Biology Now, 2e

Amazon on Fire ■ 335

moist air rises and cools, releasing moisture

as rain or snow depending on temperature,

and then sinks back to the ground as dry air

(Figure 18.13). These convection cells, in combi-

nation with the angle of sunlight striking the

Earth, play a large role in the creation of regional

environments on Earth, such as rainforests and

deserts.

In the Amazon, trees are very important

players in the water cycle, adds Coe. “The trees

pull water out of the soil and evaporate it into

the atmosphere in the process of photosyn-

thesizing (through transpiration), so they’re

the mediators between the rainfall and the

streams,” he says. “Burning the trees greatly

reduces the amount of water getting back into

the atmosphere.” (See Chapter 5 for more on

photosynthesis.)

Each year of Balch’s study, Coe joined Balch

at the test site. There, his team dug 10-meter-

deep soil pits—long, dark caverns in which they

inserted instruments to measure the moisture

content of the soil. In a healthy forest ecosys-

tem, trees absorb a lot of the water in the soil,

leaving it nice and dry, with only minimal water

runoff into streams. This is what Coe observed

in the control plot where nothing was burned. “A

healthy forest uses up almost all the water [in

the soil],” says Coe.

But in the other two plots—burned every

year or every 3 years—he found that the soil in

the pits was wet to the touch. When the forest

burned, trees died, so nothing absorbed the

moisture from the soil. Consequently, nearby

streams were overflowing with water—up to

four times the volume of water seen in healthy

forests. “That’s not a good thing,” says Coe.

“We’re circumventing the natural cycle. Instead

of this water going back into the atmosphere,

creating more rain and driving vegetation, it’s

flushing the water out of the system.”

And on the burned plots, the invasive

grasses that took the place of the trees have

very shallow roots, absorb less moisture from

the ground, and evaporate less water into the

air. In this way, deforestation of large areas—

whether through unintentional wildfires or

the intentional cutting down of trees—results

in less rainfall and hotter temperatures. “If

you deforest enough of it, you’re going to really

decrease the rainfall over a broad swath of this

region,” says Coe.

Figure 18.13

Earth has four giant convection cells

Two giant convection cells are located in the

Northern Hemisphere and two in the Southern

Hemisphere.

Q1: How do the patterns of rainfall in

the Northern and Southern Hemispheres

compare?

Q2: How do the patterns in the kinds of

environments shown in the Northern and

Southern Hemispheres compare?

Q3: What happens at the equator to make

this region so wet?

Some of

the cool,

dry air

sinks and

is then

deflected

both north

and south.

Warm,

moist

air cools

as it

rises,

causing

rainfall.

Warm, moist air

rises; rain falls.

The clash of moist,

warm air and polar cold

fronts produces rain.

Cool, dry air sinks.

Cold, dry air sinks.

60°

30°

0° Equator

30°

60°

North

Pole

Cell 2

north

Cell 1

north

Cell 1

south

Cell 2

south

South

Pole

Grassland

Grassland

Tropical

rainforest

Deciduous forest

Deciduous forest

Tundra

Desert

Desert

Boreal

forest

Tundra

The Carbon Games

In addition to measuring the hydrologic cycle

and the expansion of grasses into the region, the

teams led by Balch and Coe measured how much