The growth of forest trees has traditionally been measured in terms of wood products since that is how we used forests. These measurements are valuable since we can compare them with measurements taken decades ago, as well as forest measurements from around the world. We are learning to translate these measurements allowing us to focus on many other aspects of forests that we value today.
The events around the turn of the century that shaped our forests created vast areas of forests that are between 70 and 110 years old. These forests have reached the age where most of the trees are about 18 inches in diameter and the number of trees of smaller sizes has decreased. These large trees are used for veneer and other high quality wood products; so the value of wood products in the forest has increased in value over the last 3 decades.
Also, bigger trees are commonly considered more aesthetically pleasing than smaller trees, and big trees give us a better feeling of health and well being. These increases might reflect normal growth and size increases of the even-aged forests covering our landscape rather than responses to management techniques.
Between 1976 and 1986, the amount of forest land in Maryland supporting sawtimber sized trees increased from 1.4 to 1.7 million acres. In 1986, sawtimber sized trees dominated about 71% of all forest lands capable of producing forest products. A sawtimber tree is larger than 11 inches in diameter for hardwoods or 9 inches in diameter for softwoods. Diameter is measured at 4.5 feet above ground level on a tree or breast height. Figure 4 illustrates dbh or diameter at breast height.
By volume, the commercial wood products growing in Maryland's forests also increased between 1976 and 1986. Hardwood trees of sawtimber size increased in volume from 9.1 to 11.1 billion board feet and softwood trees of sawtimber size increased slightly from 2.0 to 2.3 billion board feet. Figures 3 and 7 illustrate a board foot, a cubic foot and tree size classes.
On an individual species basis, yellow-poplar is the most abundant tree by volume across the State. Loblolly pine and red maple are the second and third most abundant species respectively. These three species account for 44% of the volume. The red oaks, including northern red oak, southern red oak, black oak, scarlet oak, and pin oak, account for 23% of the volume. The white oaks, primarily white oak and chestnut oak, account for 17% of the volume of sawtimber trees (See chart for more information)..
Before 1986, the average annual mortality due to natural causes accounted for less than 1% of the total volume of growing stock on all lands capable of producing wood products. Since 1986, repeated gypsy moth defoliations combined with low precipitation during growing seasons has increased mortality.
Estimates of this increase are difficult to calculate. Mortality surveys were completed 2 years after defoliation by gypsy moth caterpillers on 60% of the land defoliated between 1988 and 1992. These surveys found that the number of trees dying after a defoliation decreased dramatically since the 1989 defoliation.