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 processes (i.e., senescence) and exogenic disturbances (i.e., severity, frequency, duration, spatial scale, and points of interaction with the ecosystem). Tree mortality events expressed as percent of stems or biomass per unit area, range in intensity from background (5% yr-1), in spatial scale from local to massive, and in temporal scale from gradual to sudden (hours to weeks). Absolute annual rates of background tree mortality (biomass or stem ha-1yr-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, local, and gradual tree mortality in terms of the direction of succession after the event, community dynamics, nutrient cycling, and possibly selection on trees. When standardized for the return frequency of disturbance events, area, and topography, the ranking of tree mortality events (trees ha-1century-1) in the Luquillo Experimental Forest is: background > hurricanes > individual tree fall gaps > landslides. Estimates of vegetation turnover rates require long-term and spatial analysis to yield accurate results.