Dioecious Tree Species Research Articles

Geographic parthenogenesis in a tropical forest tree

As evidenced by embryological studies, certain tropical tree species can reproduce asexually via seed; however, it has generally been assumed that such apomictic reproduction is facultative. Here I report the existence of a population of the lowland Malaysian rain forest tree Garcinia scortechinii King that consists entirely of pistillate individuals (40 female trees and no male trees recorded within a 25-ha area). This constitutes the first documented observation of a geographically segregated, obligately asexual population among wild tropical trees. Six additional Garcinia species showed a trend toward female bias relative to other dioecious tree species studied, an observation that is consistent with facultative apomixis. The more surprising result for G. scortechinii runs counter to the generalization that obligate apomicts occur only in environments in which the absence of competitors might enable persistence of taxa that lack genetic recombination through mixis.

Pistillate and staminate flower development in dioecious Pistacia vera (Anacardiaceae)

The development of staminate and pistillate flowers in the dioecious tree species Pistacia vera L. (Anacardiaceae) was studied by scanning electron microscopy with the objective of determining organogenetic patterns and phenology of floral differentiation. Flower primordia are initiated similarly in trees of both sexes. Stamen and carpel primordia are initiated in both male and female flowers, and the phenology of organ initiation is essentially identical for flowers of both sexes. Vestigial stamen primordia arise at the flanks of pistillate flower apices at the same time functional stamens are initiated in the staminate flowers. Similarly, a vestigial carpel is initiated in staminate flowers at the same time the primary, functional carpel is initiated in pistillate flower primordia. Differences between the two sexes become apparent early in development as, in both cases, development of organs of the opposite sex becomes arrested at the primordial stage. Male flowers produce between four and six mature functional stamens and female flowers produce a gynoecium with one functional and two sterile carpels.

Population Density and Fruit Set in Three Dioecious Tree Species in Australian Tropical Rain Forest

The effect of male tree density on pollen flow and fruit set in females was examined for three species of diocecious tree (Neolitsea dealbata, Littsea leefeana and Diospyros pentamera) in complex notophyll rain forest in tropical, north-eastern Australia. The spatial distribution for all reproductive individuals combined was significantly clumped in Diospyros but not in either Litsea or Neolitsea. Separate male and female groups were significantly clumped in all species. More than 80% of nearest male neighbours occured within 10 m of neolitsea females and within 20 m of Litsea and Diopsyros females. The ratio of males to females was 1.6: 1 (Neolitsea), 1.5: 1 (diopsyros) and 1.3: 1 (Litsea) (...)

Tree densities and sex ratios in breeding populations of dioecious Central Amazonian Myristicaceae

ABSTRACTThe densities of the breeding populations and the sex of all flowering individuals were recorded for five dioecious canopy tree species of Central Amazonian Myristicaceae, in 11 study areas of the Minimum Critical size of Ecosystems Project totalling 22.5 ha. Adult population densities were extremely low, ranging from 0.38 to 1.61 ha–1 for the five species studied. In a 10 ha study plot the mean distance to the nearest flowering conspecific ranged from 48 to 100 m, while the mean distance to the nearest opposite sex conspecific was 147 m. The two most abundant species, Iryanthera macrophylla and Virola calophylla, both showed male-biased sex ratios, of 23:9 and 20:6, respectively. The size class distribution of males, females and non-flowering individuals in V. calophylla suggests that earlier reproductive maturation of male plants may provide a partial explanation for this bias. In I. macrophylla, since 95% of the individuals were observed flowering, the observed ratio is representative of the population, and may be caused by sex shifts from male to female. The low reproductive densities, combined with the skewed sex ratios and overlapping generations of these species, create very small effective breeding populations, placing species such as these at great risk in the face of deforestation and habitat fragmentation.

Pollen dispersal distances for a conifer canopy species in the Knysna Forest

No data are available on pollen flow distances for wind-pollinated canopy trees in broad-leaf, closed, evergreen forests. Nevertheless, major biogeographic patterns, such as the relative absence of anemophilic tree species in lowland tropical forests and the rise of the angiosperms, have been explained in terms of the supposed relative inefficiency of anemophily compared to zoophily. In this study, fruits were found below females of the dioecious anemophilous tree species Podocarpus falcatus in the Knysna Forest, which were up to 70 m from male trees. This pollen flow is over more than twice the distance previously suggested as a maximum and indicates that wind-pollinated species could produce out-crossed progeny in species-rich communities.

SPATIAL RELATIONSHIPS BETWEEN STAMINATE AND PISTILLATE PLANTS OF DIOECIOUS TROPICAL FOREST TREES.

Alternate floral morphs in the form of dioecious, gynodioecious, distylous and tristylous plants within the same species are of widespread occurrence among flowering plants. The spatial distribution of different floral morphs, and therefore floral resources, within a population is likely to influence the foraging behavior of pollinators and concomitantly, the level of pollination. This distribution might also indicate the extent to which plants with different floral morphs exploit different microhabitats, particularly in dioecious species in which sexual dimorphism is most pronounced. Recently, Levin (1974) and Ornduff and Weller (1975) have analyzed the spatial distribution of pins and thrums among distylous Hedyotis nigricans (Rubiaceae) and Jepsonia heterandra (Saxifragaceae), respectively. However, to the best of our knowledge, no such analysis exists for a dioecious species despite the fact that dioecy is much more common than heterostyly, especially in species-rich tropical forests (see Ashton, 1969; Bawa, 1974; Bawa and Opler, 1975). Herein, we report the dispersion patterns of staminate and pistillate plants of several dioecious tree species that occur in tropical lowland forests of Costa Rica. The objective of the study was to investigate: a) if staminate and pistillate plants of the same species display spatial segregation, b) if the dispersion pattern indicates asexual reproduction by one or both sexes, and c) if the distribution of one sex might be more correlated with a particular environmental factor than the other. The study is part of a wider investigation dealing with ecological aspects of dioecism in tropical forests (Bawa and Opler, 1975, 1977; Opler and Bawa, 1977).

Differences in Endogenous Levels of Gibberellin Activity in Male and Female Partners of Two Dioecious Tree Species

Differences in Endogenous Levels of Gibberellin Activity in Male and Female Partners of Two Dioecious Tree Species

DIOECISM IN TROPICAL FOREST TREES.

The existence of dioecism among angiosperms is universally acknowledged, yet is poorly understood both from the ecological and evolutionary points of view. The reason for this hiatus might be that some have believed the relative proportion of dioecious taxa to be low (Yamplosky and Yamplosky, 1922; Lewis, 1942), while others have felt that since the number of seed-bearing individuals is halved in a dioecious population its evolution is an unlikely and perhaps inconsequential event (HeslopHarrison, 1972). However, recent studies have reported a large proportion of dioecious tree species in tropical forests (Ashton, 1969; Bawa, 1974; see also Tomlinson, 1974); this indicates that the incidence and importance of dioecism may have been underestimated in the past. These new data are significant in at least one other respect. Dioecious (and monoecious) flowers in temperate plants are presumed to have evolved in response to selective pressures favoring wind-pollination (Grant, 1951; Stebbins, 1951), yet windpollination is probably either absent or uncommon in tropical forests (Whitehead, 1969; see also Daubenmire, 1972). Here we present an overview of the adaptive and evolutionary significance of dioecism in general and its expression in certain tropical forests in particular. We first present data on the frequency of dioecious species in a lowland tropical semi-deciduous forest in Costa Rica, then proceed to an examination of available information on