RELATING GUST SPEED TO TREE DAMAGE IN HURRICANE HUGO, 1989

From 17 through 19 September 1989 Hurricane Hugo passed through the Antilles from Guadalupe to Puerto Rico, causing severe damage to ornamental and shade trees. Damage to 1226 trees of 81 species on 18 urban and rural sites was related to maximum wind gust speeds. Damage (defoliation, minor branch breakage, major branch breakage, trunk snap, and tipping) began at gust speeds of about 60 km/hr, increased rapidly with gust speeds to about 130 km/hr, and although highly variable, did not worsen at higher gust speeds. The most severe forms of damage are apparently avoided if the crown surface area is reduced quickly by loss of leaves and twigs. Palms were more wind resistant than broadleaved trees From 17 through 19 September 1989 Hurricane Hugo passed through the Antilles from Guadalupe to Puerto Rico (Fig. 1). Hugo was classified as a category four hurricane, meaning it had sustained winds of at least 210 km/hr but less than 248 km/hr. Twenty-eight deaths and more than $2 billion in material damages were directly attributed to the hurricane in the Antilles (3). Damage to the environment by such natural and cyclical events is beyond inference. It has been estimated that the forests of eastern Puerto Rico are normally affected by hurricanes every 25-30 years and severely affected every 50-60 years (12). One of the most striking evidences of a hurricane's passage is damage to trees. Although the damage is readily visible, surprisingly little is understood about how it relates to wind speed. The objective of this study was to relate damage sustained by exposed trees to measured and estimated gust speeds. Data Collection During the first 2 months following the storm, 18 sites were surveyed. Ideally, each site would have been a 1-km circle around a calibrated wind instrument. However, since so few good measurements of wind intensity were available, other sites with less reliable measurements or only estimates were used. The first five exposed trees of each species encountered were evaluated for degree of storm damage. From 11 to 24 species per site were observed, with a total of 81 species over all 18 sites. The 1226-tree sample consisted of planted ornamental and shade trees and naturally reproduced trees along fences and in undeveloped areas. Trees sheltered by buildings or groves of trees were not sampled. Thrown trees with artificially restricted root systems (such as one-sided root systems that had been cut during construction or that had grown up against a barrier) and trees toppled by the fall of other trees or structures were not sampled. Diameter at breast height (dbh) ranged from 3.5 to 216 cm, with most trees in the 10-50 cm range. The sample trees were classified according to the most severe damage sustained by that tree. The system included six classes that progressed from no damage to windthrow. The wind intensity variable used in this study was maximum gust speed. Not only are gust speeds higher than sustained wind speeds in storms, but gusts displace tree crowns more than constant wind and because of the inertia in swaying trees, repeated gusts can bend a tree crown three or more times as much as constant wind (8). 'i J XCalebra J~^-,-.I Puerto Rico p S ; r T