Big-leaf mahogany ( Swietenia macrophylla) seedling survival and growth across a topographic gradient in southeast Pará, Brazil

Abstract

Adult populations of big-leaf mahogany ( Swietenia macrophylla) occur in aggregations along seasonal streams in transitional evergreen forests of southeast Pará, Brazil. To test whether variable seedling survival and growth across topography may underlie this observed distribution pattern, we planted nursery-grown seedlings in the forest understory and in artificial gaps at opposite ends of a slope gradient where mahogany occurs (low-ground hydromorphic soils) and does not occur (high-ground dystrophic soils). At both positions seedling survival and growth were significantly greater through 42 months in gaps than in adjacent forest understories, though mean understory survival exceeded that in gaps through the first growing season. Mean seedling growth in gaps on low ground was significantly greater than growth in gaps on high ground. Under nursery conditions (well watered, 70% full sun light), growth of seedlings planted in soils from low ground was significantly higher than that of seedlings planted in soils from high ground, indicating that differences in soil nutrient status, particularly Ca and Mg, may account for results in the outplanting experiment. Ca+Mg nutrient supplement accelerated growth rates of nursery seedlings planted in high-ground soils relative to growth rates of seedlings planted in low-ground soils, nullifying significant differences between controls. Soil differentiation across topographic relief with consequent gradients in soil nutrient status complements canopy disturbance regimes (increased light levels) in shaping adult distribution patterns and population structures. This implies that recruitment success under natural and artificial regeneration management practices may vary as a function of both gap size and soil fertility.