Spatial patterns of sapling mortality in a moist tropical forest: consistency with total density-dependent effects

Density-dependent mortality (DDM) is major driver of species coexistence in tropical forests. We assessed the occurrence and strength of this mechanism among saplings (i.e. trees with a diameter of 1-4 cm) in the moist tropical forest of Barro Colorado Island (BCI), Panama, over six inter-census periods (26 years), by using spatial point pattern analysis. We considered conspecific density-dependent mortality, but also total DDM (i.e. the effects of conspecific and heterospecific neighbours all together) upon saplings of all species (i.e. at the community level), and on saplings of light-demanding and shade-tolerant ones, separately (i.e. at the light-guild level). We also analysed the effects of conspecific DDM at the community and light-guild levels. Conspecific density-dependent mortality affected a lower proportion of species than was to be expected. Its strength was not significantly related with species abundance, and it had relatively weak effects at the community and light guild levels. Conversely, we detected a significant spatial signature of total DDM among all saplings, and among saplings of the light-demanding and shade-tolerant species, but its strength showed pronounced temporal variation. Total DDM among light-demanding saplings was stronger than among shade-tolerant ones and its effects were particularly marked 10-15 years after the occurrence of unusually severe droughts associated with El Nino events. Our study indicates that conspecific DDM is relatively unimportant among saplings in comparison with total DDM at community and light-guild levels. This finding contrasts strongly with the results for seedlings, where conspecific DDM was constituted the dominant process. The pronounced temporal variations observed in the occurrence and strength of total DDM among saplings indicate that time-varying events associated with climatic disturbances such as canopy openings or episodes of high recruit emergence might be important drivers of this process. They also emphasize the importance of considering time frames longer than one decade to study density-dependent effects among saplings in tropical forests.