Forest Pests: Insects, Diseases & Other Damage Agents

Storm Damage Forest Health Guide for Georgia Foresters Written by Terry Price, Georgia Forestry Commission Adapted for the web by the Bugwood Network

Storms of various types and intensities do considerable damage to forest stands. Based on best estimates during the last three decades, over 51 million board feet and 1.6 million cords of pine and hardwood were destroyed following 30 reported storms in Georgia.

Tornadoes occur more frequently than ice storms but do not cover as large an area. Georgia Forestry Commission records indicate that 24 tornadoes did an estimated $7 million of damage that averaged over $291,666 per occurrence (1973-1994). However, four major ice storms occurring during the same period did over $26 million of damage or $6.5 million per occurrence.

Numerous other tornadoes and ice accumulations occurred but did only light damage and were not recorded. As the records indicate, tornadoes occur more frequently during April and May and ice storms are more common during January and February.

Hailstorms are usually associated with severe thunderstorms and occur more frequently during spring and summer. Most hailstorms do very little noticeable damage to forests, but occasionally will cause serious defoliation and branch damage. A hailstorm in 1985 damaged about 400 acres of pine and hardwood in northeast Georgia. The damage was so severe that wood sawyers (Monochamus spp.) and Ips beetles infested the stand that promulgated a clearcut (Figure 166).

Figure 166 photo by Bob McMurry

Land managers can reduce the impacts of high winds and ice by regulating stand densities. Pine plantations in areas likely to be hit by ice should not be thinned more than one-third of the total basal area at a time. Shortleaf and Virginia pines are more resistant to ice than loblolly. Loblolly should be maintained at denser stockings than shortleaf and Virginia pines.

Pine stands that have been damaged by winds, hail or ice should be evaluated as soon as possible. Damaged trees should be salvaged to prevent pine bark beetles (see pine bark beetles).

Stem breakage and windthrow are severe effects of wind damage that may occur depending upon the windspeed and duration of wind. Other factors contributing to wind damage are stand age and structure, species characteristics, heart decay, fusiform rust and root disease. Trees weakened by decay or cankers are more likely to snap and those infected with annosum root rot are more likely to be windthrown.

Tree species vary in their ability to withstand strong winds. Wind resistance is often determined by several factors: depth and mass of the root system, live crown ratio, wood strength and amount of rainfall. Pine trees often have deep-rooted tap roots provided they have been planted properly and the roots have not encountered any hard objects such as rock, heavy clay or hardpan.

Hardwoods in general have shallower taproot systems than pines and it is for this reason, pines are more likely to snap from strong winds and hardwoods are more prone to uprooting. Rainfall also increases the likelihood of trees being uprooted. Once the soil has become saturated with moisture, roots are easily dislodged by strong gusts of wind or sustained winds associated with hurricanes and tornadoes. The size and shape of the crown is critical in determining the amount of wind resistance. Trees with large, dense crowns catch more wind and are more likely to blow over. The amount of breakage depends a lot on the strength of the wood that varies among tree species.

Table 6 compares the relative resistance of selected trees to hurricane related damage.

Table 6: Resistance of selected tree species to hurrican related damage in descending order of resistance. (Source: United States Department of Agriculture, Forest Service, Southern Region, Management Bulletin R8-MB 63).

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