ÐÓ°ÉÔ­´´

EVERY YEAR, when the first rain falls in the Cape Verde Islands, everyone
rolls up their sleeves and goes out to plant trees. The islanders aim to
cover their nude hills with forest by dint of sheer hard work and enthusiasm.
But while the Cape Verde islanders are out planting saplings, logging companies
in other parts of the tropics continue to fell the trees of the rainforest.
Vast tracts of the tropics are now bare or have lost their largest and best
trees to the timber trade or to agriculture.

Schemes to replace the forest cannot keep pace with the destruction,
and new plantations usually rely on a handful of exotic species, such as
Eucalyptus, introduced because they grow faster than the native hardwoods.
Some countries insist that logging companies rehabilitate the forest they
have plundered, but the loggers face practical problems in doing what is
required of them, and the authorities face problems in trying to ensure
that they do.

There are also good biological and economic reasons why restoring the
forests will be a slow process. Many tropical hardwoods produce seed very
erratically, and the seed theydo produce often fails to germinate. Most
indigenous hardwoods also grow slowly and are susceptible to pests and disease.
As Ron Kemp, the senior forestry adviser at Britain’s Overseas Development
Administration (ODA), points out:’If land has been laid bare and severely
eroded, the number of native tree species which would be capable of recolonising
is extremely limited. Very often there simply will not be a native pioneer
species that could survive.’

ÐÓ°ÉÔ­´´s and development agencies now realise that conservation in
the tropics will depend on making indigenous species of trees ‘forester
and farmer friendly’. At the tropical tree improvement unit of the Institute
of Terrestrial Ecology, near Edinburgh, and field stations in Africa and
Central America, biologists are taking a leaf out of the horticulturalists’
textbook. Following the examples of foresters working on commercial plantations,
they are applying the traditional techniques of cutting and coppicing to
produce clones of hardwood trees that might form the basis of new forests.
Elsewhere, scientists are trying out micropropagation, producing new plants
by culturing tissues from the best trees.

The ODA wants developing countries to use more of their overseas aid
on conserving their forests. ‘In countries including our own, the proper
role of trees and forests has not been appreciated early enough,’ says Kemp.
But governments, forestry companies and individual farmers are not likely
to grow native hardwood trees unless there is something in it for them.
The task, then, is to make species that are most desirable in an ecological
sense desirable to growers, by making them more productive. One way to do
this is to create clones of the most productive specimens and plant these
rather than saplings grown from seed.

As the result of sexual reproduction and the mixing of genes, seed is
unpredictable: it is impossible to tell how the offspring will turn out
until the tree matures many years later. Taking cuttings creates clones
that are genetically identical to the parent plant. The principle is straightforward.
By planting clones from different parents, the characteristics of each clone
gradually become apparent until scientists can measure how fast the clones
grow, how resistant each is to disease, its yield, the flavour of its fruit
or the palatability of its leaves. The forester can then pick out the best
clones and carry on propagating trees from them ad infinitum.

The practice is more complicated. Beyond the age of 4 or 5 years, the
tissue of a tree loses its ability to generate roots. But at 5 years old
a tree has not yet shown its true colours; by the time it has proved that
it has the required characteristics it is useless to the propagator. Roger
Leakey and his colleagues at the Institute of Terrestrial Ecology are looking
at the biology of rooting in an attempt to find ways to improve the ‘rootability’
of cuttings. ‘We are aiming to understand the fundamental physiology of
rooting, but we are still a long way off that,’ he said. ‘Rootability seems
to be affected by almost everything you can think of – including light quality
and intensity, and the position on the plant of the shoot to be cropped.
The other problem is that every tree species has a different recipe for
optimum rooting, and even different genotypes of the same species can be
different.’ Leakey is trying to identify factors that are common to all
trees, which should give him and his colleagues a head start in finding
ways to root even the trickiest of tropical trees.

Leakey began his research with the West African timber tree Triplochiton
scleroxylon, which yields obeche, a lightweight hardwood and a favourite
of plywood manufacturers. He believes that careful management of stock plants
is the key to a long succession of good cuttings. Cuttings taken from different
positions on the shoot are slightly different – in the age of their leaves,
the length of the internode and how woody the tissues are. The shoots also
vary in how much carbohydrate and other nutrients they contain and probably
also inthe concentrations of other chemicals, such as plant growth substances.
All these factors affect a cutting’s abilityto take root.

After looking at Triplochiton, Leakey concluded that the size of the
stem is of overriding importance. The greater the volume of the stem, the
better the chances of the cutting rooting well – possibly because a larger
stem has a bigger store of carbohydrates. This ties in with another observation,
that keeping relatively few, larger shoots on a stock plant gives each cutting
a better chance of rooting.

Every cutting needs a coating of the growth hormone auxin on its tip
before it is planted in the rooting medium. Beyond this, Leakey has found
that he gets better results if he trims the leaves. A cutting with less
leaf uses less energy in transpiration. It also produces less energy by
photosynthesis, but there is an optimum size at which the balance between
to twois just right. Experimenting with a pair of scissors and a collection
of Triplochiton cuttings, Leakey found that the best size for leaves is
around 50 square centimetres.

Temperature also affects rooting. For the best results, the air should
be between 5 and 10 Degree C cooler than the rooting medium, which is around
30 Degree C. To minimise the physiological stress on the plants at this
vulnerable stage, they should be kept in a shady and humid environment.
Leakey is now looking at how the intensity and quality of the light affects
a stock plant’s ability to give cuttings that root successfully. It seems
that stockplants grown in shade photosynthesise more efficiently, perhaps
because they have to work much harder at it and because their leaves are
less choked up with starch. Cuttings taken from shaded stock plants inherit
this efficient photosynthesis, which, in turn, increases their chances of
rooting well.

This finding has been putto practical use at the tree improvement unit
at the National Office for Forest Regeneration in Cameroon, West Africa.
By growing stock plants of Triplochiton scleroxylon in the dappled shade
of the leguminous tree Leucaena leucocephala, the foresters have increased
the success rate of cuttings enormously – from 21 to 74 per cent.

Another of Leakey’s observations provides a means to select clones that
are likely to give the best results, without having to wait and see how
a tree matures. Trials in Nigeria with T. scleroxylon showed that the pattern
of sprouting in young seedlings is a reliable indicator of which trees will
put out the fewest branches, and so which will grow tallest and have the
broadest main trunks. ÐÓ°ÉÔ­´´s can now predict the best trees from a range
of 3- to 4-month-old seedlings. These can then be cloned by taking cuttings.

Successful propagation does not need to be an expensive and highly technical
operation. This is important because individual farmers are the key to successful
replanting in many areas. In Cameroon, scientists from Edinburgh, backed
by funds from the ODA, are rooting cuttings of African walnut inside simple
propagators made from a sheet of polythene wrapped around a wooden frame
and placed under a palm leaf or other form of shade. Inside the propagator,
a simple rooting medium of rotting sawdust and coarse sand lies atop layers
of gravel and stone, which are kept saturated with water from a feeder pipe.
All the farmer must do is to keep the water topped up daily and spray the
cuttings with a fine water spray using a knapsack sprayer.

Getting the cutting to take root may be the trickiest part of the process
but it is not the end of the scientist’s work. When it comes to planting
out, Leakey’s concept of a ‘plantation’ is rather different from the stereotyped
lines of identical trees. ‘We are looking towards a system of silviculture
which will allow us to establish highly productive ‘plantations’ while making
sure we do not damage natural ecosystems, and which do not expose us to
an explosion of pests and pathogens,’ he says. Research in Cameroon has
shown that the best approach is to plant trees in partly degraded or secondary
forest, where the canopy is only partially broken. Here, cloned trees can
be fitted in between established native trees, improving the productivity
of the partly cleared forest. ‘This prevents a complete change in the balance
of microflora and ecological succession of the vegetation. Ecologically
it is a very desirable method and we have also found that these systems
give us the best tree growth,’ says Leakey.

Leakey is less enthusiastic about growing monocultures of indigenous
hardwoods in places that have been cleared totally, where there is no leaf
litter to enrich the soil and its microbial flora is very different from
that in virgin forest or partly degraded forest. The Ivory Coast and Ghana
both tried planting Terminalia ivorensis, a relatively common timber tree,
on completely cleared land. After 8 to 10 years the young forests died.
Leakey blames the failure of the plantations on impoverished soils. He suggests
that they simply ran out of nutrients. Soils under degraded forest or on
cleared land also lack the fungi that form partnerships with the roots of
particular trees, the ‘mycorrhizae’ that are essential for the trees to
grow. When forest is cleared, different types of fungi replace those present
in a natural, healthy forest.

Despite these failures, Leakey remains optimistic that the techniques
of propagation he has helped to develop will contribute to the conservation
of tropical hardwood trees if the species are planted in mixed associations
that mimic the natural forest. ‘For the first time, conservation and forestry
in the tropics appear to be heading the same way,’ says Leakey.

Time is not on the side of conservation, however. Nigeria, for example,
has pitiful amounts of natural forest left where such systems of agroforestry
can be established. In Cameroon, prospects are better; the country still
has 20 million hectares of tropical forest. The World Bank is funding a
tree improvement unit for indigenous hardwoods, based on the methods of
vegetative propagation and cloning used for obeche. The unit is about to
begin work and will propagate a quarter of a million cuttings a year. ‘Compared
with what is needed to restore tropical forests, this is small fry,’ says
Leakey, ‘but it is a start and it shows what can be done’.

There is little doubt that vegetative propagation is vital to ensure
that forestry and agroforestry schemes include more species of tropical
forest trees. But there are drawbacks to relying too heavily on cloning
as a source of new trees. Richard Barnes, of the Oxford Forestry Institute,
warns that vegetative propagation is not a substitute for breeding programmes.
By itself, cloning can only multiply one selected set of characteristics.
The only way to improve on these is by combining the best features of two
individuals by sexual reproduction. ‘Cloning is only a tool by which you
can make maximum use of breeding. Only breeding will allow cumulative gains
over many generations,’ says Barnes.

According to Barnes, people often claim that advances made through vegetative
propagation are ‘breeding successes’, but they are not. On the spectacularly
successful eucalyptus plantation at Aracruz in Brazil, for example, foresters
planted a huge area with seed which they did not realise was hybrid. When
the trees grew, they varied enormously, and the overall yields were poor.
But by selecting and cloning individuals that grew fastest and produced
the best wood, the foresters increased the plantation’s yield by 112 per
cent. ‘They have now come up with the most productive forest in the world,
by vegetative propagation of a hybrid,’ said Barnes. But there is no scope
for further improvement because the trees are sterile.

The only way to increase productivity now is to go back to the original
parent species and find individuals that, through breeding, could give rise
to ever better clones. Realising this, the foresters from Aracruz are searching
Australia for ‘supertrees’ from natural stands of eucalyptus. This is exactly
what conservationists want to hear. The future of forestry and agroforestry
depends on the survival of natural stands of species that are important
commercially, or have great potential, and so the industry must safeguard
such stands. This is good news but Barnes still worries that spectacular
success in cloning selected trees of commercially valuable species could
jeopardise the conservation of genetic diversity of forest trees in general.
Leakey accepts that this is a danger but argues that cloning is not a major
hazard provided it is done within the context of a programme designed to
improve trees.

One way to maintain genetic diversity is to grow trees from seed. Tropical
pines, which are good pioneering species, regularly produce large amounts
of seeds, which germinate easily, so they lend themselves to breeding programmes
and foresters have been very successful with them. In contrast, many tropical
hardwoods produce seed irregularly and often fail to germinate. There is
little incentive to grow these species from seed when propagating the trees
from cuttings produces more predictable results.

Foresters growing tropical pines from seed can produce highly uniform
plantations by controlling pollination and continually selecting the best
individuals. Work on vegetative propagation of tropical pines lags far behind
the breeding programmes, but scientists at the Oxford Forestry Institute
are trying to close the gap. Alan Pottinger, of the institute’s field station
at Wytham, near Oxford, has spent the past three years visiting natural
stands of pine around the world, in Southeast Asia, South Africa, South
America and Australia. His greenhouse is full of souvenirs of his travels
– growing tips, or scions, from selected mature trees brought to England
on ice and grafted onto young rootstock of the same or a closely related
species.

Pottinger’s aim is to create cloned stock plants of superior individuals
at Wytham from which he can send cuttings to regional forestry centres around
the world to create new stock plants of prime clones – whether as pioneers
for reafforestation schemes or for new commercial plantations. He admits
that he still cannot explain some of the results he has been getting. He
has had more success with grafts since he began to pay more attention to
the health of his rootstock, but he cannot explain why different stock plants
of the same clone develop varying numbers of shoots. Nor does he yet know
how viable his cuttings will be when they are packaged and sent off to be
regrafted elsewhere.

Pottinger believes that micropropagation could solve many of his problems.
One of the biggest problems when shipping plant material around the world
is that many countries have strict regulations about the movement of plant
material to prevent the introduction of diseases. Micropropagation bypasses
this because the plants are grown in a sterile tissue culture, which could
cross national boundaries safely. The small size of tissue cultures also
makes them easier to store and to transport around the world.

As far as trees go, micropropagation is still in its infancy. At the
University of Cambridge, Tony Simmonds has been investigating the potential
of micropropagation in tree improvement programmes. He says there are very
few examples of high returns from micropropagation using the most important
tropical pines.

In Australia, scientists at the Queensland Forestry Department have
had some success in developing clones by micropropagation of tissues from
a hybrid of Pinus caribaea and Pinus tecunumanii, which outyields both parents.
But so far, according to Simmonds, production of seedlings from cuttings
far outstrips production by micropropagation.

One of the drawbacks of micropropagation is that it depends entirely
on artificial conditions. Instead of growing on the nutrients produced by
photosynthesis of the parent plant, tissues in culture are nurtured on sucrose
in the culture medium. When the plantlets are transferred to a more natural
rooting environment, the change can be traumatic and they fail. Micropropagation
is also labour intensive and expensive. Cultures require large amounts of
chemicals and hormonesto give a high yield. But high concentrations of chemicals
can cause genetic changes in the cells, so that the resulting plant is no
longer identical to its parent.

New trees from old

The biggest benefit of micropropagation is that tissues can be taken
from a mature proven tree and multiplied to form a clone of guaranteed quality.
Micropropagation is also a useful tool for screening clones for tolerance
to disease, salt and drought. And it is much easier to test 2000 tissue
cultures in a laboratory than to test 2000 clones in the field.

In the future, when genetic engineers turn their attention to trees
and find a way to introduce new genes into exotic species, they will need
a ‘delivery system’ to transform cell cultures into plantlets. Micropropagation
will be an essential part of the process.

As the great trees become rarer, botanists around the world are collecting
seed in an attempt to conserve the genetic diversity of these species. Seed
from collecting trips ends up in seed banks, dehydrated and stored at very
low temperatures. But the seeds of most tropical trees die when they are
dried. ÐÓ°ÉÔ­´´s are trying to find other ways to preserve such ‘recalcitrant’
seeds.

At the Royal Botanic Gardens’ seed bank at Wakehurst Place in West Sussex,
researchers are working with seeds from Southeast Asian dipterocarps, a
large and varied family of trees that provides much of the world’s tropical
hardwood. Traditional methods for storing seeds involve removing as much
moisture as possible then freezing at a temperature around -20 Degree C.
At Wakehurst, Hugh Pritchard and Isabelle Linington are looking at extracting
the embryo from a seed, and preserving that rather than the whole seed.
The embryo usually makes up less than 1 per cent of the volume of the seed,
and could be easier to manipulate. ‘The smaller the material the quicker
it can be dried, and the speed of drying seems to help maintain viability,’
says Pritchard.

Pritchard has successfully stored embryos taken from the seeds of some
trees that grow in temperate regions, two species of oak and a sweet chestnut.
When the embryos come out of cold storage, they need to be developed in
vitro, on a culture medium. As the cotyledons begin to develop, the propagator
can take tiny cuttings to create clones.

Linington has begun to experiment with tropical trees. Working with
two dipterocarp trees, Shorea leprosula andS. parvifolia, she has grown
seedlings from ‘fresh’ embryos. The next step is to find out if the embryos
will survive drying and freezing and then grow into trees.

There is no quick fix for the forest. Market forces have driven tropical
forests to the edge of extinction. Only market forces can drive their rehabilitation
and conservation. The only way to save the forests is to make the trees
we want to save more productive, more attractive commercially and more available.

Debbie Macklin is a freelance journalist specialising in environmental
issues.