A tropical rainforest without rain wouldn't be much of a rainforest.
I mean, all plants need water to grow, and without it, they shrivel up and die.
So what about the ancient Hawai'ian proverb,"Hahai no ka ua i ka ulula`au", which means "the
rain follows after the forest"?
How could that be?
Well, all land plants lose water when the pores on their leaves open up during photosynthesis,
and this evaporation draws more water up through their stems.
With so much rain soaking the soil in rainforests, water is nearly unlimited, and accordingly,
rain forest trees can afford to move and lose more water than other plants.
All that water vapor rising from the forest feeds moisture-laden clouds while causing
convection - together, these effects accelerate the formation of rain - which falls to the
soil and gets taken up all over again.
This cycle--absorption, evaporation, rain-- happens everywhere there are plants.
However, super-wet soil, fast-pumping trees and hot tropical sun make the cycle so fast
in the rainforest that - unlike other biomes where clouds might form in one place and rain
in another - all that water stays in the same region.
So without the forest pumping so much water into the air, rainforests wouldn't be as rainy.
And without so much rain, the forest couldn't pump so much water into the air.
So which came first, the rain, or the rainforest?
Well, before rain forests, ancestors of trees like cypress, pine and spruce dominated the
land - but they were conservative when it came to using (and losing) water - so the
air tended to be dry, meaning less rain.
However, around 130 million years ago, a new kind of plant developed that took the risk
of losing more water in return for souped-up photosynthesis - these were the flowering
plants, and their risk paid off: their faster growth enabled them to out-compete the ancestral
pines and take over the tropical regions of the globe.
Angiosperms lost so much water into the air that as they spread, they brought their own
rain with them.
And today, tropical rainforests receive more rain than if they were replaced by pine forests-
in some places as much as a meter more rain each year.
That's equivalent to an extra two and a half hours of heavy rain every week.
Not surprisingly, all that water cools off the forest, too, which is why the Amazon isn't
nearly as hot as the Sahara or even an east Texas pine forest in summer.
But the hot, dry tropics of the past may soon be a part of our future.
In parts of the Amazon where vast swaths of rainforest have been logged or cleared for
agriculture, weather stations are already observing decreased rainfall, and forest fires
have become more frequent.
Scientists worry that these changes will lead to ever hotter, drier and more flammable tropics
in the coming decades, making things tougher both for the remaining forest and for the
people who live there.
So, when in drought, plant a tree.
Seriously - Hahai no ka ua i ka ulula`au (the rain follows after the forest).