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).