ÐÓ°ÉÔ­´´

THE TROPICAL grasslands have as much to do with stable global climate
as do the rainforests, according to a study that has just been completed
by the United Nations Environment Programme. The study looked at grasslands
on five continents. Its results show that tropical grasslands turn far more
carbon dioxide into carbohydrates than anyone suspected, equalling – or
even exceeding – the productivity of tropical rainforests.

While environmentalists bemoan the destruction of the rainforests, few
people pay much attention to the less exotic plains. Yet vast tracts of
grassland are being destroyed each year, and grubbing up the world’s grasslands
could eventually have the same effect on global temperatures as felling
its forests.

Models of global warming have to take into account what happens to carbon
dioxide, the most important of the greenhouse gases. The modellers look
for sources and sinks for the gas, what is producing it, and what is removing
it from the atmosphere. The destruction of rainforests is a large source
of carbon dioxide, while the ocean, whose plankton lock up carbon and carry
it to the sea floor, is a large sink.

Every ecosystem plays some part in the global carbon budget. But, until
recently, scientists have ignored tropical grasslands, considering them
relatively unproductive.

Productivity is a measure of the rate at which plants assimilate carbon
dioxide from the atmosphere and transform it to carbohydrates using energy
from sunlight . Basic measures of productivity are essential if modellers
are to predict future levels of carbon dioxide. They can also provide a
baseline from which to assess the effect of climatic changes on the growth
of plants.

The dried and withered stalks of grasslands in the dry season may not
look as productive as the luxuriant trees of the rainforests. But it is
the quantity of organic material that plants produce by photosynthesis that
is important.

What information there is on bioproductivity is, it seems, wildly inaccurate.
Much of the data on tropical grasslands come from an extensive series of
studies carried out for the International Biological Programme (IBP) between
1967 and 1972. In these studies, the researchers first measured the increase
in growth of plant shoots over the year. From this measurement, they estimated
the productivity of the plant community either by adding together the increases
recorded over the year (the ‘standard’ method), or by calculating the difference
between the highest and lowest weights recorded (the ‘maximum-minimum’ method).

Both methods have disadvantages. First, they suppose that it is enough
to measure only those parts of the plant growing above the ground, and ignore
roots and rhizomes. Secondly, they assume that plants are growing and dying
at different times. This may be true of temperate grasslands, where growth
and death tie in with the seasons; but in a community of perennial plants,
like many tropical grasslands, some plants are growing while others are
dying.

Even in a single plant, some leaves and shoots may be growing while
other organs are dying. The IBP’s methods ignore any part of the plant that
dies during the growing season, and take into account only living plant
material. The neglect of dead or decomposed organic material leads to a
serious underestimate of productivity.

When part of a plant dies naturally, the plant withdraws valuable organic
materials for storage, often in underground organs. Those parts of plants
that are eaten or decomposed by microorganisms are converted into other
organic materials.

As the IBP’s measurements are concerned only with living material, they
‘lose’ this part of a plant’s productivity. Because turnover is high in
the tropics, the omission of death and decomposition leads to a considerable
underestimate of productivity.

The ‘max-min’ method also has a drawback: it assumes that growth reaches
a peak at one time in the year. Again, this may be true for grasslands in
temperate climates, but the perennial species that make up tropical grasslands
have different growing phases. Growth in the tropics often takes place during
two distinct rainy seasons, for example.

Measurements of maximum and minimum weight ignore these growth cycles
and so give a low value for productivity. Also, tropical climates are variable
and so productivity fluctuates from year to year, making it necessary to
collect data over several years.

The UNEP project aimed to provide reliable, long-term data from contrasting
sites around the tropics. The study began with three types of grassland:
dry savanna in Nairobi National Park, Kenya; a saline grassland near Mexico
City; and a wet savanna-type grassland near Hat Yai in southern Thailand.
The scientists at each site used the same methods so that their data were
directly comparable.

Over a period of at least three years at each site, they collected standard
samples of plant material above ground and in the soil every month, weighing
both living and dead material. They estimated the rates at which plant material
decomposed by putting weighed samples of dead plant material into fine meshed
bags, and leaving some bags on the surface and burying others in the soil.
The loss in weight of the plant material inside the bags over a month indicated
the rate of decomposition.

The results of the UNEP study are remarkable. They show that natural
tropical grasslands are between three and five times as productive as earlier
studies indicated. About 50 per cent of the plant growth was in the roots
and rhizomes – the very parts other studies ignored.

During the severe drought that hit northern Africa in 1984, Simeon Imbamba
and Jenesio Kinyamario, of the University of Nairobi, found that as much
as three-quarters of the biomass was underground. Previous estimates suggested
that, worldwide, tropical grasslands account for 9 per cent of photosynthetic
productivity on land. Taking the UNEP study’s lower estimate – that grasslands
are three times as productive as previously thought – this contribution
would rise to some 25 per cent.

The study also considered a bamboo forest near Hangzhou in China and
wetland grasses near Manaus in Brazil. Shen Yun-Kang, from the Shanghai
Institute of Plant Physiology, and Huang Qi-Min, from the Subtropical Forestry
Institute at Fuyung, found that subtropical bamboo is about as productive
as other grasslands, even though Hangzhou receives only half as much sunlight.

At the flooded Manaus site, Nunes de Mello and Maria-Teresa Fernandez,
of the National Institute for Amazon Research in Manaus, found that the
aquatic grasses are at least five times as productive as their dryland relatives,
and are almost as productive as record-breaking agricultural crops.

These findings promote tropical grasslands to equal ranking with the
rainforests as consumers of carbon from the atmosphere. ‘Modellers of climate
change have not previously realised this and have tended to concentrate
on rainforest,’ said David Hall, who coordinated the project.

The UNEP team believes that its figures for the productivity of grasslands
may still be too low. Many of the most delicate roots are lost when samples
are washed before weighing. These could account for as much as half the
weight of the root system. Nor do the measurements take into account organic
matter exuded from roots, or carbon consumed in symbiotic relationships
with soil microorganisms.

UNEP’s estimates of rates of decomposition are also probably too low,
perhaps by around 10 per cent, because larger insects that help to break
down plant material cannot reach it inside the fine-meshed bags.

In total, Hall, and Steve Long, technical coordinator of the project,
from the University of Essex, suggest that UNEP’s figures could underestimate
productivity by as much as 25 per cent.

The discovery that tropical grasslands extract at least as much carbon
dioxidefrom the atmosphere as the rainforestshas other important implications.
Apinan Kamnalrut, of Songkla University, working at the site in Thailand,
and Edmundo Garcia Moya, of the Postgraduate College in Chapingo, working
in Mexico, both produced a startling finding: they measured annual productivities
of 20 (dry) tonnes per hectare, at least twice the yield from wheat grown
intensively with fertilisers and pesticides in Britain.

Tropical grasslands have evolved to provide consistently high productivity
in a wide range of conditions. Yet many of the areas studied are under pressure
from agriculture. In Kenya, for example, there is pressure to grow sorghum,
cotton and pineapple, and in Thailand, rice and sorghum.

When grasslands are cleared for agriculture, the new crops are likely
to be less productive than the original species of wild grasses. Without
irrigation, agriculture typically produces around 3 to 5 tonnes per hectare.
Turning savanna over to agriculture also increases soil erosion, which in
turn reduces the productivity of the land.

The loss of natural grasses also leads to a serious loss of genetic
diversity. Today’s food crops could be improved with genetic material extracted
from native grasses that are able to maintain their productivity under extreme
conditions of drought and flooding, for example.

Finally, underrating productivity undervalues the input of organic material
that grasslands provide for the soil and the ecosystem. Natural grasslands
provide a constant supply of carbon in the form of leaf litter and other
organic matter, constantly adding to the carbon stored in the soil.

But when the land is ploughed up for agriculture this carbon is gradually
lost, leached into rivers and oceans, or oxidised to the atmosphere. Burning
grasslands leads to similar losses of carbon from the soil. Little is known
about the fate of this carbon, but much of it probably ends up in the atmosphere.

Worldwide, 700 millionhectares of savanna are burnt each year. In the
moister regions of the tropics, farmers who own cattle burn grasslands every
one to two years to encourage the growth of new shoots and destroy pests.
About 75 per cent of the African grasslands are burnt annually.

In West Germany, Paul Crutzen and his colleagues at the Max Planck Institute
for Chemistry in Mainz have come up with some surprising estimates for the
amount of carbon dioxide that this burning produces. Taking data from a
survey of changing land use carried out by the UN Food and Agriculture Organization
between 1975 and 1980, they calculate that burning the savannas contributes
three times as much carbon dioxide to the atmosphere as burning the rainforests.

What happens to this carbon is not clear. Grasslands regenerate quickly,
so some carbon is incorporated back into the grassland biomass. But if grasslands
are burnt too often, the grasses cannot recuperate.

Burning and overgrazing also lead to soil erosion, which lowers the
productivity of the land. So destroying grasslands affects the concentration
of carbon dioxide in the atmosphere in both the short term and the long
term.

Where modellers have tried to take account of biological productivity,
they often based their calculations on information collected by remote sensing.
But, as the UNEP studies show, much of the productivity of plants lies underground,
and scientists can measure true levels of productivity only by going into
the field.

Now that global warming has become a prominent issue in international
politics, people are beginning to look more closely at what is happening
to the rainforest. So far most people have failed to see the grass for the
trees.

Peter de Groot is a researcher and freelance science writer.