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For years biologists have dreamt of visiting the top surface of the
tropical rainforest. This delicate and intricate mosaic of leaves and twigs
is the setting for many of the most important aspects of life in the forest.
Animal life abounds there, as do the flowers and fruits of many trees and
climbing plants.

Yet the treetops have proved inaccessible by conventional methods. Researchers
have constructed aerial walkways and towers and borrowed rope-climbing techniques
from mountaineers. Because these methods depend on stout branches for support,
the last few metres of fragile canopy have remained just out of reach. The
towers do provide a viewpoint, but one that is limited and confined to fixed
locations.

The ‘raft of the treetops’ approaches the problem from above by actually
landing on top of the trees. The raft’s story begins almost 10 years ago,
when Francis Halle of the Botanical Institute at the University of Montpellier,
France, came to the conclusion that the best method of reaching the surface
of the canopy would be to settle gently on it in the basket of a hot-air
balloon. Such an arrival would cause a minimum of disturbance and noise.

In 1983, balloonists were marking the bicentenary of the invention of
the hot-air balloon in France by the Montgolfier brothers. Halle took the
opportunity to attend a gathering of balloonists at Annonay – the home of
the inventors – where he was able to discuss his dreams with the experts.
He found a sympathetic listener in Dany Clayet-Marrel, a graphic designer
and an enthusiastic balloonist. Ideas began to blossom. The first hurdle
was to decide whether the canopy of the forest could support the kind of
structure that Halle and Clayet-Marrel had in mind.

A year later, a chance meeting brought them into contact with Gilles
Ebersolt, an inventor who specialises in bizarre creations made from inflated
plastic. At the time of the meeting in 1985, Ebersolt had deposited a structure
made from plastic tubing and nylon netting on the surface of a conifer wood
in France. The structure, which had been conveyed to and from the forest
by helicopter, had not collapsed under the weight of a person, and neither
had the trees. Could one of Clayet-Marrel’s large balloons carry Ebersolt’s
treetop ‘raft’?

In October 1986, an expedition set off for French Guiana sponsored by
UNESCO. Ebersolt’s company Aeroscope had constructed a new radeau des cimes
(raft of the treetops) as large as two tennis courts. A hot-air balloon
with a capacity of 3000 cubic metres was on hand, with Clayet-Marrel as
pilot. The operation went according to plan and the surface of the rainforest
proved capable of supporting the weight of the raft (some 750 kilograms).
Once the raft was in position, the balloon was deflated and stowed away.
A harness, rope and rachet system allowed researchers to scale the 30 metres
from the dark forest floor to the light and sky above. Once on the raft,
it was possible to move about freely. The rubber ‘sausages’ of the raft
were wide enough to walk along and the PVC netting acted as a somewhat bouncy
thoroughfare. During my time with the expedition I was able to build up
a photographic record of a wide range of forest plants.

The success of the 1986 mission prompted further plans for the future.
The key rrequirement was for the balloon to be replaced by an airship. On
each flight a bolloon has just one opportunity to deposit the raft as a
favourable and safe site. It has to be reinflated if the raft is to be moved
over the forest to another base. Wind direction determines the balloon’s
path and restricts the choice of landing sites. Use of an airship would
solve all of these problems. It would be highly manoeuvrable and could offer
a shuttle service for equipment and researchers once it had lowered the
raft into place.

Such ventures are not cheap. A number of commercial companies provided
funds for the first raft and its attendant balloon, partly because they
saw the potential for advertising. Plans to build an airship finally came
to fruition in the spring of 1989, with the help of sponsorship from a consortium
of Japanese companies collectively named ‘Sankinkai’. In return, the consortium
had exclusive rights to produce a film, which would go on show to the public
at a grand exposition in Osaka entitled ‘Man and the Natural Environment’.

Most airships are filled with helium and kept permanently inflated,
but this was clearly impracticable at a remote site in the rainforest. Instead
the airship would have to be filled with hot air. Thunder and Colt, a company
based in Oswestry, in Britain, designed and built an airship holding 7500
cubic metres of hot air – the largest ever attempted. Ebersolt’s company
in Paris contributed a new, larger raft.

Successful test flying took place in May 1989 and everything was set
for a five-month expedition to Brazil and Peru, beginning in July. There
could hardly be anything more innocuous than a gentle airship with a cargo
of botanists, but the expedition went awry. The Peruvian leg of the trip
was abandoned because of guerrilla activities at the location of the proposed
camp site, and the Brazilian component was thwarted by bureaucracy. At the
end of August, the entire expedition was asked to leave the country. All
equipment was shipped via Belem to Cayenne in French Guiana, where no more
troubles were encountered.

An airship is particularly vulnerable when the envelope is being inflated
or deflated. It can act as a huge spinnaker sail, catching the slightest
breeze. So activities at the new camp, 50 kilometres inland from Kourou,
had to begin before dawn in windless conditions. Inflation is a lengthy
procedure. The burners can only be lit after the envelope has been partially
filled with air supplied by a powerful petrol driven fan – a process that
takes at least three-quarters of an hour. Manoeuvring the airship into position
to lift the raft is rather like docking a large ship – the airship has considerable
momentum. A ground crew of six has to be on hand to pull on ropes.

The gondola, which has a steel frame, provides seating for five people
(two crew and three passengers). Behind the passengers sit the bottles of
propane gas for the triple burners. At the very rear is a 100-horsepower
Continental aircraft engine. The airship is capable of lifting 2100 kilograms
and so is well able to cope with the raft (750 kilograms) and gondola (530
kilograms) together. Despite its size and lifting powers, the airship is
relatively easy to transport. The envelope, which is some 50 metres in length,
folds up into a canvas bag that can be loaded manually into a truck.

The principal advantage of the airship, compared to a balloon, is its
ability to survey for landing sites. The forest surface is irregular and
contains numerous holes and gullies, so not all areas offer a safe landing.
Once a suitable site has been identified, the ship hovers into the breeze
in order to deposit the raft. A mechanical winch lowers the centre of the
raft, before a crew member on the raft detaches the peripheral connections.

A typical landing site on the forest surface in French Guiana would
cover 10 to 15 trees. Each landing site is different, but the raft’s edge
might be expected to give access to the crowns of 30 trees or climbers.
The total length of the edge of the raft is about 80 metres, so a transect
(observation strip) of this length around the treetops is available to biologists
on the raft. They can reach surrounding vegetation by stretching out from
the raft’s edge, which is shaped like a sausage 1 metre in diameter and
so provides a comfortable and reassuring rim. Naturally, everyone on the
raft, which carries up to six people at a time, wears a safety line.

When not involved in moving the raft, the airship is free to survey
the forest canopy and to transport various devices for collecting plants
and insects. A small raft, constructed in the same manner as the main one,
can be suspended 10 metres below the airship to act as a mobile observation
platform and collecting base. Given a slight headwind, the airship can stop
with the small raft positioned next to any selected treetop, permitting
the botanist on board to lean out in the manner of a yachtsman at sea. A
second device consists of a pyramid of sausages with netting on three sides,
the fourth side being left open, leading into an insect trap. The airship
transports the pyramid to a suitable sampling site and retrieves it after
a week or so.

Night flights in the airship are both spectacular and scientifically
rewarding, especially for entomologists. The airship patrols a few metres
above the canopy surface, trawling for small nocturnal insects with the
help of a collecting net rigged up beneath the gondola. The entomologist
on board wields another net from within the open-sided gondola. Larger nocturnal
insects can be attracted to the main raft by setting up a floodlight (driven
by a generator), but the entomologist must remain there throughout the night.

Sleeping on the main raft is a remarkable experience. On humid, rainless
nights there is the constant sound – from below – of water dripping from
leaf to leaf as it condenses from the damp air or exudes from leaves with
special glands. The netting suspended between the radial sausages offers
ample hammock facilities, although the size of the mesh (three centimetres)
does allow small objects such as pens and pencils to fall through. The overall
impression is one of a vast safety net with a secure peripheral wall. Progress
across the raft is possible via the netting, although the unhindered view
of the treetops below is a little disconcerting. The fastest way to cross
the raft is via the radial sasages, which are wide enough to walk on. If
you fall off you simply land on the bouncy netting.

The 1989 expedition completed five different treetop landings, proving
the reliability of the airship and raft. Only one landing site proved insecure.
Some hours after positioning, the raft folded up on itself and the central
semi-rigid platform slipped a few metres into the trees. Yet the incident
had its positive aspects. It gave an indication of the limits of support
to be expected from trees and also provided the airship with an opportunity
to retrieve the raft from an awkward situation. Damage was confined to a
slow puncture and several tears in the netting, which were easily repaired.

The expedition also demonstrated the raft’s potential as a scientific
tool. The airship’s manoeuvrability means that the raft is able to return
to precisely the same site n several visits, allowing researchers to make
a series of observations over time. When not servicing the raft, the airship
can proceed along parallel transects, permitting botanists to make a detailed
record of the structure of the forest. It is possible to film, or take photographs
directly downwards. Stereoscopic photographs would provide a wealth of accurate
information about the canopy surface.

In recent years botanists have learnt a good deal about the architecture
of forest trees. Francis Halle is one of the pioneers in the field. The
main branching framework develops in an orderly and predictable manner in
many tropical species. Studies of these developmental sequences have yielded
insights into the processes at work in less orderly trees. Nevertheless
the outer ‘skin’ of older trees – where there is often a renewal of small
twigs year after year – remains an unexplored ecosystem. The raft of the
treetops is now opening up this new territory.

The shape of a tree’s crown is governed by the activities of the buds
at the extremities of its shoots. The buds may be subject to internal influences
originating within the tree, external ones from the local environment, or
a combination of the two. The point where two adjacent crowns meet can shed
light on the processes involved. In temperate woodland, the boundary between
two crowns is often marked by a zone of broken twigs and damaged buds. This
damage, which is caused by the wind, results in a kind of forbidden zone.

Trees too shy to touch

However, many tropical trees present an entirely different picture.
The crowns of neighbouring trees are often well delineated and a gap of
perhaps half a metre is visible between them. Buds stop developing at the
edges of this zone, well before the trees are actually touching – a process
rather charmingly referred to as crown shyness. Some change in the environment
must influence their development – but only when they come within some critical
distance of an adjoining crown. One hypothesis is that the buds are responding
to a subtle difference in light quality as they draw closer to the foliage
of the neighbouring tree. Analysis of the developmental anatomy of buds
from different zones in the crown should provide a clue to the timing of
the mechanisms at work. With this in mind, I collected hundreds of buds
during the raft’s second expedition and preserved them for anatomical investigation
back at Bangor.

The stand-off phenomenon is not confined to situations where two trees
meet. Some trees display a more subtle version in which crownlets within
the crown of a single tree are separated by an obvious barrier of air. Therefore,
I also collected a crownlet from a specimen of Couepia and preserved 200
buds for analysis. Obtaining this sample was anything but straightforward.
The tree, which was growing on a ridge, was taller than its neighbours.
The plan was that the airship would approach the tree slowly from below
and then, as the gondola passed the crown, I would lean out and sever a
crownlet with secateurs or a machete. The plan failed, because air currents
flowing over the ridge prevented the airship from pausing while it was alongside
the tree. Eventually, we tackled the tree head-on, with the gondola coming
to rest momentarily in the tree’s crown. The pilot was then able to reach
forward and detach a suitable specimen.

Subsequently, we have sectioned 80 buds and examined them under a microscope;
half of the buds were from the summit of the crownlet and half from the
shyness zone. Surprisingly, we found that these buds are augmented by several
hundred hidden accessory buds. These hidden buds seem more numerous and
less likely to die at the crownlet surface compared to those at the shyness
edge.

In all, 49 scientists representing eight nationalities and a wide range
of disciplines have now spent time on the raft. All have been impressed
by its potential. However, future activities will depend on funding. The
Japanese sponsors have donated the airship and raft to the organisers of
the enterprise, but support is needed for transport and logistics. If all
goes well, the airship may one day be able to position and service an entire
fleet of rafts. There is no shortage of researchers eager to visit the treetops,
and a new expedition based in Congo is likely to go ahead this year.

Dr Adrian Bell lectures in the school of biological sciences at the
University of Wales at Bangor. His book Plant Form has just been published
by Oxford University Press.