Geology Reference
In-Depth Information
of trees in the rain-forests of south and central America. Bromeliads are not parasites;
they don't use their roots to suck life-giving sap from their host, but use them instead to
girdle a branch in a knotty tangle of tightly gripping lassos which keeps the bromeliad
firmly in place. Bromeliads are merely epiphytes: they use the tree only for support, and
must therefore get all their moisture and nutrients from rainwater. Bromeliads are per-
fectly shaped for water collection. Their thick, fleshy leaves act like spear-shaped gutters
which together make a set of nested cup-like receptacles at the base of the plant where
water collects in miniature ponds inhabited by complex ecological communities, each
differing slightly from bract to bract and bromeliad to bromeliad. The water in these bro-
meliad ponds swarms with microbes, many of them algae that provide photosynthetic
food for the whole community. Strangely enough, the algae live in a medium somewhat
resembling sea water, and face an osmotic challenge similar to that encountered by their
marine relatives. The algae appear to solve their osmotic problems by making DMSP.
Thus large quantities of DMS waft from bromeliads into the air above the forest, where
oxygen converts it into the cloud-seeding sulphate aerosols. It isn't just the bromeliad
microbes that seed clouds; the trees do it too, by emitting not DMS but complex organic
molecular beings called turpenes and isoprenes that oxygen dismantles into cloud con-
densation nuclei of another sort.
The entire forest not only seeds its own clouds, but also recycles the very water that
makes the clouds by capturing rainfall with its roots before sending it out into the air
through countless billions of microscopic pores on the undersides of its leaves. Most of
the Amazon's rainfall comes from outside its basin, mainly on moist, warm winds from
the Atlantic, but 25% of the Amazon's water is recycled as rain by the forest itself. This
is far more than the Mississippi basin, which can only manage to recycle 10% of its wa-
ter. The Amazon is a very efficient water-harvesting organism: of the water that falls on
it, about half is returned to the air via leaves, whilst the rest feeds its extensive network
of rivers and oxbow lakes. The clouds the forest seeds are planet-cooling—they are the
dense white kind that bounce the sun's rays right back to space. When clouds condense
over the Amazon forest, a vast amount of energy is released—about 40 times the en-
tire energy use by humanity in a year. Much of this energy is shunted around the globe
in great waves of air to affect the climates of far-flung regions such as North America,
South Africa, South-east Asia and parts of Europe.
Cutting the forest down is a catastrophe, not only for the millions of species living in
them, but also for the world's climate. The latest models from the Hadley Centre in Bri-
tain predict that the Amazon could soon collapse into tropical savannah, even without
the cutting down of a single tree. Why? The Amazon seems to do best during an ice
age, when the high latitudes are covered in ice but the tropics are deliciously balmy and
just right for the cloud-seeding forests. In between ice ages, during the warm intergla-
