There are lots of excellent reasons to plant trees, but doing so to sequester carbon is a reflection of short-term thinking. A tree does harvest CO2 from the air and converts it to various organic structures. If it is a deciduous tree, all the leaves that sequestered carbon for the summer will return that carbon to the environment when the leaves decay in the fall and winter.
The CO2 that was sequestered in cellulose and lignin will be trapped in the wood for a longer period of time, but when that tree dies, the cycle reverses: CO2 is released as the wood decays. The CO2 sequestration can be prolonged by cutting the tree down while it is alive, using the wood to make furniture for example, and covering the furniture with a layer of lacquer or paint or other preservative. The ultimate sequestration of course is to bury the dead tree underground and let it turn into coal.
That old-growth forest of towering giant trees is not a net oxygen producer. The living trees utilize photosynthesis and release O2, but the dead ones on the forest floor are decaying, consuming oxygen as the wood is "burned" back to CO2. If you want to glorify a trees as an oxygen source, point to a stand of quick-growing "rubbish" trees or a tree farm of new plantings, not the ancient rain forest or the taiga.
I've held this view for a long time, but have seldom expressed it in social settings because the sentiment is so unpopular and frankly a "downer" in conversation, so why bother. But the recent uproar about Amazonian fires has produced so much hyperbole that the Atlantic boldly posted an article entitled The Amazon is Not Earth's Lungs. Herewith some excerpts:
The Amazon is a vast, ineffable, vital, living wonder. It does not, however, supply the planet with 20 percent of its oxygen.The rest of the article is worth reading in toto.
As the biochemist Nick Lane wrote in his 2003 book Oxygen, “Even the most foolhardy destruction of world forests could hardly dint our oxygen supply, though in other respects such short-sighted idiocy is an unspeakable tragedy.”
The Amazon produces about 6 percent of the oxygen currently being made by photosynthetic organisms alive on the planet today. But surprisingly, this is not where most of our oxygen comes from. In fact, from a broader Earth-system perspective, in which the biosphere not only creates but also consumes free oxygen, the Amazon’s contribution to our planet’s unusual abundance of the stuff is more or less zero...
That is, Peters wanted to know what would happen to the atmosphere if you burned down not just the Amazon, but every forest on Earth, every blade of grass, every moss and lichen-spackled patch of rock, all the flowers and bees, all the orchids and hummingbirds, all the phytoplankton, zooplankton, whales, starfish, bacteria, giraffes, hyraxes, coatimundis, oarfish, albatrosses, mushrooms, placozoans—all of it, besides the humans.
Peters pulled up the next slide. After this unthinkable planetary immolation, the concentration of oxygen in the atmosphere dropped from 20.9 percent to 20.4 percent. CO2 rose from 400 parts per million to 900—less, even, than it does in the worst-case scenarios for fossil-fuel emissions by 2100. By burning every living thing on Earth.
“Virtually no change,” he said. “Generations of humans would live out their lives, breathing the air around them, probably struggling to find food, but not worried about their next breath.”
On their own, then, trees—and even entire forests and seas of plankton—are not enough to fill the atmosphere with a surplus of oxygen. If 99.99 percent of the vast reservoir of oxygen created by the living world is consumed by the living world, that gets you an atmosphere with 0.01 percent oxygen, not our modern 20.9 percent. Photosynthesis is a necessary but not sufficient condition for a world that is hospitable to white-hot oxygen-burning furnaces like us.
“The notion that we owe the breath we breathe to the rain forest, or the [phytoplankton] off the rain forests’ coasts, is just a little bit misinformed on the long timescale,” says Peters.
You don’t get to 20.9 percent, or an atmosphere that can host animal life, without geologic time, and without the fossil record. The tiny remainder of photosynthetic stuff that isn’t consumed and respired again by life—that 0.01 percent of plants and phytoplankton that manages to escape from this cycle of creation and destruction—is responsible for the existence of complex life on Earth. It’s the organic carbon that, once created, doesn’t get consumed again.