老司机传媒

Located within Redwood National Park in California, resides the tallest tree in the world. Known as Hyperion, this behemoth is over 380 feet tall. The most massive tree in the world is the General Sherman Tree, whose trunk has a circumference of almost 103 feet, meaning you would need almost 20 people wrapping their arms around it, fingertip to fingertip, to reach around the entire tree! It is incredible to think about how big these trees truly can grow, but the absolute size of these trees raises an interesting question: where do trees get all of the matter that they use to grow so large? One obvious guess might be that trees absorb material from the ground using their roots, but that is incorrect. Trees don’t need to get their building materials from the ground, because they get it from a different source: the air itself.
We hear about how trees use carbon dioxide in photosynthesis, but for trees to get large amounts of building material from the air just doesn’t seem immediately obvious. After all, the air is something that, in our everyday lives, feels extremely immaterial. The air just kind of flows around us, not getting in our way too much. Hearing that trees mostly build themselves up with material from the air is like hearing that someone built a house out of salt crystals that they extracted by boiling tons upon tons of ocean water. With trees, only small amounts of minerals are actually absorbed through the roots–and as it turns out, there is a fairly simple observation that shows this. When we look at a tree and think about the trunk and all of the leaves and branches, everything besides the roots is above the ground. If the tree was getting matter to build itself up from the soil, then we would expect that the ground around the tree would sink down from material being pulled from the ground upward to build up the trunk, but we all know that often around trees the ground is actually often elevated by their roots extending outward and pushing up the soil. While the roots do indeed absorb water and small amounts of minerals from the ground necessary for growth, that does not make up most of the tree.

Within a tree, most of the dry mass of the wood (that is, the mass excluding water) comes from chemicals that provide structure to the tree, specifically, cellulose and lignin, among others. These structural compounds are composed of carbon, oxygen, and hydrogen–and while oxygen and hydrogen are easily obtained from water and other simple chemicals, carbon is best absorbed from carbon dioxide in the air. As it turns out, since about half of a tree’s mass is water, about one fifth of the mass of a tree is from carbon that it absorbs from the air. Thinking about this, it really goes to show how great trees are for fighting greenhouse gas emissions. Every year, an average-sized tree absorbs on average about 48 pounds of carbon dioxide from the air, which it uses to sustain itself and build itself up. And yet, while trees are essential for regulating the atmosphere’s carbon composition, human deforestation has destroyed about one third of all of the earth’s forests. This deforestation not only removes trees that would otherwise be absorbing carbon dioxide, but the deforestation and often burning of the biofuel obtained from the trees releases the carbon that was previously stored within the tree back into the atmosphere. If growing a tree absorbs large amounts of carbon, then naturally destroying it will release it again. As such, it is essential that we not only stop deforestation, but begin replanting trees, expanding forests. This is especially important since trees provide many other environmental and economical benefits besides carbon capture. It is essential that any strategy for combating climate change contain, as one part of the approach, reforestation.

However, we can’t just plant trees and expect that to solve all of our problems. In 2019 43.1 billion tons of carbon dioxide was released into the atmosphere by human activity, which, assuming 48 pounds of carbon dioxide per year is absorbed per tree, would require almost 2 trillion additional trees to be planted to absorb it all. Planting that many trees is, to put it mildly, not really feasible. As such, we need to be taking major actions to reduce carbon emissions. This includes, of course, actions such as switching to renewable energy sources away from fossil fuels, changing manufacturing processes and electrical consumption to be more efficient so less energy is needed in the first place, as well as many other areas. Once we are able to reduce emissions, then trees will be essential in absorbing the carbon that has already been emitted, and bringing the planet back to its proper balance.