Some people claim that when mature street trees are cut down and replaced with young trees, the urban canopy is just as good as it was before. Others go further, claiming that leveling a whole forest and replanting the area is a smart move, because the young trees will absorb more carbon than the older trees would have. Is this good logic?
The second claim has some appeal to it. Surely small trees, which are actively growing, pull more carbon out of the air than mature trees that are not going to get any bigger. But new research shows that this is not true. Large trees can and do continue to absorb significant quantities of carbon from the air. It’s also important to note that if the cleared trees are burned, or otherwise disposed of in a way that doesn’t keep their carbon safely sequestered, the next generation of trees is just re-absorbing the same carbon that their predecessors were already doing a perfectly good job of storing. That’s clearly no victory.
What about the claim that cutting down a street tree and planting a new one results in no loss to the community? A basic understanding of what trees do shows that this cannot be correct. A young tree cannot filter as much air as a mature tree; it absorbs pollutants less effectively. A young tree cannot take up as much water as a mature tree; it mitigates flooding less effectively. A young tree does not cast as much shade as a mature tree; it moderates temperatures less effectively. A young tree cannot host as many bird nests or produce as much fruit as a mature tree; it provides habitat less effectively.
For all of these reasons, replacing a mature tree with a young tree means a significant loss of ecosystem services over the next several decades, until the new tree catches up to the size of the tree that had already been there. (For the same reasons, when city officials cut down a large tree, replace it with a tree that will never get more than about fifteen feet tall, and say it is just as good, that is also not true.)
Imagine a company that fired all of its employees and replaced them with new staff. Would that company continue to run successfully? Probably not; the loss of experience and institutional knowledge would be too disruptive.
To put it even more starkly, imagine a company that fired all of its employees and replaced them with children. It’s obvious that this company would struggle to continue providing high-quality products and services to its customers.
It’s no different when we destroy mature trees and replace them with young ones. It’s simply not realistic to expect an adolescent tree to do what a fully-grown one can do.