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How do trees die?


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How and why do trees die? – Anish K., age 11, Boston, Massachusetts

Trees can die suddenly or quite slowly.

Fire, flood or wind can cause rapid death from a tree’s ability to do serious damage transporting water and nutrients up and down its trunk.

Sometimes one serious insect infestation or disease can kill a tree. This kind of death usually lasts for several months to a few years. Again, a tree loses its ability to move water and nutrients, but does so in stages, more slowly.

A tree can also die of what you might call old age.

I’m a scientist studying trees and the web of living things that surrounds them. The death of a tree is not exactly what it seems, because it immediately leads to new life.

Different trees, different life spans

Photo of a huge old living tree.
An old bristlecone pine (Pinus longeva) in Patriarch Grove in the White Mountains of California.
Nicholas Turland/flickr, CC BY-ND

Trees can live incredibly long, depending on what kind they are. Some brush cone pinefor example, are among the oldest known trees and are more than 4,000 years old. Others, such as logs or poplars, will have a much shorter lifespan, from 20 to 200 years. The largest trees in your neighborhood or city are probably somewhere in that range.

You’ve probably noticed that different living things have different lifespans – a hamster generally won’t live as long as a cat, which won’t live as long as a human. Trees are no different. Their lifespan is determined by their DNA, which you can think of as the operating system embedded in their genes. Trees programmed to grow very quickly will be less strong – and have a shorter lifespan – than those that grow very slowly.

But even a tough old tree eventually dies. The years and years of damage done by insects and microscopic critters, combined with abuse from the weather, will slowly end its life. The process of death may begin with a single branch, but will eventually spread to the entire tree. It may take an observer a while to realize that a tree has finally died.

You could see death as a passive process. But in the case of trees, it is surprisingly active.

The underground network

Roots do more than anchor a tree to the ground. They are the place where microscopic fungi attach and act as a second root system for a tree.

Photo of thin cobweb-like filaments attached to roots.
Some fungi look like fragile cobwebs, but these tiny tubes act like superhighways underground.
André-Ph. D Picard, CC BY-SA

Fungi form long, superfine threads called hyphae. Fungal hyphae can reach far beyond the roots of a tree. They extract nutrients from the soil that a tree needs. In return, the tree pays back fungi sugars it makes from sunlight in a process known as photosynthesis.

You may have heard that fungi can also pass nutrients from one tree to another. Almost every tree you see is connected to other trees through a complex underground network of fungi enables trees to communicate and help each other. Many scientists call this underground network the “wood wide web.”

When an old tree begins to die, begins to give away its nutrients to nearby treesincluding baby trees, through its fungal network.

Afterlife of a tree

Before falling over, a dead tree can stand for many years and provide a safe home for bees, squirrels, owls and many more animals. Once it falls and becomes a tree trunk, it can house other living creatures such as badgers, moles and reptiles.

A mossy trunk of a dead tree lies in the forest.
One day the remains of this tree will be completely gone.
Swen Pförtner/photo alliance via Getty Images

Tree trunks also harbor another type of fungi and bacteria called decomposers. This tiny organisms help break down large dead trees to the point where you would never know they existed. Depending on the circumstances, this process can be from a several years to a century or more. As wood breaks down, the nutrients return to the soil and become available to other living things, including nearby trees and fungal networks.

A tree leaves a legacy. During life it provides shade, a home for many animals and a lifeline for fungi and other trees. When it dies, it continues to play an important role. It gives a boost to new trees ready to take their place, shelter for another group of animals, and ultimately nourishment for the next generation of living creatures.

It’s almost as if a tree never really dies, but just passes on its life to others.

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