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).