Tree Mortality in Mangrove Forests

Twenty-eight worldwide reports of massive mangrove tree mortalities are reviewed. Massive mortality is defined as tree mortalities that occur in response to rapid environmental change and affect all size dasses. Massive mortality occurs in addition to normal tree mortality. Normal tree mortality was described using structural data from 114 mangrove stands. This mortality is density dependent, follows orderly time dependent patterns dictated by stand maturation (related to average tree diameter), and usually occurs in the smaller diameter size classes. Disease and other biotic factors do not appear to be primary causes of massive mangrove mortalities. Instead, these factors appear to attack forests weakened by changes in the physical environment. Mangrove environments are dynamic and cyclical and mangrove associations adapt to such environments by both growing and dying fast. Mangrove species' characteristics such as the capacity to produce large quantities of propagules that take advantage of dispersal agents, sharp species zonations, and even-aged populations contribute to the rapid growth-mortality cycles in mangroves. Humans may tilt the balance towards higher mortality rates by introducing chronic stressors that inhibit regeneration mechanisms. MANGROVE FORESTS ARE DYNAMIC ECOSYSTEMS. Factors such as sedimentation rates, soil subsidence, freshwater run-off, tidal forces, and changes in sea level influence mangrove growth and survival. In response, mangrove species arrange themselves in zones that reflect geomorphologic and hydrologic gradients (Lugo 1980). Consequently, significant changes in environmental conditions are generally followed by alterations in the vigor or zonation of vegetation. These alterations may include widespread tree mortality.

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