Maintaining genetic diversity in the Amazon: Species-specific strategies are more effective for managed forests than generalist criteria in Brazilian legislation

Abstract

Pollen and seed dispersal affects both gene flow and the spatial distribution of tree species and, consequently, their population. Forest management operations, in their different intensities and selectivities, cause alterations in the species population density and can negatively affect these dispersal processes. In the present study, we evaluated in four Amazonian forest areas the percentage of trees at distances less than the mean pollen dispersal distance (PDD) and the volume of wood harvested after simulating five cutting criteria for the species Manilkara elata, Hymenaea courbaril and Handroanthus serratifolius. We simulated the cutting criteria of the Brazilian law of 1980 (minimum cutting diameter, MCD >45 cm) and 2006 (MCD >50 cm). We also propose three additional criteria using distance (CD) that consider an MCD of 45 cm and the minimum distance between pairs of the same species that we call minimum cutting distance (MCDistance). In CD1, the harmonic mean of the PDD of the three studied species was considered MCDistance. In CD2, MCDistance was the mean PDD for each species across all areas. In CD3, MCDistance was the mean of the PDD of the first quartile of distances in the no-cut situation for each species in each area. Results indicate that both MCD and MCDistance can be used as criteria to increase the percentage of trees with adequate distances for pollination, promoting genetic diversity and forest regeneration. It was also observed that as the distance between plants decreased in the simulations, the collected volume was reduced, indicating that more restrictive harvesting criteria — allowing fewer trees to be cut — resulted in more trees remaining at a distance smaller than the pollen dispersal distance. However, MCD does not approach the problem directly; it is influenced by the distribution of diameter at breast height (DBH) in an area and affects the reproductive and genetic characteristics of the population. In contrast, the species-specific MCDistance criterion is more effective since it directly addresses the issue of trees being too sparsely spaced to exchange pollen. The general MCD allows only smaller, thinner individuals — often younger and not yet in the reproductive age — to persist, disrupting population dynamics by preventing larger, older trees from exchanging pollen. To determine the best MCDistance values for each species, new studies are necessary to characterize the pollen dispersion for different Amazon species. Also, it would be helpful to create a unified platform where this information could be archived and accessed and to improve communication between scientific studies, legislation, and logging in practice.