Abiotic and Biotic Influences on Early-Stage Survival in Two Shade-Tolerant Tree Species in Brazil's Atlantic Forest

Regeneration dynamics in tropical forests are driven by many abiotic and biotic factors, including light availability, litter depth, and density-dependent interactions with enemies. Whether ontogenetic stage also can play a critical role, however, is seldom considered. We address how early-stage survival of two shade-tolerant species is affected by canopy openness, litter depth, ontogenetic stage, and conspecific neighborhood in the understories of secondary forest fragments in the Brazilian Atlantic Forest. We monitored the survival of naturally occurring early-stage individuals of one canopy and one understory tree species in six forest fragments for over 2yr. We then compared how different abiotic and biotic variables, as well as the initial height of seedlings and the length of time interval, influenced variation in survival using generalized linear mixed-effects models. Survival of the canopy species was negatively affected by light availability, while for the understory species increasing light availability either increased (for seedlings) or decreased (for saplings) survival. In addition, survival of both species at the seedling stage was positively related to litter depth. Finally, we found that conspecific neighbors were an important biotic factor reducing survival. Our results suggest different regeneration niches for these two tree species related to light availability. Moreover, we found that the effect of both abiotic factors depends on ontogenetic stage, a critical factor for understanding regeneration niches in the forest understory.