Dear Dr. Universe: How do some trees survive after being burned in a wildfire?
-S.P., Quilcene, WA
While it might seem like wildfires only cause destruction, they are actually a natural and important part of keeping forests healthy. After many years, trees have adapted to their homes. Some are pretty invincible when it comes to surviving a wildfire.
There are a few ways they can survive, says my friend Andy Perleberg. He’s a forestry expert at Washington State University.
One thing that protects trees from wildfire is thick bark. In Washington state, the most common trees with really thick bark are the western larch and ponderosa pine. Ponderosa pine actually has jigsaw-puzzle shaped pieces of bark. Maybe you have seen these in your neighborhood. Some people call the pieces “scales,” Perleberg said. When on fire, these scales peel back and fall to the floor, taking the fire back to the ground.
The tree makes sugar—its food— through a process called photosynthesis. Under the bark is a very important part of the tree that helps the tree mobilize sugar called the phloem.
It helps move sugars around the tree and to the roots. The thick layer of bark also helps protect the tree’s food-processing system from fire and other damage so it can get the energy it needs to survive.
When a fire happens, some trees will release a kind of sticky, honey-like substance called sap, or pitch. The pitch will flow into cracks where fire could reach that very fragile phloem. It’s kind of like smearing putty over a crack in a wall, Perleberg adds. This leaves the tree with a fire scar, he says, but the tree survives and keeps growing.
Fire ecologists can use these fire scars to trace the patterns of historic fires and how often they happened. Sometimes, fires occur naturally through lightning strikes. Sometimes, fires are man-made, and Native Americans traditionally burned areas to help people survive, encourage certain plants and keep ecosystems healthy.
Some trees have also adapted to shed their lower limbs. As the tree grows higher and higher, some limbs don’t grow anymore. The fire can’t climb up the tree as quickly without the source of fuel to help it along.
After a fire, the trees left standing likely had thick bark or another one of these adaptations. Meanwhile, the rest of the dead trees will also have a new purpose in life.
Dead trees and old plants that turn into ash return important things, called nutrients, to the soil. The old trees also become habitats to some kinds of wildlife that live in the forest. Bark beetles like the weak trees and go in to eat the sugary layer beneath the bark. Other critters, like flying squirrels or tree frogs, might turn a dead tree into their new home in the forest.