¿ìè¶ÌÊÓƵ

In a tropical forest near Panama City, a bright red construction crane
rises conspicuously out of the olive treetops. Once it hoisted steel girders
and cement. Now its daily load is somewhat lighter: a gondola full of biologists.
Up in the canopy, they can see the skyline of Panama City with its own swarm
of cranes. So they call theirs the ‘feral crane’.

Formally known as the Forest Canopy Access System, the crane is the
first such mechanism to serve as laboratory equipment for tropical biologists.
¿ìè¶ÌÊÓƵs at the Smithsonian Tropical Research Institute (STRI) in Panama
had the idea of using a crane to get into the treetops, in 1990. With $40
000 from the Smithsonian Institution, Washington DC, they rented a crane
and hauled it down a dirt road to a clearing in Metropolitan Park, a secondary
dry forest about 70 years old near Panama City. Biologists have been riding
up and down by crane ever since.

‘It sure beats climbing a rope,’ says Kevin Hogan of the Smithsonian,
one of the several score ecologists who work at STRI. The crane gives scientists
such as Hogan, who studies photosynthesis in plants, a view of the most
interesting but most inaccessible part of the forest – the canopy. ‘You
can throw ropes up and haul yourself up, but you don’t know how secure they
are,’ says Hogan, a former, and unenthusiastic, rope climber. ‘And when
you get up there you aren’t really where you want to be, at the ends of
the branches where the leaves are.’

French ecologists tried to solve this problem by constructing a huge
plastic doughnut with mesh across the central opening (‘On the roof of the
rainforest’, ¿ìè¶ÌÊÓƵ, 2 February 1991). In French Guiana, they lifted
the doughnut into the trees by helicopter and laid it on to the canopy.
¿ìè¶ÌÊÓƵs walk around on it, reaching through the mesh or over the side
to measure, mark or take samples from the treetops.

The Smithsonian team thinks its crane works better. With a boom 35 metres
long, the crane can hang biologists anywhere in the forest within a cylindrical
space 30 metres high by 70 metres across. The steel and wire mesh gondola,
about 1.5 metres square and 2.5 metres high, can hold three people (or two,
with plenty of equipment). And though not as mobile as the French doughnut,
the crane can be moved by rail or road. Its operator, Jos Herrera, who learned
his trade when building skyscrapers, can place the gondola precisely, to
reach practically anywhere. Doors open down to waist height on all four
sides, giving clear access to the part of the canopy biologists want to
examine. ‘You can reach out and touch just about anything,’ says George
Angehr, an ornithologist at STRI.

Unfortunately, some things can reach in and touch you. When the doors
are open a preoccupied passenger can be rudely surprised. David Roubik,
STRI’s expert on Africanised (better known as ‘killer’) bees, was recently
set upon by a swarm of angry wasps while 20 metres up. He thought better
of jumping, but it was some minutes before Herrera could get the gondola
down. Roubik was stung about 70 times. A rope ladder has since been recommended,
although no one is sure how helpful that would be in the event of airborne
attacks. ‘It would be a good idea to have a fire extinguisher in here to
anaesthetise them,’ said Angehr during a recent ascent. Since then, STRI
scientists assure, head nets and a gas canister known as a ‘bee bopper’
have become standard equipment.

Once among the treetops, however, one forgets about that sort of thing.
During his first crane trip, Richard Condit, a botanist from Princeton University,
rattled off species he had never seen from this perspective before. ‘Those
white flowers are from a liana. I think that’s Leuhea seemanii, with the
gold undersides there; it’s a fast growing tree that loses its leaves in
the dry season. This tree here is a wild cashew, and there is a whole forest
of ficus (fig) trees. And there are heliconia, not to be confused with the
butterfly heliconid.’

After the novelty of their first trip aloft wears off, STRI’s scientists
return with their instruments. They are beginning to take the pulse of the
forest from itsextremities. In the tropics, that pulse races like a runner’s.
A rainforest, for example, produces about 2000 grams of dry plant material
per square metre annually, compared with an average of 1250 grams for temperate
deciduous forests. Over a year, 1 square metre of rainforest captures about
6700 kilojoules of energy from sunlight, using 2000 kilojoules for new growth
and reproduction. A rainforest’s leaf area index – the total area of leaves
above each square metre of forest floor – can reach 8; in other words, 8
square metres of leaves cover 1 square metre of forest floor. The index
for temperate forests in North America is usually about 6.

Kevin Hogan, his colleague Alan Smith, and assistants Solby Chavarria
and Mirna Samaniego are now measuring carbon dioxide and water vapour exchange
at the surfaces of leaves of six species of tree in different light regimes.
They clamp a small chamber, called a cuvette, over the leaf. A gas analyser,
consisting of an infrared detector, measures the uptake of carbon dioxide
by the leaf, which in turn reflects the rate of photosynthesis.

Botanists know that some trees need far more light than others. Many
of these are called ‘gap species’, because they grow where gaps in the canopy
have been created by fallen or cut trees. The Metropolitan Park forest is
especially interesting because it contains many gap species as well as those
which tolerate shadier environments. The site also harbours bothdeciduous
and evergreen trees. Hogan and Smith suspect that deciduous trees, such
as Pseudobombax septenatum (locally known as barrigon), may have a higher
photosynthetic rate than evergreens because its leaves, which drop off in
the dry season, have a shorter lifespan. Another question is how very large
leaves, like the palmate, parasol-sized ones of the gap species Cecropia
longipes, respond to the stress of the dry season.

Trees vary in how efficiently they use water. Hogan and Smith have found
that one rule of temperate climates – that evergreens use water more efficiently
– appears not to apply in the tropical dry forest. Nor does seasonality
appear to matter as much as they expected.

Twenty-five metres up in the canopy, the scientists cut the end of a
twig along with some terminal leaves and put the cut section in a ‘pressure
bomb’. This instrument pressurises the sample until water is squeezed out
of the xylem. That level of pressure marks the minimum amount of force the
plant needs to pull water up from the soil. A low reading indicates plenty
of water available to the tree. While there is a marked difference between
dawn and midday, Hogan said that they were surprised at how little effect
the dry season has.

The inevitability of some global warming and deforestation of the tropics
has heightened STRI’s interest in projects related to the carbon cycle.
One effect of rising levels of carbondioxide could be an increase in plant
growth. But deforestation removes these carbon ‘sinks’, and burning or decomposition
returns the carbon to the atmosphere. The influence of forests on the global
carbon cycle is huge; forests are estimated to store almost half the world’s
living terrestrial carbon and 11 per cent of the world’s soil carbon.

STRI is trying to learn how to model this complex carbon cycle for an
entire forest from basic information on light, humidity, water balance,
respiration, photosynthesis and other plant functions. For example, if Smith
heats a tree stem at the ground, he can measure how the heat spreads up
the trunk, an indication of the rate of water loss by the entire tree. Readings
of water loss in the leaves of the canopy also reflect carbon uptake. High
rates of water loss mean the stomata – tiny openings in the plant’s epidermis
that allow passage of gas and water vapour – are more open. The more open
they are, the greater their capacity to take up carbon dioxide. Sophisticated
techniques such as these go hand in hand with more laborious efforts to
tag and measure every stem and leaf in parts of canopies. The figures that
emerge are multiplied to create models for whole trees and forests.

One potential drawback is that experiments of this kind may entail a
kind of Heisenberg effect, when the mere act of observing something alters
its behaviour. ‘Some of these species respond to being touched,’ says Hogan.
‘They shut down.’ However, Smith adds that the crane is more gentle than
the French balloon, which disturbs and breaks branches and can change readings
by creating shade. Moreover, the gondola can be returned to exactly the
same spot months after the last observation. This makes the crane particularly
useful for monitoring the effects on canopies of sporadic climatic disturbances
such as El Nino events.

The Smithsonian regards the crane as a success and, with $165 000 from
the UN, has bought it outright. STRI plans more sophisticated cranes with
radii of 80 metres. The first would go on Barro Colorado Island. Sitting
in Gatun Lake, the island was created during the building of the Panama
Canal. It has been a biological reserve since 1923 and run by the Smithsonian
since 1946. Much of its forest has grown untouched for 200 years, and its
1564 hectares are the most intensely studied of any tropical forest in the
world.

STRI wants to put cranes in several of a planned string of 50-hectare
plots in forests around the world, experimental areas that will be modelled
after Barro Colorado. Sites in Malaysia and India are already agreed upon;
others in the Malaysian state of Sarawak on the island of Borneo, Thailand,
Sri Lanka and the Ecuadoran Amazon are in negotiation.

Smith would like to see portable computers put into the gondolas. These
would preserve spatial coordinates for each project so that passengers could
automatically be returned to their chosen spot, day or night. Computers
would also allow researchers to superimpose one set of data, such as a
map of tree species within the crane’s catchment area, on to another, such
as insect distribution.

In the meantime, the STRI team is cooking up more projects for their
crane in the Metropolitan Park. One researcher plans to spend nights in
the trees watching bats employ echolocation to find their insect prey. Others
plan to watch insects, many of which feed only at the canopy tops where
the freshest young leaves abound. The total amount of herbivorous activity
in tropical forests could be much greater than earthbound biologists can
detect. The number and appetite of arboreal insects are often inferred by
the number of partly eaten leaves that fall from the canopy. ‘But if entire
leaves are eaten,’ Smith notes, ‘then there is nothing to catch, except
perhaps insect droppings.’ Also of interest is at what point in a leaf’s
life it is eaten. A tree has not invested much energy in a newly expanding
leaf and may be able to spare it more easily, an hypothesis reinforced by
the relative lack of toxic compounds in new versus older leaves of some
species.

If the Smithsonian’s plan succeeds, the big red crane in Metropolitan
Park might be the first of a new megaspecies appearing in tropical forests
around the world – a fitting conclusion to its ‘feral’ beginnings.