The Importance of Tree Size and Fecundity for Wind Dispersal of Big-Leaf Mahogany

Seed dispersal by wind is a critical yet poorly understood process in tropical forest trees. How tree size and fecundity affect this process at the population level remains largely unknown because of insufficient replication across adults. We measured seed dispersal by the endangered neotropical timber species big-leaf mahogany (Swietenia macrophylla King, Meliaceae) in the Brazilian Amazon at 25 relatively isolated trees using multiple 1-m wide belt transects extended 100 m downwind. Tree diameter and fecundity correlated positively with increased seed shadow extent; but in combination large, high fecundity trees contributed disproportionately to longer-distance dispersal events (>60 m). Among three empirical models fitted to seed density vs. distance in one dimension, the Student-t (2Dt) generally fit best (compared to the negative exponential and inverse power). When seedfall downwind was modelled in two dimensions using a normalised sample, it peaked furthest downwind (c. 25 m) for large, high-fecundity trees; with the inverse Gaussian and Weibull functions providing comparable fits that were slightly better than the lognormal. Although most seeds fell within 30 m of parent trees, relatively few juveniles were found within this distance, resulting in juvenile-to-seed ratios peaking at c. 35–45 m. Using the 2Dt model fits to predict seed densities downwind, coupled with known fecundity data for 2000–2009, we evaluated potential Swietenia regeneration near adults (≤30 m dispersal) and beyond 30 m. Mean seed arrival into canopy gaps >30 m downwind was more than 3× greater for large, high fecundity trees than small, high-fecundity trees. Tree seed production did not necessarily scale up proportionately with diameter, and was not consistent across years, and this resulting intraspecific variation can have important consequences for local patterns of dispersal in forests. Our results have important implications for management and conservation of big-leaf mahogany populations, and may apply to other threatened wind-dispersed Meliaceae trees

Seed Dispersal of Big-Leaf Mahogany (Swietenia Macrophylla) and its Role in Natural Forest Management in the Yucatan Peninsula, Mexico

Seed dispersal of big-leaf mahogany (Swietenia macrophylla) and its role in natural forest management in the Yucatán Peninsula, Mexico. As part of efforts to design appropriate silvicultural methods for sustainable forest management of big-leaf mahogany (Swietenia macrophylla: Meliaceae) in natural mixed forests of Quintana Roo, Mexico, a study of seed dispersal of 11 mahogany seed trees was undertaken. Both small (50−74 cm diameter at breast height, dbh) and large (75−100 cm dbh) seed trees showed seed distributions that were skewed to the west, generally matching the prevailing easterly trade winds. Total mean seed production of trees in the smaller size class was one-half of that of the large trees. Maximum seed dispersal distance was 50 m in the westerly directions (NW−W−SW) and only 20−30 m in other directions. Total seed dispersal areas were 0.4 and 0.5 ha for the small and large trees respectively. These results are important for designing seed tree or shelterwood regeneration methods for mahogany; they provide guidelines for creating the size and spatial layout of an overstorey felling and site preparation treatment that would match the dispersal area

Six years of fruit production by mahogany trees (Swietenia macrophylla King): patterns of variation and implications for sustainability

Although mahogany, the most commercially important timber tree in Neotropical forests, is widely acknowledged to be threatened by unsustainable logging which does not provide for its regeneration, its fruiting dynamics are poorly understood. During each of six successive years, the authors measured tree diameters and counted woody fruit capsule segments that fell below the crowns of 82 mahogany trees in natural forest in central Quintana Roo, Mexico. Sample trees ranged from <20 cm to more than 100 cm DBH. Fruit production increased with diameter, and trees 75 cm produced significantly more fruits each year than did trees of smaller diameters. Large trees could produce more than 700 fruits/year. Trees 75 cm DBH were also more consistent producers: while up to 27% of trees <75 cm DBH produced <1 fruit/year in any year, at least 93% of larger trees produced fruit every year. Over the 6 years, individual trees 75 cm produced a total of 367 34 fruits, as compared to 91 8 fruits among trees <75 cm (an average of 61 7 and 15 2 fruits/year, respectively). However, the number of fruit produced per unit crown volume was not significantly different between the two size categories. Fruit production varied among years, with the highest production in 1998 and the lowest in 1999 and 2000. The population of sampled mahogany trees produced approximately three times more fruit, and individual trees produced up to five times more fruit, in the year of highest production, as compared to the lowest. Mahogany does not exhibit the fruit production patterns found in mast fruiting species. Inter-annual variability in size of fruit crops at both the population and individual-tree levels are lower than in masting species, and the synchronocity among trees was low. Basal area growth averaged 76.5 18.6 cm2/year for trees 75 cm and 29.9 1.7 cm2/year for trees <75 cm DBH, and varied significantly among years. Years of high fruit production were also years of good growth, implying ‘‘resource matching’’ rather than the ‘‘resource switching’’ associated with masting. To ensure seed production for the regeneration of mahogany it is important to retain mahogany trees 75 cm DBH as seed sources. This presents a challenge, since currently mahogany trees in this region are harvested down to a minimum diameter as low as 55 cm

Six years of fruit production by mahogany trees (Swietenia macrophylla King): patterns of variation and implications for sustainability

Although mahogany, the most commercially important timber tree in Neotropical forests, is widely acknowledged to be threatened by unsustainable logging which does not provide for its regeneration, its fruiting dynamics are poorly understood. During each of six successive years, we measured tree diameters and counted woody fruit capsule segments that fell below the crowns of 82 mahogany trees in natural forest in central Quintana Roo, Mexico. Sample trees ranged from \u3c20 cm to more than 100 cm DBH. Fruit production increased with diameter, and trees ≥75 cm produced significantly more fruits each year than did trees of smaller diameters. Large trees could produce more than 700 fruits/year. Trees ≥75 cm DBH were also more consistent producers: while up to 27% of trees \u3c75 cm DBH produced \u3c1 fruit/year in any year, at least 93% of larger trees produced fruit every year. Over the 6 years, individual trees ≥75 cm produced a total of 367 ± 34 fruits, as compared to 91 ± 8 fruits among trees \u3c75 cm (an average of 61 ± 7 and 15 ± 2 fruits/year, respectively). However, the number of fruit produced per unit crown volume was not significantly different between the two size categories. Fruit production varied among years, with the highest production in 1998 and the lowest in 1999 and 2000. The population of sampled mahogany trees produced approximately three times more fruit, and individual trees produced up to five times more fruit, in the year of highest production, as compared to the lowest. Mahogany does not exhibit the fruit production patterns found in mast fruiting species. Inter-annual variability in size of fruit crops at both the population and individual-tree levels are lower than in masting species, and the synchronocity among trees was low. Basal area growth averaged 76.5 ± 18.6 cm2/year for trees ≥75 cm and 29.9 ± 1.7 cm2/year for trees \u3c75 cm DBH, and varied significantly among years. Years of high fruit production were also years of good growth, implying “resource matching” rather than the “resource switching” associated with masting. To ensure seed production for the regeneration of mahogany it is important to retain mahogany trees ≥75 cm DBH as seed sources. This presents a challenge, since currently mahogany trees in this region are harvested down to a minimum diameter as low as 55 cm