Trees Have an Amazing Resistance to Decay Fungi

May 14, 2019

In this month's installment of Field Notes, Scott Bowe of Kemp Station discusses trees and their resistance to decay fungi.

If you have had the opportunity to travel the forests across Wisconsin, you will notice that we have an impressive variety of tree species.  These tree species are an important source of food and shelter for wildlife, but also an important raw material for human use.  But not all wood from trees is created equally. We favor some species for their aesthetic beauty such as the rich chocolate brown color of walnut or the bright white color of sugar maple.  We favor other tree species for its durability in use such as the sturdiness of an oak beam in a 100 year old barn, or the black locust fence post that refuses to rot away by decay fungi.  Let’s take a look at Wisconsin tree species and their ability to resist decay fungi.

Among Wisconsin trees you will find a wide range of resistance to decay fungi.  There is an excellent resource called the Wood Handbook that is published by the Forest Products Laboratory on the UW-Madison campus.  The Wood Handbook is available for free online and lists everything you want to know about wood.  Specific to today’s conversation, it categorizes tree species by decay resistance.  For example, aspen and birch are two species groups that are the least decay resistant.  If a wind storm blows over an aspen stand, that aspen will have to salvaged within a year to have any value as a raw material.  An example of a moderately resistant species would be tamarack.  Our original log buildings at Kemp Station are made from tamarack logs and have held up well for nearly 100 years.  Finally, two examples of Wisconsin species that are very decay resistant are Eastern redcedar and black locust.  Farmers learned early on that black locust make excellent fence posts, which can last for decades.

What makes some tree species more decay resistant than others?  It all comes down to chemistry. Within the main trunk and branches, trees have sapwood and heartwood.  Sapwood consists of the newest growth rings, contains living and dead cells, and measures in 1 to 2 inches from the bark.  Heartwood consists of the older growth rings, contains only dead cells, and continues from the sapwood to the center of the tree.  As an aside, people are often surprised to learn that all of the cells in the heartwood portion of the tree are dead.  This is one of the reasons why a tree cannot heal a wound, it can only cover it up as the bark closes over the damaged area.  As you move from the sapwood to the heartwood you find increased extractive content, which often darkens the color of the heartwood.  In some species, these extractives make the heartwood resistant to decay and insect attack.  Extractives include tannins, coloring matter, essential oils, fats, resins, waxes, and other metabolics.  The name extractives comes from the fact that these chemicals can be removed or extracted from the wood using solvents such as water, alcohol, or acetone.

I mentioned before that black locust is very decay resistant.  What I should have said is the heartwood of black locust is very decay resistant.  If you look at a cross section of a black locust log, you will see that the sapwood is nearly white, while the heartwood is very dark.  The dark color and decay resistance of the heartwood comes from the extractives.  The sapwood portion of black locust is susceptible to decay just like aspen.  The extractives in the heartwood act as a natural chemical defense.

Humans have figured out how to chemically treat wood to increase its performance when used in decay prone environments.  In fact there are dozens of chemicals that are used to treat wood today.  You are all familiar with the green treated lumber at the local building center.  In Wisconsin, we commonly treat red pine to extend its service life.  Copper salts are the main component in green treated lumber.  The copper is what produces the green color and decay resistance.

Some emerging technology that is growing here in the United States, but is more common in Europe is called thermally modified wood.  Thermally modified wood is heated to a very high temperature in the absence of oxygen.  If oxygen were present, the wood would combust during treatment.  After treatment, the chemical structure of wood is modified so it is no longer palatable to decay fungi or insects. Thermally modified wood is popular because you get decay resistance without the use of added chemicals.

Trees are amazing organisms.  They have evolved with very specific adaptations to deal with decay fungi, a quality that mankind has found useful for thousands of years.

For Field Notes, this is Scott Bowe from the University of Wisconsin-Madison’s Kemp Natural Resources Station.