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ecology

Fleeting encounters

The survival of trees as tall as 40 meters depend on tiny insects and pollinating birds

In silence, and after having spent 12 successive hours at the top of the 30-meter tall wooden towers, spread over the Tapajós National Forest, in Pará, biologists from Embrapa Eastern Amazon begin to unveil the reproductive mechanisms of some species of tropical trees that are commercially important as a source of noble wood. This observation work is a way of identifying who are the pollinating agents, normally insects – and how they work, of a group of seven tree species with popular trees with sonorously Brazilian popular names: jatobá, andiroba, parapará, maçaranduba, cumaru, anani and tatajuba. Until recently, there had been no precise studies about the process of pollination of the majority of these trees, but there is now something to celebrate.

The team from Embrapa discovered that a species of woodpecker of theCeleus genus, with orange plumage with black spots, is one of the pollinizers of the manni (Symphonia globulifera), a tree up to 40 meters in height that releases an intensely yellow latex. “It was a surprise”, explains biologist Márcia Motta Maués, the coordinator of the group of six researchers. “We saw the woodpecker look for the red flowers of the tree three times on the same day.” There had already been records of woodpeckers, above all of birds from the Passeriformes order, like the silver-beaked tanager (Ramphocelus carbo), which act as pollinators of the manni, but not yet of such a large bird as this 25 centimeter long woodpecker, seen from the heights of one of the six observation towers.

When animals visit flowers, they are usually in search of food the flowers of the manni, for example, even without any perfume, which would make it easier to locate, are rich in nectar, a mixture of sugars. While they feed, birds and insects fulfill a vital task for plants, pollination, by carrying grains of pollen from a flower, with the male sexual cells, to the stigma, the female structure, of another flower. And so living beings or even the wind and the rain spark off the reproductive process, uniting the male and female sexual cells. This is how fruits and seeds are formed, guaranteeing the survival of plants and their genetic diversity.

Getting to know the pollinizers of the trees most threatened by the action of man in Amazonia is fundamental for determining precisely the upper limit for the rational exploitation of the species of trees. “If we discover that a species of trees is pollinated by just one given insect of animal, the preservation of the pollinizer becomes essential for the survival of the plant”, explains Márcia, whose work is part of the Dendrogene project, for Genetic Conservation within Managed Forest in Amazonia, managed by Embrapa Eastern Amazon and with the participation of institutions from abroad, like the British Government’s Department for International Development (DFID).

The researchers are becoming increasingly concerned, as they get to know better the profound dependence between the trees and the pollinizers the disappearance of one group implies the disappearance of the other. In 1998, specialists from the entire world, gathered during the Biodiversity Convention, launched the São Paulo Declaration, with recommendations for protection and more intensive research into the decline of the pollinating insects, with an emphasis on the bees.

Abundance and scarcity
The massaranduba (Manilkara huberi), a tree that reaches a height of 40 meters and is endowed with little white flowers, has shown a wealth of pollinators unmatched amongst the species studied. Butterflies, flies, wasps, beetles, birds, and at least 15 different species of bees, amongst them the Apis mellifera and stingless bees, visit the tree’s flowers. “With the exception of the bats, there were representatives of all the main pollinating agents on the massaranduba”, the biologist says. This strong attraction probably derives from the easy access to the nutritive rewards from the flowers of this tree, in which the nectar and the pollen are to be found unprotected by any of the flower’s structures. But if the abundance of pollinators of the Manilkara huberi is good news, the possible scarcity of flowers to be fertilized is a worry.

The team from Embrapa found out that the massaranduba may go four years without blossoming, probably because of the changes in the climate brought about by El Niño, the abnormal warming of the surface waters of the South Pacific which alters the regime of rainfall and the temperatures in a part of the planet. Another species that shows a remarkable variety of pollinators is the cumaru (tonka bean tree), which is between 30 and 35 meters in height and from which the coumarin used in the cosmetics industry is extracted. At least 25 species of moths, wasps, beetles, hummingbirds and bees from several families, including the Epicharis, visit its flowers and can carry its pollen. Besides the nectar, the white flowers with a violet-colored detail offer another attraction for insects and animals: they are very aromatic, as Dipteryx odorata, the scientific name of this species of tree, leads one to believe.

To study the pollination of the jatoba (Hymenaea courbaril), the biologists had to pass the small hours in the tower located alongside a specimen of this species, which can reach as much as 50 meters in height. Finally, they discovered that the flowers of the jatoba in Amazonia only open after 10 p.m., which means that the ideal conditions for pollination are concentrated on the night. Having night habits, bats seem to constitute an important pollinating group for the jatoba. The problem is that these winged mammals destroy the flowers, which makes them rather uninteresting for daytime pollinators like bees. A similar situation occurs with the purple flower of the Jacaranda copaia , sought after by several groups of bees. Except that, in this case, the role of the pollinating villain is played by a species of corpulent bumblebee of up to 5 centimeters in length, the Xylocopa frontalis, besides a few butterflies, which perforate the flowers in search of nectar.

Hermaphrodite flowers
The reproduction of trees seems to be simple, but sometimes complications arise. The pollen is in the male structure of the flowers, and the stigma is to be found in the female part. But, with the exception of the andiroba (Carapa guianensis) and of the tatajuba (Bagassa guianensis), the other five species of trees studied have hermaphrodite flowers, with reproductive structures of both sexes in other words, they are male and female at the same time. But hermaphroditism does not necessarily mean that what is known as autofecundation can occur, with the male sexual cells finding the female ones of the same flower. “The pollen from a hermaphrodite flower of a massaranduba is not able to fertilize the stigma of this same flower”, says Márcia. “Not does it meet with success in fertilizing other hermaphrodite flowers of the same massaranduba.” The pollen is only capable of fecundating flowers of another massaranduba and if it is carried by a pollinator.

Apparently pollinated by stingless bees, beetles and moths, the situation of the andiroba is different. Its flowers are called monoecious: in the same plant, there are two sets of flowers, the males ones and the female ones. At the beginning of the flowering period, which lasts about five months, the andiroba shows only male flowers, and then flowers of both sexes but never hermaphrodite flowers. Except that the flowers of one and the same andiroba, even though they are of different sexes, are unable to carry out fecundation between themselves. Preliminary tests have indicated that for fecundation to occur in this species, the pollen from one andiroba has to be taken to a female flower of another andiroba.

“Like what happens with tropical trees that have hermaphrodite flowers, the species of trees with monoecious flowers seem to have mechanisms that protect their genetic diversity by preventing autofecundation”, Márcia explains. Trees that fecundate themselves with their own pollen tend to lose genetic variability and jeopardize the survival of the species. The researchers noted a peculiarity of the male flowers of the andiroba, slightly larger than the female ones, which may make the work of the pollinators more difficult. In the period of flowering, they remain open for less than 24 hours and then drop off the tree. The insects that spread the pollen of this species have to act quickly. Otherwise, there is no fertilization.

Perhaps the tatajuba, a relatively rare species in Amazonia, from the same family as the fig tree, shows the most complicated reproductive context, amongst the varieties of trees studied. It is a dioecious species. Some tatajubas are male trees, with only male flowers. Other tatajubas are female trees, with just female flowers. “We saw that only one kind of insect, the thrips, visits the flowers, which have a strong and sweet aroma”, says Márcia. Thrips are insects that measure a few millimeters, from the order of Thysanoptera, whose efficiency in pollination depends on a natural factor: the wind. If it is blowing strongly, many thrips disembark onto the tatajuba. If there is little wind, this number comes down a lot.

To complicate things, there is the question of the sex of the trees. Marivana Borges Silva, a researcher from the Federal University of Pará (UFPA) associated with the project, is trying to find out whether the proportion of male and female tatajubas is the same. What has already been found out is that the male and female sets of flowers (inflorescences) are different: the male inflorescences are in the shape of a spike, with extremely simple flowers, with neither petals nor sepals, the protective structures at the base of the flowers, while the female ones are also bare and are reminiscent of a miniature golf ball. But the lumbermen, when they fell trees of this species, rarely pay any attention to these details. The result is that one still does not know for certain, but the tatajuba may be running the risk of disappearing, above all in the areas of intensive extraction of timber, due to the lack of balance between the male and female plants.

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