1996 National Technical
Forest Health Issues
Damage caused by pathogens, insects, storms, and human activities can significantly affect the growth, reproduction, and mortality of trees.2 In the FHM program, tree damage is recorded only if it is considered serious enough to increase the probability that a tree will be infected by lethal pathogens (such as open wounds or broken branches), that a tree will die prematurely (presence of pathogenic conks, cankers, or broken roots), or that the growth and/or reproduction of the tree will be seriously depressed (such as high defoliation or broken branches). To be recorded, damages must meet or exceed set thresholds; i.e., >20 percent bole circumference with an open wound; >30 percent of the foliage damaged more than 50 percent (Milke and others 1995). Only serious damage is recorded, and damage estimates are repeatable and comparable from State to State. This approach to quantifying tree damage produces an estimate of damage severity at the plot level.
Tree damage is analyzed as incidence of damage and severity of damage. Incidence of damage is an analysis of the number of trees affected by significant damage, thus indicating the frequency of encountering damaged individuals. The severity of damage is a quantification of the amount of damage at the plot level. Analyzing both incidence and severity of damage gives some indication about the proportion of trees injured and the gravity of the damage to the affected trees.
Damage incidence Incidence of tree damage is simply the number of trees on a plot with one or more damages divided by the total number of trees on the plot. Tree-damage incidence can help differentiate whether damage severity at the plot level represents a few trees badly damaged or many trees with less severe damage. Because only serious damages are reported, incidence of damage is a good indicator of the percentage of trees on a plot with an increased probability of reduced growth or premature mortality.
The spatial pattern of damage incidence was visually evaluated for the Eastern States in 1995 and evaluated using plot-level values for the Western States in 1993 through 1995. Damage data before 1993 were not used because field methods changed significantly in 1993. The spatial pattern of tree damage incidence for all species indicated that more than 20 percent of the trees per plot had significant damage in many plots in the Northeast and Great Lakes States (fig. 12). Some plots in the mid-Atlantic States, Colorado, and California had similar average numbers of damaged trees.
Tables 3, 4, and 5 show the average percent of trees damaged in the East and West for the years 1993 through 1995. Because the number of plots in this class decrease from north to south, the high number of trees injured may reflect the more severe winters that occur in northern forests each year.
Areas with a relatively low percentage of trees damaged include southern Virginia, northern Alabama and Georgia, and northern California (fig. 12). When all species are stratified into hardwood and softwood species, hardwood species in California have more plots with higher levels of percent trees damaged (fig. 13) than softwood trees (fig. 14). In Colorado, hardwood and softwood trees had similar amounts and distribution of tree damage.
Damage severity Tree damage severity is composed of three variables: location on the tree, type of symptom, and severity of the symptom. Location of the injury affects the severity; for example, injury near the base of the tree is more serious than injury near the apex of the tree because parts of the crown can be lost without killing the tree. Similarly, some damage symptoms are more serious than others; for example, open wounds can heal if they do not become infected and therefore are not as serious as cankers, which are caused by fungi that kill the bark and cambium. The severity of the symptom is simply an estimate of the area affected; for example, a canker affecting 80 percent of the tree-bole circumference is more serious than a similar canker affecting 30 percent of the tree-bole circumference. These three variables are combined in a multiplicative index (see footnote 2) to score each tree for damage. This index was developed following several workshops of Federal, State, and university experts in forest pathology and entomology.
Tree scores are aggregated to plot-level scores. Up to three damages per tree can be scored. The damage index has a range of 0 to 21.7; that is, a tree with three serious damages of maximum severity occurring near the base of the tree would have a damage index score approaching 21.7. In general, a tree (or plot-level aggregates of tree species, types, etc.) with a high damage index indicates multiple damages, severe types of damage, and extensive damages with the damages occurring near the base of the tree. The damage index (DI) for the plot is computed as:
For evaluation of spatial patterns, the range of plot-level damage index scores was divided into three classes: 0 to 1.8, 1.9 to 3.6, and 3.7 to 14.0. Because most trees typically have only one serious damage (see footnote 2), the 0 to 1.8 range can be broadly interpreted to represent plots where, at a maximum, 25 percent of the trees have one serious damage in a critical location.
The spatial pattern of tree damage severity for all species indicates that the highest numbers of plots with relatively severe damage occurred in the Great Lakes States (fig. 15). Hardwood species had the highest relative damage severity in the Great Lakes States, in the northern parts of New England, and to a lesser extent, in the two Western States (fig. 16). Softwood species in general were not as damaged as hardwood species, although some parts of the Great Lakes States, northern New England, and southern Alabama had groups of plots with relatively high damage levels (fig. 17).
The combination of biotic and abiotic damage to trees was highest for hardwood species and most common in the Great Lakes States and parts of New England. While the number of trees damaged was relatively high in several areas of the country, the damage severity was relatively high only in parts of the Great Lakes States and New England, suggesting that, in many cases, the number and severity of the damages overall were relatively low. That is, even though more than 20 or 40 percent of the trees on a plot might be damaged, the damage to most trees was not very severe.
Return to Table of Contents
Return to FHM Home Page