674 The Impact of Tropical Cyclones on Perennial Species in the Mariana Islands

Abstract

The frequency of tropical cyclones is a major factor affecting the vegetation of the Mariana Islands, where these storms are called typhoons. An average of about one typhoon per year has passed within ≈100 km of Guam during the past 50 years. The physiognomy of Guam's natural and urban forests is largely determined by these typhoons. The impact of each typhoon is determined by a long list of interacting factors such as species characteristics; environmental and horticultural conditions preceding the typhoon; the intensity, direction, and duration of winds; the amount of rainfall associated with the typhoon; and the environmental and horticultural conditions following the disturbance. Many species survive typhoons by reducing aerodynamic drag of the canopy by abscising inexpensive leaves or breakage of small stems which results in an intact major structural framework. Speed of recovery for nonlethal damage following disturbance depends on nonlimiting conditions during recovery. Thus, the most destructive typhoons are those that occur in sequence with other environmental stresses. The most common of these may be heat and high-light stress, associated with subsequent high pressure systems, and severe drought conditions. For example, the 230–298 km·h–1 winds of Typhoon Paka in Dec. 1997 were followed by the driest year on record for Guam. Typhoon debris and drought generated 1400 forest and grassland fires from Jan. through May 1998. Sequential typhoons are also severely damaging. For example, Guam experienced three direct eye passages and two more typhoons within 113 km during the months Aug. to Nov. 1992. Damage susceptibility and recovery dynamics will be discussed in relation to these and other physical, chemical, biological, and human-induced factors.