15 August 2020 | New Scientist | 41minuscule fraction of the Congo forest,
which is twice the size of France, Spain
and Germany combined. “That’s a lot of
extrapolation,” says Scott Denning, a climate
scientist at Colorado State University. The
decline in carbon uptake found on these
plots may not be replicated everywhere else,
so “you have to be appropriately sceptical”.
There is a glimmer of hope that higher
CO2 levels will instead help tropical forests
to continue soaking up carbon, even as other
conditions deteriorate. The concept, known
as carbon fertilisation, makes sense in
principle: add more of one crucial ingredient
to the photosynthetic recipe, and you can
expect improved growth. Commercial
greenhouse operators have long piped in the
gas to boost the growth of certain crops. The
trouble is that the extent to which it works
in the jungle, with so many other factors
influencing forest health, is hard to establish.
It isn’t that people haven’t tried. Richard
Norby at Oak Ridge National Laboratory
in Tennessee has run several experiments
studying carbon fertilisation in full-size
trees in natural conditions. As early as the
PHOTO ESSAY1990s, he erected a ring of towers around a
plot slightly larger than the centre circle of a
football field in an old sweet gum plantation
near his office and proceeded to pump in
CO2-enriched air through pipes hanging
above the plot. Sure enough, the trees grew
faster – at first. But they eventually slowed
when the soil ran low on nitrogen, more of
which appears to be required for those trees
to benefit from extra CO2.
Scientists have since run similar studies,
dubbed free-air CO2 enrichment, or FACE,
experiments elsewhere in the US, Europe
and Australia. Sometimes the extra CO2
helps. Sometimes it doesn’t. But no such
experiment has ever been conducted in a
tropical forest – which is critical, says Norby,
because every ecosystem has its own quirks.
David Lapola is among those trying to
change that. A biologist at the University of
Campinas in Brazil, Lapola has spent the best
part of a decade setting up a FACE experiment
on a jungle plot 90 kilometres north of
Manaus, the Amazon’s largest city. When I
visited last year, workers trudged back and
forth under a dense canopy hauling long,rigid panels of clear plastic mounted in
aluminium frames. They were assembling
the pieces into eight roofless chambers,
each the size of a small garden shed.
Lapola was getting set to start supplying
the chambers with CO2-enhanced air. He later
told me that, although some plants do seem
to grow better with extra CO2, provisional
results “do not seem to show much of a
difference between enriched and control
chambers”. This suggests carbon fertilisation
might not take up the slack. Norby says that
may be because the soil at the site, as in much
of the Amazon, is deficient in phosphorus,
another essential plant nutrient.
But results from this experiment are far
from definitive. Lapola is the first to admit
that huge, unenclosed plots are superior. His
small chambers let in normal light and rain,
but beyond that the conditions inside them
are highly artificial. For instance, the plastic
walls block out wind and restrict access to
birds, insects and rodents. Worse, they are
only big enough for short plants – and Lapola
says full-size trees in bigger plots could
behave very differently. >