Report on Wind Dispersal in a Lowland Moist Forest in Central French Guiana

by

Scott A Mori and John L. B.rown

Reprinted with permission from Brittonia 46(2): 105-125. 1994. ©1994. The New York Botanical Garden. See additions and corrections for name changes and additional species added since the publication of this paper. Please report comments and corrections to Scott A. Mori (smori@nybg.org).

Abstract | Introduction | Study Site | Methods | Results | Discussion
Table 1 | Figure 1 | Figure 2 | Figure 3 | Figure 4
Acknowledgements | Literature Cited | Additions and Corrections

MORI, S. A. and J. L. BROWN (Institute of Systematic Botany, The New York Botanical Garden, Bronx, New York 10458-5126 U.S.A.). Report on wind dispersal in a lowland moist forest in central French Guiana. Brittonia 46(2): 105-125. 1994.— An analysis of the flowering plant flora of a lowland moist forest in central French Guiana reveals 298 species with adaptations for wind dispersal. This represents 16.2% of the flowering plant flora and 9.8% of the class Magnoliopsida (dicotyledons). The most diverse wind-dispersed families are the Orchidaceae in the Liliopsida (monocotyledons) with 135 species and the Bignoniaceae in the Magnoliopsida with 37 species. The wind-dispersed species of central French Guiana have evolved either small, dust-like seeds, fruits or seeds with various kinds of wings, fruits or seeds with tufts of hairs, or expanded wing- or parachute-like persistent calyces. Most wind-dispersed species, among the liliopsids, are epiphytes and, among the magnoliopsids, trees or lianas. In central French Guiana, collections of these species with mature diaspores have been gathered most often in October and March, the months with peak wind velocities. In contrast, collections from June and July, when wind velocities are at a minimum, are rare.

Lowland tropical forests are recognized as being among the most species-diverse habitats of the world (see papers in Gentry, 1990), and much, but not all, of the diversity can be attributed to coevolution among the plants and animals that inhabit these forests (Emmons, 1989; Hladik & Miquel, 1990; Janson, 1983; Ridley, 1930). The fruits and seeds of many of the flowering plants in lowland, tropical forests have responded to the myriad interactions with animal seed dispersers. For example, the seeds of many tropical species are dispersed by animals attracted to the fruits by edible pericarps (e.g., Myrtaceae and Sapotaceae), brightly colored, fat-rich arils, e.g. species of Virola ( Howe & Westley, 1988), sugar-rich arils (e.g., Tetragastris panamensis Kuntze; Howe & Westley, 1988), the edible seeds themselves (e.g., Bertholletia excelsa Humboldt & Bonpland; Mori, 1992), and even false arils (e.g., some species of Ormosia). Although the interactions between plants and their animal dispersers are extremely important in tropical forests (Howe & Westley, 1988), abiotic agents also play a significant role in seed dispersal. The purpose of this paper is to describe the role of wind in seed dispersal (anemochory) among the flowering plants of a lowland moist forest in central French Guiana.

This report is a byproduct of a floristic study of the forests surrounding the village of Saül (3°37'N, 53°12'W) in central French Guiana that began in 1965 under the auspices of ORSTOM (Institut Français de Recherche Scientifique pour le Développement en Coopération). The New York Botanical Garden became a participant in this project in 1976.

The village of Saül is surrounded by nearly undisturbed lowland, moist forest located, for the most part, between 200 and 400 meters above sea level. However, Mont Galbao, ca. 20 kilometers WSW of Saül, reaches 762 meters in elevation.

Descriptions of the forest surrounding the village can be found in Mori and Boom (1987) and Oldeman (1974). Minor disturbance to the forest is caused by mining for gold, hunting, subsistence agriculture, selective removal of timber, and clearings opened for the airport landing strip and for homesteads. Nevertheless, the low population of the area (less than 100 individuals) has kept forest destruction to a minimum.

Monthly rainfall for Saül averages 2413 mm and there is a distinct dry season from July to November and a less pronounced drier period sometime in February and March. Temperature is relatively constant throughout the year with the daily fluctuation greater than the annual. In 1982, for example, the average monthly temperature was 27.1° C and the average monthly minimum and maximum temperatures were 20.9° C and 31.5° C, respectively (Mori & Prance, 1987).

Depending on the time of year, French Guiana is under the influence of NE and SE trade winds. During the dry season 30-50% of the winds are from the E, whereas in the wet season the winds are from the NE, ENE, and E. Average annual wind speed is moderate and decreases from the coast to the interior. For example, it is 3.7 m/sec near the coast at the Rochambeau airport and 1.6 m/sec in the interior at Maripasoula. There are slight peaks of wind velocity in October, at the peak of the dry season, and in March, in the less pronounced dry season. The highest winds recorded are 21 m/sec at Rochambeau, 16 m/sec at St. Laurent, and 17 m/sec at Maripasoula. Violent winds are rare. Between 1956 and 1960, there were only 12 days with winds over 16 m/sec. The calmest months of the year are May, June, and July, particularly in the interior (CEGET-ORSTOM, 1979).

Nearly 12,000 collections of vascular plants have been gathered as part of the Flora of central French Guiana project. Based on these collections, a total of 133 families, 699 genera, and 1,840 species (405 monocots and 1435 dicots) of flowering plants have been identified. However, ongoing study of extant collections continually adds to the number of species, as does every new expedition to the area. As a result, the exact number of species in the flora, as well as the exact number of wind-dispersed species, will not be known for many years to come. Nevertheless, we feel that the overall percentage of wind-dispersed plants in relation to plants with other types of dispersal will not change significantly with continued botanical exploration.

A checklist of the flora based on the "AUBLET" database (Cremers et al., 1990; Hoff et al. 1989) was examined to identify possible wind-dispersed species among the flowering plants. Gymnosperms (only one in the flora) and pteridophytes (ca. 191 species in the flora) were not included in this analysis. Collections of the wind-dispersed species housed at The New York Botanical Garden (NY) and the Centre de ORSTOM at Cayenne (CAY) were then examined to determine 1) habit, 2) the dispersal unit (fruit versus seed), 3) the type of adaptation for wind dispersal (wings, hairs, microspermous seeds, etc.) and 3) the time of year the species was collected in fruit. A list of voucher specimens is available from the senior author.

One of the difficulties in classifying propagules is that the seeds of some species are dispersed in more than one way (Plitmann, 1986). For example, Pseudobombax munguba (Mart. & Zucc.) Dugand (Bombacaceae), a common plant of periodically flooded forest in Amazonia, has comose seeds which are carried away by the wind when the capsule opens (Pijl, 1982). Many of its seeds, if not most, land in the water where some float away with the currents and others are eaten by fish (Ridley, 1930). This species, however, would be classified by us as wind-, and not water-, dispersed because wind is the mechanism by which the seed departs from the mother plant.

The systematic concepts of Cronquist (1981, 1988) have been followed rigorously in our analysis.

Distribution of Wind Dispersal among the Flowering Plants

There are 298 species of flowering plants in the flora of central French Guiana with wind-dispersed diaspores (diaspore = unit of dispersal) (Table I). This represents about 16.2% of the flowering plant flora. This figure is greatly inflated, however, by the 135 Orchidaceae with microspermous, wind-dispersed seeds. The other important family of Liliopsida with adaptations for wind dispersal, the Bromeliaceae, has 21 species with anemochorous diaspores. Among the Magnoliopsida, the Bignoniaceae (37 species), legumes sensu lato (Mimosaceae, Caesalpiniaceae, and Fabaceae) (21), Apocynaceae (17), Malpighiaceae (15), Vochysiaceae (9), and Asteraceae (7) are the most diverse wind-dispersed groups (Table I).

Among the monocotyledons (Liliopsida sensu Cronquist) in central French Guiana, only the orchid (Orchidaceae), pineapple (Bromeliaceae), and yam (Dioscoreaceae) families are known to have species with wind-dispersed propagules. This represents three families (12%) and 158 species (39%) among the 25 native families and 405 species of monocotyledons known for the flora.

Among the dicotyledons (Magnoliopsida sensu Cronquist) in central French Guiana, 24 of the 108 families (22.2%) are known to have species with wind-dispersed propagules. The 140 species of dicotyledons with wind-dispersed propagules represent 9.8% of the species of dicotyledons presently known from the flora of central French Guiana.

In central French Guiana, wind-dispersed species are found exclusively in the more advanced subclasses as defined by Cronquist (1981, 1988). The more primitive Alismatidae, Arecidae, and Commelinidae in the Liliopsida do not possess wind-dispersed species. On the other hand, the Zingiberidae, with one family and 21 species, and the Liliidae, with two families and 137 species, do possess wind-dispersed species. In the Magnoliopsida, the Magnoliidae, Caryophyllidae, and Hamamelidae completely lack wind-dispersed species and the Dilleniidae have only eleven species in three families with wind dispersal. In contrast, the two most advanced subclasses of Magnoliopsida, the Rosidae and the Asteridae, have 62 species in 14 families and 67 species in 7 families with wind dispersal, respectively (Fig. 1).

Adaptations

Species of flowering plants in central French Guiana have adapted for dispersal by wind in the following ways: 1) by small, almost dust-like seeds; 2) by wings on the fruit (Fig. 2); 3) by wings on the seed (Fig. 3); 4) by hairs (a coma) on the fruit derived from the calyx (Fig. 4); 5) by hairs (a coma) on the seed (Fig. 4); or 6) by the expansion of the calyx (Fig. 3) into wing-like structures, or pterophylls (Bremer & Eriksson, 1992; Leppik, 1977).

Species with small, wind-dispersed seeds are found in central French Guiana mostly among the monocotyledons. Most species of Orchidaceae and Pitcairnia of the Bromeliaceae possess microspermous, wind-dispersed seeds. Wind dispersal of the small seeds of Pitcairnia (Bromeliaceae) may also be aided by their seed appendages. A few species of dicotyledons with small seeds (Adelobotrys adscendens and Begonia spp.) are also probably anemochorous.

Winged fruits are found in the legumes (Mimosaceae, Caesalpiniaceae, and Fabaceae), Combretaceae, Rhamnaceae, Malpighiaceae, Polygalaceae, and Sapindaceae. The wings may be unilateral (e.g., Machaerium spp., Securidaca spp., and Vataireopsis surinamensis; Fig. 2) or the entire fruit may be flattened into a single wing (e.g., Martiodendron parviflorum and Sclerolobium melinonii; Fig. 2). In some species, the fruit is a schizocarp which separates into winged units (mericarps) which are carried away by the wind. Certain species of Malpighiaceae (Fig. 2), Rhamnaceae, and Sapindaceae (Fig. 2) provide the best examples of schizocarps in the flora of central French Guiana. A special kind of winged fruit is that of the hemi-legume (Augspurger, 1989). In this type of fruit, the legume splits open and each half, with its attached seeds, serves as the dispersal unit (diaspore). Acacia tenuifolia (Fig. 2), Peltogyne paniculata (Fig. 2), and Recordoxylon speciosum (Loubry, 1993, cited as Melanoxylum speciosum Benoist) are examples of hemi-legumes in the flora of central French Guiana.

In the flora of central French Guiana, winged seeds (Fig. 3) are found in species of Aspidosperma and Himatanthus (Apocynaceae), Couratari (Lecythidaceae), Roupala (Proteaceae), Qualea, Ruizterania, Vochysia (Vochysiaceae), Cedrela odorata (Meliaceae), Huberodendron swietenioides (Bombacaceae), Hippocratea volubilis and Prionostemma aspera (Hippocrateaceae) and many species of Bignoniaceae. The fruits of these genera are dehiscent, opening to release the winged seeds. The seeds of Aspidosperma and Couratari are surrounded by the wing (Fig. 3), the seeds of Bignoniaceae have a wing on each side (Fig. 3), and the wings of the other genera are unilateral (Fig. 3). The wing of Couratari is probably derived from the aril and the wings of species of Vochysiaceae are made up of hairs, perhaps arising from the testa.

Fruits with a modified, hair-like calyx are found only in the Asteraceae (Fig. 4). Not all species of Asteraceae in our flora, however, are adapted for wind dispersal — species of Bidens stick to the fur of animals and the fruits of Wulffia baccata are berry-like and presumably dispersed by animals.

Seeds with terminal tufts of hairs (Fig. 4) are found in some species of Bromeliaceae (subfamily Tillandsioideae), Apocynaceae (species of Forsteronia, Mandevilla, and Odontadenia), Asclepiadaceae (Asclepias curassavica, Matelia sp.), and Rubiaceae (Hillia illustris). Bombacaceae (species of Bombacopsis, Ceiba, and Eriotheca) have seeds surrounded by hairs which are not attached to the seeds themselves.

In three genera, the calyx has developed into wings which aid in wind dispersal. The calyces of both species of Erisma (Vochysiaceae) are expanded into unilateral wings of unequal length (Fig. 3), that of Chaunochiton kappleri (Olacaceae) is developed into a circular, membranous, parachute-like structure (Fig. 3), and that of both species of Petrea possesses wing-like lobes of uniform size.

Of the 161 species of flowering plants, excluding the orchids, with wind dispersal in central French Guiana, the diaspore of 102 is the seed, whereas of the other 59 species it is the fruit, some part of the fruit, or the fruit plus the calyx.

Habit

When the orchids are taken into account, the most common habit of wind-dispersed species in central French Guiana is that of an epiphytic herb. When the orchids are excluded, however, only 18 other species of monocots, all Bromeliaceae, are epiphytic herbs. The only dicotyledonous epiphytic herbs in the flora of central French Guiana are species of Begonia. However, dispersal in this genus may be a combination of wind dispersal and adherence of the microspermous seeds to the bodies of animals (Lange & Bouman, 1991).

Among the dicotyledons, it is clear that the liana habit, with 79 species, and the tree habit, with 52 species, are the dominant habit types with wind dispersal. Five of the eight species of terrestrial herbs with wind dispersal are weedy species of Asteraceae.

Phenology

Our phenological data on wind-dispersed species is not adequate for determining seasonal patterns of wind-dispersed species in central French Guiana. It is noteworthy, however, that few collections of a wind-dispersed species have been gathered in June and July even though at least one expedition has been carried out in each of these months. Because botanical expeditions have not been evenly spaced throughout the year, this observation must be verified through additional collecting in June and July.

The wide range of variation in seed size, from 10^-6 grams in orchids to more than 10⁶ g in the double coconut, Lodoicea maldivica (Gmelin) Persoon (Ridley, 1930; Westoby et al., 1992), is an important factor in the dispersal of flowering plants. Species with dust-like seeds may be dispersed by the wind without the evolution of special aids, whereas species at the other end of the spectrum are excluded from wind dispersal because of the heavier weight of their seeds. Those species which have evolved aids for wind dispersal, such as hairs and wings (see Ridley, 1930), have diaspores which are neither exceptionally large or small. The hairs and wings have been shown to increase the efficiency of wind dispersal (Andersen, 1993) by decreasing fall velocity, either by increasing drag or by creating lift (Augspurger & Franson, 1993; Matlack, 1987).

In the flora of central French Guiana, 152 species of flowering plants (8.3%) are probably aided in seed dispersal by the presence of either hairs or wings on the diaspores. However, as Augspurger (1986) has pointed out, generalizations about tropical plants employing wind dispersal are subject to modification by many other factors, such as number of seeds per fruit (Augspurger & Hogan, 1983; Loubry, 1993).

The percentage of species of angiosperms with wind- dispersed diaspores varies with the type of plant community. For example, López and Ramírez (1989) found that 42.1% of the species from a savanna in Venezuela possess wind-dispersed diaspores. Prance (1978), based on the work of Macedo (1977), calculated that 13.5% of the species of campina in central-Amazonian Brazil are adapted for wind dispersal. Hladik and Miquel (1990) found that 14% of the species of trees and lianas in a non-random sample from a Gabon rain forest possessed winged seeds or fruits. Pijl (1982) summarized reports on wind dispersal from the literature for an alpine flora (60% wind-dispersed species), a Mediterranean flora (50%), and the flora of Mt. Kinabalu (25-28%). Willson et al. (1990) have demonstrated, in their study of wind dispersal in temperate plant communities, that as much as 70% (Alaska), but usually 10-30%, of the species are wind dispersed. In the dry vegetations of central Australia and Israel, Jurado et al. (1991) and Ellner and Schmida (1981) conclude that wind dispersal dominates. A review of all aspects of seed dispersal, which includes additional references to dispersal spectra in communities, has been prepared by Renner (1992).

The relatively low percentage of wind-dispersed species, 16.2% of all of the flowering plants and 9.8% of the dicotyledons, in central French Guiana may be due to the region's closed forest and relatively wet climate. Nevertheless, at least 298 species of flowering plants are adapted for dispersal by the wind in this flora.

In contrast to wind dispersal, wind pollination plays a negligible role and is often even absent in tropical, evergreen forests (Regal, 1982). In the forests of central French Guiana, not a single species of angiosperm is known to be wind- pollinated. The presence of wind-dispersed species suggests that sufficient winds are present in this forest to carry pollen as well as diaspores. The absence of wind pollination is probably attributable to the uncertainty of having pollen carried by wind from one individual of a species to another individual of the same species in tropical lowland forests (Regal, 1982). Pollination requires much more specificity than seed dispersal (Howe & Westley, 1988 and references cited therein).

Wind dispersal predominates in the Orchidaceae of central French Guiana. Only a few species of orchids (three species of Vanilla, two of Palmorchis, and Selenipedium palmifolium) have fleshy fruits that are most likely dispersed by animals and not by wind. Small seeds, which are easily transported within the canopy and which are readily trapped in fissures, cracks, and holes in bark, may partially explain the great success of the orchids in exploiting the epiphytic habit in tropical forests (Benzing, 1981).

Wind dispersal in central French Guiana is restricted to the most advanced subclasses of flowering plants. This is true in both the Liliopsida and the Magnoliopsida (Fig. 1) but this may not be an overall trend in the flowering plants (S. Renner, pers. comm., 1993).

In the Liliopsida, wind dispersal in our flora is found only in three families, the Bromeliaceae (subfamily Tillandsioideae), the Dioscoreaceae, and the Orchidaceae, whereas 24 different families of Magnoliopsida in this flora possess wind-dispersed species. The reason for this difference is two-fold. In the first place, there are five times as many families of magnoliopsids as there are families of liliopsids. In the second place, two common habits of wind-dispersed plants, trees and lianas, are poorly represented among the region's liliopsids.

Very few families having wind-dispersed species in central French Guiana have all of their species dispersed by the wind. All of the Vochysiaceae and nearly all the orchids are wind dispersed. Other families may have all of the species of a part of the family dispersed by the wind, such as the Bromeliaceae subfamily Tillandsioideae, but most families have a mixture of wind-dispersed and non wind-dispersed species. For example, most of the lianas of the Malpighiaceae are wind dispersed, whereas the trees, represented by the genus Byrsonima, have drupaceous fruits probably dispersed by animals. Some families, such as the Lecythidaceae and the Hippocrateaceae, have only one or two species (usually belonging to separate genera) that are dispersed by wind while the great majority of their species are dispersed by a variety of vectors. Even within the same genus, wind-dispersed and non-wind-dispersed species may occur. For example, in our flora, Securidaca (Polygalaceae) has three species with str ongly winged fruits adapted for wind dispersal, while the third, S. spinifix, has a very reduced wing that does not appear to be effective for wind dispersal. The sporadic occurrence of wind-dispersed species with non-wind-dispersed species in the same families or even genera suggests that adaptations for wind dispersal evolved within different phylogenetic lines without excluding adaptations for dispersal by other means. Bremer and Eriksson (1992) point out that the evolution of wind-dispersed diaspores in the Rubiaceae has evolved independently in several different lineages.

The ways in which species of flowering plants in central French Guiana have adapted to dispersal by the wind are not diverse and similar adaptations have evolved in very distantly related groups (see also Ridley, 1930). For example, tufts of hairs on the seeds are found in the Bromeliaceae as well as in the Apocynaceae (Table I) although the tufts of hairs in these taxa are not homologous. On the other hand, divergent methods for wind dispersal may be found within the same family. For example, the unilateral wing of the seed of Huberodendron swietenioides is different from the seeds of the other wind-dispersed species of Bombacaceae, which are surrounded by a light fluff that, along with the seeds, is carried away by wind.

In central French Guiana, wind dispersal is mostly limited to epiphytes, lianas, and trees. This supports other studies that have shown that wind dispersal is most effective for those plants which grow in the upper strata of the forest (Sinha & Davidar, 1992). Studies of African forests by Jones (1956) and Keay (1957) have demonstrated that 46-56% of the emergent trees and lianas are wind dispersed, whereas lower story plants have considerably fewer wind-dispersed species.

It is generally held that wind-dispersed species fruit predominantly during the dry season. Croat (1969) has demonstrated this for a lowland, moist forest in central Panama. Although our data do not reveal any distinct patterns in fruiting phenology, very few collections of plants with wind-dispersed diaspores have been gathered in June and July, which, along with May, are the months of the year with the calmest winds. In addition, April-June is the wettest time of the year in central French Guiana, and this may inhibit the flowering that would lead to a June and July crop of fruits. Numerous fruiting collections of wind-dispersed species have been gathered in the prolonged dry season (August to December) as well as in the shorter dry season in February and March. It is noteworthy that March and October are the months of the year with the strongest winds.

Gentry (1993) suggests that in the Neotropics the percentage of species of wind-dispersed lianas decreases from drier to wetter habitats and that, for plants in general, Africa has less wind dispersal than the Neotropics. Therefore, comparison of our data with that of other tropical communities must take climatic and geographic figures into consideration.

This report on wind dispersal in a lowland, moist forest in central French Guiana indicates that species with wind-dispersed diaspores, although not as abundant as reported from open tropical and subtropical vegetation types, do make up a significant proportion of the flowering plant species in this habitat. Further studies are now needed to establish the presence or absence of phenological patterns as well as to elucidate the ontogeny of the wings and hairs that enable a species to be dispersed by the wind.

We thank Bobbi Angell for preparing the illustrations, Michel Hoff for developing the database upon which this study is based, Eric Christenson for his comments on the systematics and dispersal of Orchidaceae, John D. Mitchell for his numerous useful suggestions and leads into the literature, and William R. Anderson, Rupert Barneby, Carol Gracie, James Grimes, Sabine Huhndorf, Jacquelyn Kallunki, John D. Mitchell, Susanne Renner, and Michael Rothman for reviewing the manuscript. We are grateful to the Beneficia Foundation, The Fund for Neotropical Plant Research of The New York Botanical Garden, and the National Science Foundation (BSR-9024530) for providing financial support for various aspects of this study.

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The following name changes have been made since the publication of this paper: Aspidosperma macrophyllum Müll. Arg. is a synonym of A. schultesii Woodson; Heteropterys siderosa is replaced by H. oligantha W. Anderson; Mandevilla is now represented by two species, M. rugellosa (Richard) L. Allorge and M. scaberula N. E. Brown, neither of which was listed in the table, M. lutea is not known from our flora; Matelea sp. (Mori et al. 23412) has been described as Matelea gracieae Morillo; Roupala (Mori et al. 20968) has been identified as R. montana Aublet; Schlerolobium melinonii in Figure 2 is now Tachigali sp. and all Schlerolobium spp. have been placed in synonomy under Tachigali; Tetrapterys crispa in Figure 2 is T. glabrifolia and the latter should be added to Table I; Securidaca diversifolia is S. volubilis Linnaeus; Vernonia cinerea is now considered Cyanthillium cinereum (Linnaeus) H. Robinson; and Wulffia baccata is now considered Tilesia baccata (Linnaeus) Pruski var. baccata.

Several wind-dispersed Rubiaceae were overlooked when the initial analysis was made. They are: Alseis longifolia Ducke, a tree with winged seeds; Capirona decorticans Spruce, a tree with winged seeds; Chimarrhis microcarpa Standley and C. turbinata de Candolle, trees with winged seeds; Ferdinandusa paraensis Ducke, a tree with winged seeds; Hillia parasitica Jacquin, an epiphytic shrub with a tuft of hair on one end of the seed; Manettia alba (Aublet) Wernham and M. reclinata Linnaeus, climbers with winged seeds; and Uncaria guianensis (Aublet) Gmelin, a liana with winged seeds.

In addition, Astronium ulei Mattick (Anacardiaceae), with a modified calyx aiding in wind dispersal, has been recently collected in central French Guiana.

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