Kevin T. Smith, PhD. Supervisory Plant Physiologist Northern Research Station, USDA Forest Service Durham, New Hampshire Handout Page 1 of 2

Tree Decay, Wood Durability, and Strength Wood is an amazing material with a wide range of properties that enable trees to become the largest and oldest individual organisms. Because of the use of wood products, society developed concepts of wood strength and durability for products from structural timbers to toothpicks. These expectations were usually based on idealized wood: consistent, homogenous, and uniform. In past decades, foresters understood wood decay as a source of lost timber volume and economic value. In contrast, today’s arborists recognize that within a tree species and even an individual tree, wood structure can vary greatly. More important than the presence of a tree hollow or cavity is the amount and position of the remaining wood. Branches and stems frequently break at areas of attachment or structural variation and not along the length of an idealized beam. Still, the engineers and researchers in biomechanics have a lot to teach us!

Trees contribute to the livability of our cities and communities.

Dissection can show the relationship of growth and decay that result in both the benefits and the potential liabilities that trees contribute.

Smith—Page 2

Wood properties such as strength (such as stiffness, resistance to tension and compression, hardness, cleavability) vary depending on the orientation of the wood piece.

Wood testing techniques are usually performed on small pieces that are as uniform and homogeneous as possible. However, failures of stem, branches, and roots usually have little relation to those properties as measured in small, clear samples.