Experimental establishment of big-leaf mahogany (Swietenia macrophylla King) seedlings on two soil types in native forest of Para, Brazil

Research into the mechanisms underlying edaphic associations reported for many tropical forest tree species, including those in the Meliaceae, has paid little attention to the earliest post-dispersal life stages. We conducted a reciprocal sowing experiment to examine germination and establishment rates of big-leaf mahogany (Swietenia macrophylla King) during the first seedling growing season in two local edaphic habitats in southeastern Amazonia. In this region, adult populations are restricted primarily to low-ground soils associated with seasonal streams, but rarely are found on high-ground soils. Soil analyses revealed that low-ground soils at the study site were silty loams whereas high-ground soils were clays. High-ground soils were less acidic and more fertile than low-ground soils, especially with respect to total nitrogen, Mg, and sum of basic cations. In October 2003, at 30 locations evenly split between the two soil types, we added 40 seeds to paired canopy gap and closed understory plots on low- and high-ground soils. Two months after sowing, no significant differences between the two soil types were found for the proportion of seeds: (1) ungerminated yet viable, (2) still germinating, (3) already established, or (4) surviving (variables 1, 2, and 3 combined). Nearly twice as many seedlings had established in the understory (43%) than in canopy gaps (24%), and overall survival was lower in canopy gaps than in the understory (46% vs. 58%, respectively). Seven months after sowing, survival was similar between gaps and understory; however, seedling recruitment on high-ground soils was nearly twice that on low-ground soils (38% vs. 20%, respectively; P = 0.006). Localised flooding in low-ground plots drowned seedlings at four locations (16 plots). Difference in seedling performance between soil types weakened when these flooded plots were discarded (corresponding recruitment was 38% vs. 27%, P = 0.083). On average, seedlings showed small differences in leaf damage, leaf numbers, and foliar conditions between edaphic habitats. As expected, seedlings had more leaves and greater between-census survival rates in canopy gaps compared to understory conditions. Collectively, we found little evidence of enhanced early post-dispersal performance on mahogany's preferred soil type in the region. Any ecological processes driving mahogany's association with low-ground forest soils presumably emerge beyond the first growing season. (C) 2007 Elsevier B.V. All fights reserved.