29. Trees Make Cents

Plant a tree: it will keep you warm in winter, cool in summer, and help you save money all year round.

Money really does grow on trees. That's a message 14-year-old Candice Ray of New Haven, Connecticut, could have used when she testified before Congress two years ago.

"I wanted Washington to know my fears about global warming, pollution, and especially my fears about all the carbon dioxide in the air," says Candice, who testified as part of an environmental campaign called "Beat the Heat." Her proposed solution: Get people to care about trees, Earth's natural air conditioners and pollution filters. "Unless more people care about saving trees," Candice says, problems like pollution and maybe global warming "will get worse."

The results of a new study on trees may help boost Candice's argument—and hit members of Congress where they'll feel it: in the wallet. The study, conducted by the U.S. Forest Service, found that trees make good economic sense. "Trees can actually save cities millions of dollars each year by making it less costly to heat and air-condition buildings, and by reducing air pollution," says Greg McPherson of the Forest Service.

If, like Candice, you want to convince some difficult-to-inspire adults, here are some facts from the study to back up these claims:

First, there's the obvious. Trees provide shade to help cool us in summer. And they block cold winter winds. That saves money on air-conditioning and heating bills—and the fuel it takes to power cooling and heating systems. "By planting three mature trees for shade and windbreak," says McPherson, "a home owner can save $50 to $90 per year on air-conditioning and heating costs alone." Over time, the money saved would far exceed the cost of the trees.

ALL NATURAL AC

But there's more to the cooling effect of trees than simple shade. Trees provide natural air-conditioning through a process called evapotranspiration, or "tree sweating."

We perspire, trees transpire—give off moisture when they get hot. Perspiration and transpiration both produce a cooling effect when the moisture evaporates—changes from liquid to gas. Why? Because this phase change requires energy.

When you sweat, the energy required for evaporation comes from your body as heat. So evaporating sweat "pulls" heat from your body.

But trees don't have warm bodies like us. So their evaporating "sweat" pulls heat from the surrounding air. That lowers the air temperature so you can turn down the AC. Money in the bank!

OXYGEN PUMP

The coolest part is that while trees are doing all that cooling, they're also making the oxygen we need to breathe. The process is called photosynthesis, a series of chemical reactions trees use to make their food. Basically, leaves take in carbon dioxide (CO₂) from the air and combine it with water (H₂O) using the sun's energy. The products: sugar (C₆H₁₂O₆) and oxygen (O₂). The reaction looks like this:

CO₂+ H₂O ⎼⎼⎼⎼⎼→sunlightC₆H₁₂O₆+ O₂

Eventually the trees use the energy in the sugar to build new leaves, branches, and roots—in other words, to grow.

The more trees grow, the more energy they need—and the more O₂ they produce. Plus, if you look closely at the equation you'll see that photosynthesis helps use up one of our wastes—CO₂.

We're not just talking about the CO₂ our bodies exhale, but also the many tons of this gas we spew into Earth's atmosphere when we burn fossil fuels, such as oil and coal. Without trees the CO₂ in car and factory exhaust would accumulate, or build up, in our atmosphere. That would be a problem, scientists say, because CO₂(along with some other gases) forms a layer in our atmosphere that traps the sun's heat just like a greenhouse ceiling. That's why CO₂ is sometimes called a "greenhouse" gas. Some scientists say these gases could eventually trap so much heat that our entire planet will heat up.

Trees can help counteract this "global warming" by taking in the CO₂ and locking it up in the carbon compounds that make up their leaves, roots, and trunks.

POLLUTANT FILTERS

Trees soak up countless other pollutants too, says McPherson—things like smoke, ozone, and particulate matter. "One large, healthy tree can save over $2 per year in pollution removal," he says. Not impressed? Try multiplying to see how much money 2 000 trees would save a city: $4 000.

Of course, adds McPherson, "trees can become stressed if pollution levels get too high." and stressed-out trees won't do any of their jobs well.

Which brings us back to Candice's bottom line: "We need to take care of trees and keep them healthy, so they can keep us healthy," she says. "With more trees, we'll have cleaner, fresher air to breathe, and a healthier planet to live on."

From Science World, Nov. 18, 1994.