Canopy exploration in tropical rainforests
Extract of a speech given at the Centenary lecture series of the University of Pretoria on 18
November 2008 – by: Professor Francis Hallé
I am an admirer of the tropical rain forests (TRF) and have been studying them for 50 years. Twenty
five years ago, in South America, I began exploring the canopy – the upper surface of the highest
crowns. The canopy is so different from the undergrowth that it is like having two different forests,
one on top of the other.
I like the undergrowth, but I must admit that it is not the most pleasant part of the forest. What do we
find down there? Darkness, mud, humidity, dead leaves, rotten wood, musty smell and almost no
flowers. As for the animals, there are few of them and they are usually dark and slow – big snails,
some leeches, cockroaches, big spiders, big millipedes, some fire-flies by night and a few birds which
prefer to run away rather than to fly. If there is one word to describe undergrowth it must be „austerity‟.
Now if you look at the canopy, 40 or 50 metres above your head, you immediately realize that it is
another world. Beautiful things are occurring or happening up there – bright sun, trade winds, flowers
and fruits, orchids and begonias, ferns and species of Rhododendron, butterflies, magnificent beetles,
colourful birds, monkeys, sloths and flying squirrels.
These animals are beautiful and rapid, they have to move quickly and to hide from the many
predators looking for them.
An interesting point is that the mammals and birds of the canopy do not know man. For them we are
not predators, just newcomers, and they are not shy of us : they try to understand what we are doing
in this part of the forest.
The canopy is gorgeous and brilliant. To me it is the real face of the forest. Now when I am in the
undergrowth I feel as if I am under the carpet !
What are the biological attributes of the TRF canopy ?
The most important one is its extraordinary richness in Plants and Animals.
Let me remind you of a few figures concerning the global biodiversity. How many species do we have
on our planet?
The critical date is 1982. Before 1982, before knowing the canopy‟s biodiversity, the answer was 3
million. In 1982, Terry Erwin (Smithsonian Institution, USA) conducted the first attempt to quantify the
insect species in a tropical canopy. Then he published his one-page paper on canopy entomology –
and the world estimate jumped from 3 to 30 million.
Let‟s have a look at the canopy‟s biodiversity.
First the plants: Besides the biodiversity of trees and lianas, there is also a huge amount of epiphytic
plants. According to Ozanne (2003) “10% of all vascular plants are epiphytic canopy dwellers”. That‟s
why on every branch you find hanging gardens, absolutely superb, with all the colours of the rainbow.
Then the animals: Here the figures are much higher. On the basis of long term canopy works, Basset
(2003) gave an estimate of 6 million insect species in the tropical canopies of the world, meaning that
20 to 25% of all arthropods could dwell in the canopy only.
Up there we are actually surrounded by insects, but many other groups of animals inhabit the canopy
: monkeys, hyrax, bats, reptiles, tree frogs and even crabs. The crabs (in the TRF of Madagascar)
spend their lives in some epiphytes full of water or holes in old trees and they feed on tadpoles !
It is not enough to state that many living species dwell in the canopy. Actually many of them are born
up there. Why are there so many species in the TRF‟s canopy? Where do they come from?
The most efficient speciation mechanism is the Plant/Animal co-evolution, as described by Gilbert
(1975). How does co-evolution work ? I‟ll give some details, as it is the most fascinating feature in
It is never cold up there, never dry, the winds are never strong (Equatorial doldrums) and the light is
never dim. In other words, plants and animals do not face physical or a-biotic constraints. The
constraints they face are biotic instead.
Let me give an example; the co-evolution between Passiflora (the plant-group to which the granadilla
belongs) and the butterfly Heliconius in the Brazilian Amazon. At the beginning of that story, there is
only one plant, Passiflora, and one animal, Heliconius. The caterpillars of Heliconius feed on the
A first genetic switch occurs in Passiflora: due to the synthesis of an alkaloid, it becomes toxic.
The second genetic switch occurs in Heliconius: its caterpillars become able to feed on the toxic
Passiflora which, by the way, is no longer toxic for them.
These Heliconius caterpillars are very strange indeed; they can also feed on each other and this
cannibalism is of importance as we will see later.
At this stage the caterpillars eat large quantities of alkaloid and they become toxic. Then they will
change into toxic butterflies.
To let the predators (birds) know it is toxic, Heliconius adopts brilliant colours. It is actually one of the
most beautiful butterflies in the Amazon. Here we also find another, very similar-looking butterfly, but
it is a mimic, and it belongs to another family than Heliconius. This other butterfly is not toxic, but the
more it looks like Heliconius, the more it will be protected from the predators. It is a classical example
of Batesian mimicry.
The third genetic switch gives the Passiflora the possibility of diversification in the form of its leaves.
Passiflora is the plant genus that by far has the highest diversity in leaf forms. So much so, that at
least for a short period, the Heliconius gets lost when looking for the right place to deposit its eggs.
The fourth switch occurs in Heliconius which becomes able to memorize forms. It has experimentally
been demonstrated that Heliconius is the only butterfly which can distinguish a circle from a square,
or a triangle from a trapezium ! Therefore it learns how to find the right Passiflora leaf amongst the
The Passiflora‟s response is to produce false eggs! As I said, Heliconius are cannibals as the big
caterpillars eat the smaller ones. So, false eggs deliver to the Heliconius a very clear message: “Go
away! The place is not free, another butterfly has already dropped its eggs on me and if you do the
same, your caterpillars will be eaten.” And the butterfly flies away.
Now we are waiting for the next genetic switch on the butterfly side.
Every genetic change doesn‟t give birth to a new species, but on both sides, by accumulating small
changes, new species are obviously going to spring up. In the Amazonian canopy, one can collect at
least 50 species of Passiflora and 50 species of Heliconius.
I find it fascinating that, at the canopy level, the driving forces of the Darwinian evolution are biological
forces, not abiotic (physical) ones. In other words, life is driving itself. This is co-evolution and this is
the key issue explaining why TRF canopies display the highest biological diversity on earth.
This diversity can be traced in the genome of one single tree. In 1996, my colleague Darlyne
Murawski, geneticist at Harvard, went on a canopy exploration in French Guyana (South America). By
using this hot-air balloon and this sledge, she collected many leafy shoots around the crown of a big
Back in the USA, she analysed the DNA and got an unexpected result. Several genomic variants do
coexist within one and the same tree. At least some tree‟s genomes are actually polymorphic.
What do such variants look like? Most of them are not directly visible, as they are purely
physiological. But in some cases the variants are visible. This Photinia tree, a member of the rose
family, has one square metre of the canopy flowering 15 days before the rest of the crown. As a mater
of fact, some trees may display a polymorphic genome.
Another biological attribute of the tropical canopy is its richness in active molecules. As you know,
some secondary metabolites (such as terpenes, saponins, alkaloids, etc) may have molecular activity
and may be used for medicinal purposes.
Unfortunately most of the research conducted by phytochemists on TRF molecules are conducted in
the forest undergrowth. So what happens? They apply an activity test to the young tree whose leaves
are most accessible, but most of the young trees are devoid of any molecular activity.
But let the young tree grow and at more or less 30 metres it will start producing some active
molecules. And when its crown reaches the canopy, it is usually full of active molecules.
Consequently we tend to underestimate the forest potential in terms of biochemistry and in the future,
when looking for new medicinal plants, it would make sense to give up on the undergrowth and to
look at the canopy level instead.
Why are there more active molecules in the canopy than in the undergrowth? I see two reasons.
Firstly, when the tree reaches the canopy, it gets an enormous increase of energy, as its crown
becomes broader and the sun radiation rises to 100%. Paradoxically, instead of growing faster, it
reduces its annual growth to a few centimetres, or even millimetres.
Therefore, a large amount of energy is no longer used for growth and becomes available for the
synthesis of complex molecules which can be seen as a form of energy storage.
The predation by herbivores is a second reason. As most of the insects dwell in the canopy, many of
them being herbivores, the canopy is where the trees must synthesize active molecules. They are
dissuasive molecules that have a deterrent effect.
Before coming to the end, I want to show you the canopy and the equipment we use. Hundreds of
scientists of more than 25 nationalities have participated in biological explorations with the Canopy
Raft Group. But so far we have never seen any South African scientists up there.
If anyone is interested ask Dr Emmanuel Torquebiau (email@example.com ) from UP for
my e-mail address.
Prof. Hallé is a tropical botanist of international renown for his research on tree architecture and the
biodiversity and ecology of tropical rainforests. He has studied tropical plant life in 45 countries for
more than 40 years and is famous for being the man who pioneered the exploration of the rainforest
canopy using specially-designed rafts that are suspended from hot-air balloons.
From 1986 to 2003, Prof. Hallé was the team leader of several canopy raft expeditions to many parts
of the world. A recipient of the prestigious Fairchild Award for tropical botany, Prof. Hallé has
published many scientific papers and several books.
For many scientists and laypersons, however, Prof. Hallé is perhaps best known for his fascinating
and thought-provoking, book, In Praise of Plants (Timber Press, 2002). This remarkable book,
translated from the original French, presents a highly original appreciation of plants from Prof. Hallé‟s
unique viewpoint – both scientific and emotional. It is a must-read book which is without equal
amongst literature on plants.
All photos shown are accredited as follows:
‘Canopy Raft Collection, Paris, France’ (Photos used with special permission & thanks), except the
photograph showing Prof Hallé accredited as „University of Pretoria‟.
The content for this article was supplied by the University of Pretoria – for more interesting information
visit their website – http://web.up.ac.za