The density of a tree significantly affects the properties of wood products. Density is determined by a number of factors, including wood species, growth condition (such as type of soil, available water, and sunlight) and competition from other trees and plants.
This image of two southern pine logs below shows the drastic effect growth conditions can have on wood density. Although both were of the same species, the tree on the left grew slowly and has high density, whereas the tree on the right grew quickly and has low density.
The density of a tree can be determined before it’s cut down using an increment borer to sample the wood in the tree. This photo from the 1960s show’s Forest Service employee Richard Nielsen taking a sample with an increment borer.
Here’s what the increment borer wood sample looks like: Note the annual growth rings. By counting the rings, the age of the tree can be determined; measuring the space between the rings can estimate the rate of growth. The density can also be estimated by weighing the sample and knowing its volume.
A western wood density survey was conducted in the 1960s by FPL along with industry and university partners. Some timber was commercially inaccessible, such as this sample of Englemann spruce, which had to be packed out of the forest on horseback.
Back at the Lab, the researchers got to work. Here, Dimitri Pronin and Arnie Okkonen weigh disks of wood to determine specific gravity.