this data at average amounts of C. stroudagnesia herbivory increases the duration of the pre-reproductive sapling life stage by about 40% relative to undamaged plants. This is an underestimate, as top shoot herbivory by C. stroudagnesia also increased the probability of canopy gap saplings being overtopped and shaded by surrounding vegetation. Sapling mortality was increased by C. stroudagnesia herbivory, with 11.8% of the most heavily damaged young saplings dying in 3 years while no undamaged saplings died. Cromarcha stroudagnesia herbivory strongly increases with the number of conspecific T. ochracea saplings and the number of conspecific shoots within 50 m of focal saplings. It is therefore likely to disproportionately decrease the number of saplings and rate of recruitment to reproductive age in areas of high conspecific sapling density. These results suggest that sapling herbivory, especially herbivory of terminal meristems, has an important but largely unexplored influence on the population dynamics of tropical tree species. They further demonstrate that sapling herbivory by insects, in addition to the well-studied insect predation and herbivory of seedlings, is likely to influence tree species coexistence in tropical forests.     

Fine-scale genetic structure and gene dispersal inferences in 10 neotropical tree species

The extent of gene dispersal is a fundamental factor of the population and evolutionary dynamics of tropical tree species, but directly monitoring seed and pollen movement is a difficult task. However, indirect estimates of historical gene dispersal can be obtained from the fine-scale spatial genetic structure of populations at drift-dispersal equilibrium. Using an approach that is based on the slope of the regression of pairwise kinship coefficients on spatial distance and estimates of the effective population density, we compare indirect gene dispersal estimates of sympatric populations of 10 tropical tree species. We re-analysed 26 data sets consisting of mapped allozyme, SSR (simple sequence repeat), RAPD (random amplified polymorphic DNA) or AFLP (amplified fragment length polymorphism) genotypes from two rainforest sites in French Guiana. Gene dispersal estimates were obtained for at least one marker in each species, although the estimation procedure failed under insufficient marker polymorphism, limited sample size, or inappropriate sampling area. Estimates generally suffered low precision and were affected by assumptions regarding the effective population density. Averaging estimates over data sets, the extent of gene dispersal ranged from 150 m to 1200 m according to species. Smaller gene dispersal estimates were obtained in species with heavy diaspores, which are presumably not well dispersed, and in populations with high local adult density. We suggest that limited seed dispersal could indirectly limit effective pollen dispersal by creating higher local tree densities, thereby increasing the positive correlation between pollen and seed dispersal distances. We discuss the potential and limitations of our indirect estimation procedure and suggest guidelines for future studies.     

Population genetic structure of Vallisneria americana,a dioecious clonal macrophyte

The population genetic structure and spatial genetic structure of the dioecious aquatic macrophyte Vallisneria americana was examined near Turkey Island in the Detroit River in late summer of 1991. The density and distribution of male, female, and vegetative ramets was determined for three transects parallel, and three transects perpendicular to the island, at mean water depths of 38–306 cm. A strong male-biased sex ratio was observed in shallow water, while sex ratios became increasingly female-biased, and plant density increased, at greater water depths. Cellulose acetate gel electrophoresis was used to characterize the allozyme phenotype of each ramet that had been screened for seven enzyme systems. Overall, 91 allozyme phenotypes were identified. A single allozyme phenotype accounted for 33%–55% of all the ramets (depending on the transect), indicating extensive regional vegetative growth. However, the population as a whole displayed high genetic diversity (H_T = 0.3403), with most of the diversity occurring within transects (H_s = 0.3297) rather than between transects (D_ST = 0.0106). The possible factors accounting for the high small-scale genetic diversity of this clonal aquatic plant species are discussed.     

Nuclear and chloroplast genetic structure indicate fine-scale spatial dynamics in a neotropical tree population

Dicorynia guianensis is a canopy tree, endemic to the tropical rain forest of French Guiana. We compared generational and spatial genetic structure for maternally and biparentally inherited markers in two cohorts (adult and seedling) in order to infer processes shaping the distribution of genetic diversity. The study was conducted on a 40 ha study plot located at Paracou near Kourou, where 172 adults trees and 375 saplings were sampled. Aggregation of trees was therefore suggested at different distances, ranging from 100 to 400 m. There was a strong link between demographic and genetic spatial structures at small distances (less than 100 m) that is likely to be the consequence of restricted seed dispersal. Genetic differentiation was more pronounced between spatial aggregates than between cohorts. Despite the spatial differentiation, the species was able to maintain high levels of diversity for maternal genomes, suggesting rapid turnover of aggregates. Spatial autocorrelation was larger for chloroplast than nuclear markers indicating a strong asymmetry between pollen and seed flow. Fixation indices indicated a lower heterozygote deficiency for the adults, maybe because of gradual elimination of selfed trees. Genetic relatedness at lower distances was higher in adult trees than in saplings, as a result of generation overlapping in the adult cohort. Overall, our results confirm earlier biological knowledge about the dispersion mechanisms of the species, and lead to an enhanced role of spatial processes in the dynamics of genetic diversity of D. guianensis.     

A comparison of growth and reproduction, under laboratory conditions, of males and females of a dioecious fig tree

The interaction between Ficus spp. (Moraceae) and their pollinating wasps (Chalcidoidae: Agaonidae) is a highly co-evolved mutualism. Approximately half of all fig species are monoecious and produce a mixture of wasps and seeds within the same fig. In functionally dioecious fig trees male and female functions are separate. Figs on male trees produce wasps and pollen, whereas figs borne on female plants produce only seeds. Dioecious fig phenology provides an excellent opportunity to investigate the effect of sexual specialization on the obligate fig?Cfig wasp interaction and the non-pollinators associated with the system. Here we describe laboratory studies of phenological variation between two sexes in terms of vegetative growth and fig production in a dioecious fig tree Ficus montana. We also describe reproductive output in terms of wasp production in males and seeds in females. Intrasexual asynchrony was observed for the plants, with synchrony between the sexes with year-round production of figs. Male plants grew more rapidly, but leaf phenology was very similar. Crop sizes and development times were the same for males and females. Seasonal effects were strong for leaf phenology and fig initiation, but had a very limited effect on fig composition. The results show that the phenological differences described for other dioecious figs do not apply to all species.     

Population structure and dynamics of a neotropical palm in fire-impacted fragments of the Brazilian Atlantic Forest

The demography of the acaulescent palm Attalea humilis was studied from 1996 to 1999 in three forest fragments (1.6, 6.4, and 9.9ha) in southeastern Brazil (22°30–22°33 S, 42°15–42°19 W). A human-induced fire damaged the populations in August–September 1997. Palm density was higher in the medium-sized fragment, where it was concentrated in a single disturbed sector. Population ontogenetic stage structure was dependent on fragment and year, but in general later stages were more abundant than initial ones. Population finite growth rates projected from linear, stage-structured demographic models indicated that the populations were increasing before fire. Population growth rate in the smallest fragment was larger due to a single recruitment event, being most sensitive to changes in survival and growth of seedlings, virgins and the immature, and relatively insensitive to changes in shrinkage and fecundity. Observed growth rates for the entire study period showed that populations remained in equilibrium after fire, but the population in the medium-sized fragment showed a declining tendency. Population structure was resilient to fire, which caused a transitory concentration of individuals in the medium-sized stage. The self-sustainability of the population in the largest, most preserved fragment suggests that habitat-protection measures may suffice to ensure the persistence of populations in small fragments. However, population fluctuations and exclusion from hyper-disturbed sectors in the smallest fragment suggest that reduced fragment area, habitat degradation and human-induced fires can interact to threaten the persistence of even abundant, preadapted to fire, species like A. humilis.     

Interactions in the patterns of vegetative growth and reproduction in woody dioecious plants

Summary Interactions between vegetative growth and reproduction were evaluated in Peumus boldus, Lithraea caustica and Laretia acaulis, three woody dioecious species in central Chile. Phenological observations were made periodically on marked branches of male and female plants, and biomass allocation (dry weight) to vegetative and reproductive tissues was measured. The magnitude of flowering was evaluated in groups of plants in three successive seasons. The patterns of activities are species- and sex-dependent, and cycles of 2–4 years have been established. Branches that produce flowers either do not grow or grow less than branches without flowers, and males and females have differential resource allocation: male branches attain higher biomass values. Groups of plants show seasonal behavior that suggest synchrony in their reproductive activities.     

Population abundance and sex ratio in dioecious helminth parasites

Parasite populations are highly fragmented in space and time, and consist of aggregates of genetically similar individuals sharing the same host. To avoid inbreeding, theory predicts that female-biased sex ratios should be strongly favoured when either or both prevalence and intensity of infection are low. Other models indicate that if sex ratios are selected to increase the probability of mating, they should be less biased at a high intensity of infection in polygamous parasites, since at high intensities all females are mated. To test these predictions, the relationship between sex ratio and both the prevalence and intensity of infection was examined in comparative studies across 193 populations of nematode and acanthocephalan parasites. Sex ratios in these two dioecious, polygamous taxa are usually female biased. Among natural populations, no significant relationship was observed once the confounding effects of phylogeny had been removed. However, among experimental populations of nematodes, a negative relationship was found between intensity of infection and sex ratio, even after controlling for phylogeny. In other words, at high intensities, populations of nematodes are less female biased. This result must be treated with caution because of the unusually high numbers of worms per host in experimental infections. Nevertheless, combined with information on the proximate mechanisms regulating sex ratios in these parasites, it suggests a link between the characteristics of parasite populations and their sex ratio. Received: 21 November 1996 / Accepted: 26 March 1997     

Population structure, reproduction and sex-change in a tropical East Atlantic grouper

Spawning time and sizes at first sexual maturity and at sex-reversal are described in the tropical blue-spotted grouper Cephalopholis taeniops . All juvenile fish possess non-functional ovaries. At sexual maturity, most juvenile fish become functional females. Some of these later change sex to become males. The remaining juvenile females change immediately to males, without time as functional females. This is a new scheme for sexual differentiation of groupers.     

Ovule discounting in an outcrossing, cryptically dioecious tree

Ovule discounting denotes the reduction in the number of ovules available for cross-fertilization due to the interference of inferior pollen. Traditionally, ovule discounting has been discussed solely from the perspective of compromised outcrossing opportunities as a result of selfing, but the principle is more general. Here, we extend its applicability beyond the simple contrast between selfing and outcrossing by showing that, in the cryptically dioecious tree species Fraxinus ornus, ovule discounting through frequent outcrossing with inferior fathers also constitutes a substantial cost of mating. In F. ornus, hermaphrodites produce pollen capable of siring offspring, but these offspring are less viable than those sired by males and are inferred to produce few, if any, surviving progeny. In this paper, we used microsatellite markers to analyze the mating system and paternity in a wild population of F. ornus. We found that the effective number of sires per mother was low (N(ep) = 2.93 to 4.95), and that paternity was correlated among progeny sampled from the same mother, but not among progeny sampled from neighboring mothers. Despite the existence of a local spatial genetic structure (up to 30 m), we found no evidence of biparental inbreeding. There was negligible selfing by hermaphrodites, but they sired approximately one fourth of the seeds produced by other hermaphrodites. Given that these progeny are not inferred to reach reproductive maturity, this constitutes a substantial cost of ovule discounting in the broad sense. We discuss the possible reasons for why hermaphrodites invest resources into inferior pollen.     

Population genetics of Cordia alliodora (Boraginaceae), a neotropical tree. 2. Mating system

A multilocus mixed mating model was used to evaluate the mating system of a natural population of Cordia alliodora (Boraginaceae), a neotropical tree. The population was highly outcrossed (t_m = 0.966 ± 0.027), in agreement with results from controlled crosses. Departures from the mixed mating model were evident, suggesting some nonrandom, correlated mating. Pollen pool heterogeneity and variation in estimates of individual outcrossing rates indicated that the population may be genetically substructured. Individual outcrossing rates obtained for the samples taken from within different parts of the same tree indicated reduced levels of outcrossing due to limited sampling of the pollen pool. The incompatibility mechanism in C. alliodora, combined with variation in flowering and stand density, appears to lead to both temporal and spatial substructuring of the population.     

Regulation of reproduction in the neotropical ponerine ant,Pachycondyla villosa

Workers in queenless groups of the antPachycondyla villosa engage in antennal boxing and biting and by these interactions establish social dominance hierarchies, in which several high-ranking individuals may lay eggs. We observed egg cannibalism by dominant workers. The presence of larvae negatively effects the number of worker-produced eggs in queenless colony fragments in two ways. First, larvae feed on the eggs, and second, workers lay fewer eggs when larvae are present. In queenright colonies, workers lay eggs at a low rate, which are eaten by the queen or fed to the brood, but apparently they do not compete aggressively for egg-laying.     

Community dynamic models of two dioecious tree species

The effects of dioecy on community dynamics were examined by using transition matrix models for two dioecious tree species, one a superior competitor with a narrow dispersal range and the other an inferior competitor with a wide dispersal range. The models are based on tree-by-tree replacements in each identical microsite occupied by either male or female canopy trees of the superior competitor and canopy trees of the inferior competitor. Coexistence of the two species is possible not only because of a trade-off between competitive and dispersal abilities but also because of the existence of a competitor gap, which the superior competitor cannot occupy. The competitor gap is created under the male trees of the superior competitor. The inferior competitor occupies the competitor gap because of its wide dispersal range. The relative abundance of the two species depends on the dispersal ability and sex ratios of the superior competitor. The decreasing dispersal ability and the female abundance of the superior competitor increase the competitor gap, which allows the regeneration of the inferior competitor.An erratum to this article can be found at     

Post-burn reproduction of woody plants in a neotropical savanna: the relative importance of sexual and vegetative reproduction

1. Burning typically occurs at intervals of 1–3-years in the Brazilian cerrado, a rate that exceeds the precolonization fire regime. To determine if woody plants of the cerrado successfully reproduce within the short span of time between burns, experimental burns were used to quantify the effects of fire on sexual and vegetative reproduction of six species of resprouting trees and shrubs.
2. Four of the six species reproduce vegetatively by producing root suckers. For three of these species, Rourea induta , Myrsine guianensis and Roupala montana , sucker production was seven to 15 times greater in burned plots than in unburned controls.
3. Fire had a negative impact on sexual reproduction. Fire caused an immediate reduction in sexual reproductive success by destroying developing reproductive structures and seeds. Additionally, five of the six study species exhibited overall reductions in seed production in the years following fire. Fire had this effect by reducing the individual size of all species and, for three species, by reducing size-specific reproductive output. Only the tree Piptocarpha rotundifolia exhibited increased seed production following burning.
4. Fire caused substantial mortality to both seedlings and suckers. Suckers were larger than seedlings and experienced lower mortality rates for two of three species. Fire-induced mortality of seedlings varied greatly among species, ranging from 33% to 100%.
5. The results indicate that vegetative reproduction is much more successful than sexual reproduction under the high fire frequency typical of current fire regimes. It is concluded that current fire regimes must be causing a shift in species composition, favouring species capable of vegetative reproduction.