Dr. Universe,

Do trees still create oxygen and clean the air after their leaves fall off?

Nova, 8, Palouse

Dear Nova,

The trees that lose their leaves in fall, such as chestnuts, oaks, aspens and maples, are called deciduous trees. Once they lose their leaves, most aren’t able to take in carbon dioxide gas from the air or produce any oxygen.

That’s what I found out from my friend, Kevin Zobrist, a professor of forestry at Washington State University.

“Don’t fret, though,” Zobrist said. “For they more than make up for it in the summer.”

Leaves play a big part in how trees take in carbon dioxide gas from the air and create the oxygen gas that we all breathe. These gases come in and out of a tree through tiny pores on its leaves called stomata.

These gases are part of a process called photosynthesis. Trees take in carbon dioxide from the air, use sunlight as energy to turn that carbon dioxide into sugars, and then use those sugars as their food. In this process, trees also make oxygen.

Photosynthesis actually occurs in the green parts of the leaf called chloroplasts. These chloroplasts are what give leaves their color.

But as leaves start to lose their green colors in fall and winter, they can no longer do photosynthesis. However, there are some deciduous trees, such as aspens, that have green stems.

Zobrist told me some of these stems can actually do photosynthesis, as well. If the temperatures are warm enough in winter, the stems start to photosynthesize.

But in this case, the tree doesn’t take carbon dioxide from the air. Instead, it uses some carbon dioxide that it makes on its own.

In addition to photosynthesis, trees also go through a process called respiration. The tree will use some of the sugars it makes from photosynthesis to carry out different jobs in their daily lives.

As the sugar molecules break apart, they release energy. This process requires trees to take in oxygen and release carbon dioxide. If you’re thinking that sounds just like the opposite of photosynthesis, you are right. This process happens in both the leaves and the stems.

Trees with green stems may use some of this tree-made carbon dioxide to do photosynthesis after their leaves fall off. Still, there’s not nearly as much photosynthesis going on in winter as there is in summer.

Trees do most of the work creating oxygen and cleaning the air of gases like carbon dioxide in the spring and summer. For the most part, they take a kind of fall and winter vacation.

Still, at any given moment there is a tree on our planet creating the oxygen that we breathe. After all, even though it might be winter where you live, that means it is summer elsewhere on the planet.

While the branches in your neighborhood might be bare, in other parts of the world people are starting to see trees growing their new leaves.

Have a science question? Ask Dr. Wendy Sue Universe, WSU’s resident science cat and writer, by email at Dr.Universe@wsu.edu.

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