Growth response by big-leaf mahogany ( Swietenia macrophylla) advance seedling regeneration to overhead canopy release in southeast Pará, Brazil

Abstract

Big-leaf mahogany ( Swietenia macrophylla) is a valuable neotropical timber species whose seedling survival and growth dynamics in natural forests are poorly understood. To document regeneration dynamics of mahogany in seasonal transitional evergreen forests of southeast Pará, Brazil, we followed naturally established seedlings in the forest understory and in experimental canopy gaps. We found that only 1–2% of seedlings in the forest understory survived 8 years after germination; mean height growth was approximately 4 cm year −1 of height over this period, confirming mahogany's classification as a light demanding species. To test whether seedlings could survive and respond to canopy release after a period of suppression beneath competing secondary vegetation, we opened experimental canopy gaps above naturally established seedling regeneration in 2–3-year old logging gaps; seedlings were monitored at intervals through 80 months. Seedling survival probability and seedling growth rates demonstrated significant positive correlation with initial seeding height, canopy opening size, and seedling distance to gap edge. Of these factors, initial seedling height (indicating performance in logging gaps prior to experimental release) was the most important predictor of survival and growth response. More seedlings died during the first dry season beginning 6 months after canopy release than during any other census interval. Mean height growth rate of seedlings in gaps peaked at 126.1 ± 23.9 cm year −1 during the third-year census interval, declining afterwards as secondary vegetation overtook seedling crowns. Attack rates by the mahogany shootborer ( Hypsipyla grandella) were low amidst dense secondary vegetation that grew up after canopy opening. These results indicate that (1) advance regeneration experiencing early vigorous growth beneath disturbed canopies in logging gaps may respond to subsequent overhead canopy release, and (2) response vigor may depend on vigor of initial seedling growth.