BACKGROUND AND CATASTROPHIC TREE MORTALITY IN TROPICAL MOIST, WET, AND RAIN-FORESTS

The process of tree mortality has dimensions of intensity spatial, and temporal scales that reflect the characteristics of endogenic process es (i.e., senescence) and exogenic disturbances (i.e., severity, frequ ency, duration, spatial scale, and points of interaction with the ecos ystem. Tree mortality events expressed as percent of stems or biomass per unit area, range in intensity from background (<5% yr(-1)) to cata strophic (>5% yr-l), in spatial scale from local to massive, and in te mporal scale from gradual to sudden (hours to weeks). Absolute annual rates of background tree mortality (biomass or stem ha(-1) yr(-1)) can vary several fold depending on stand conditions and tend to increase with stem density. The ecological effects of a catastrophic, massive, and sudden tree mortality event contrast with those of background, loc al, and gradual tree mortality in terms of the direction of succession after the event, community dynamics, nutrient cycling, and possibly s election on trees. When standardized for the return frequency of distu rbance events, area, and topography, the ranking of tree mortality eve nts (trees ha(-1) century(-1)) in the Luquillo Experimental Forest is: background > hurricanes > individual tree fall gaps > landslides. Est imates of vegetation turnover rates require long-term and spatial anal ysis to yield accurate results.