What if a rainforest had little rain?

In a rainforest, it rains a lot. Tall trees grow close together.

Monkeys, birds and frogs live there. What if the rain stopped coming so often?

A rainforest is an area of tall, mostly evergreen trees that gets a lot of rain. Rainforests cover only a small part of Earth, yet more than half of the world's plant and animal species live in them.

The rain is part of what makes a rainforest a rainforest. So what would happen if one got much less rain, year after year? This is a thought experiment, but it is built on real droughts that scientists have studied in the Amazon. Choose a path to explore.

Rainforests cover just 2 to 6 percent of Earth's surface but hold more than half of its plant and animal species. Their trees also return water vapor to the air, helping clouds form, and store large amounts of carbon.

This thought experiment asks how a rainforest would change under sustained low rainfall. Severe Amazon droughts in 2005 and 2010, tracked by satellites and field plots, give real evidence for the first steps; beyond that, scientists rely on climate models and their answers are uncertain.

Rainforest trees grow very tall. Some are as tall as a big building!

The treetops make a leafy roof. Under it, the forest is damp, dark and still.

A rainforest has layers. The tallest trees, in the emergent layer, can reach 60 meters (200 feet). Below them, the canopy forms a thick leafy roof that blocks wind, rain and sunlight, so the layers underneath stay humid, still and dark.

More animals live in the canopy than in any other layer, because it has so much fruit. Toucans, macaws and monkeys eat the fruit and drop the seeds on the forest floor, helping new trees grow.

Most rainforests have four layers: emergent trees up to about 60 meters, a canopy roughly 6 meters thick that forms a roof, the understory and the forest floor. The canopy blocks wind, rainfall and sunlight, creating a humid, still and dark environment beneath; many canopy plants wrap their seeds in fruit that animals eat and disperse.

The trees are deep-rooted, and in the Amazon scientists have observed them sprouting more leaves in the ordinary dry season. Rainforests also help regulate climate by storing carbon in trees and soil and by supplying water vapor for clouds.

Sometimes a rainforest has a very dry year. It happened in the Amazon in 2005 and 2010.

The leaves got less green. Rivers got low. The forest needed the rain to come back.

Scientists don't have to only imagine this. In 2005, a drought slowed tree growth and killed trees in carefully watched patches of the Amazon. In 2010 an even bigger drought hit. The Rio Negro, a large river that flows into the Amazon, fell to its lowest level in 109 years of records.

Satellites showed the forest losing greenness over about 965,000 square miles, more than four times the area hit in 2005, and it did not fully recover even after the rains returned.

Field plots showed that the 2005 drought reduced growth and killed trees. In July to September 2010, satellite vegetation indices (MODIS) showed 1.68 million square kilometers of forest with signs of drought stress or reduced photosynthesis, consistent with fewer leaves or less chlorophyll. A Boston University and NASA study mapped about 965,000 square miles of reduced greenness, more than four times the 2005 area, that had not recovered after the drought ended in late October 2010.

The Rio Negro at Manaus reached its lowest level in 109 years of records, and uncontrolled fires spread smoke across the basin.

Trees need water to make their food and grow green leaves.

Rainforest trees also help make rain clouds. So trees and rain help each other.

Trees need water to keep their leaves green and make food. In a drought, stressed trees grow fewer leaves or less green ones. Dry forests also burn more easily.

Rainforest trees give water vapor back to the air, which helps clouds form. So a forest that loses trees might also lose some of its own rain. Scientists think a long dry spell could change parts of a rainforest into a more open, grassy place, but they are still studying how likely that is.

Drought stress lowers leaf area and chlorophyll, and therefore photosynthesis. Dead trees release their stored carbon as wood rots, which can add to warming; NASA notes the Amazon is one of Earth's great carbon stores.

Computer models predict that warmer temperatures and altered rainfall could cause moisture stress severe enough to replace some rainforest with grassland or woody savanna. But the evidence is mixed: deep roots may give some drought resistance, and NASA describes the forest's large-scale response to drought as an open scientific question. These are ideas, not certainties.

Scientists watch the rainforest from space with satellites. They check how green the trees are.

Many people work to protect rainforests, so the trees and animals stay healthy.

NASA satellites measure how green the Amazon is, and scientists compare dry years with normal ones. They study droughts now so they can understand what might happen if droughts become more common in a changing climate.

Rainforests face other pressures too, such as land cleared for farming and industry. People, governments and conservation groups work to protect them, which keeps more trees standing to store carbon and help make rain.

Satellite instruments such as MODIS and the Tropical Rainfall Measuring Mission let researchers compare greenness and rainfall across the Amazon year by year; tropical forests are hard to measure because dense foliage absorbs visible light and smoke can mimic stress, so scenes with haze are screened out.

Scientists study current droughts to predict forest responses in a climate where rainforest droughts are expected to be more common. National Geographic notes that unsustainable industrial and agricultural development has degraded rainforests, and that people, governments and conservation groups are working to protect them.