Seed and pollen dispersal distances in two African legume timber trees and their reproductive potential under selective logging

Abstract

The natural regeneration of tree species depends on seed and pollen dispersal. To assess whether limited dispersal could be critical for the sustainability of selective logging practices, we performed parentage analyses in two Central African legume canopy species displaying contrasted floral and fruit traits: Distemonanthus benthamianus and Erythrophleum suaveolens. We also developed new tools linking forward dispersal kernels with backward migration rates to better characterize long-distance dispersal. Much longer pollen dispersal in D. benthamianus (mean distance d p = 700 m, m p = 52% immigration rate in 6 km 2 plot, s = 7% selfing rate) than in E. suaveolens (d p = 294 m, m p = 22% in 2 km 2 plot, s = 20%) might reflect different insect pollinators.At a local scale, secondary seed dispersal by vertebrates led to larger seed dispersal distances in the barochorous E. suaveolens (d s = 175 m) than in the wind-dispersed D. benthamianus (d s = 71 m). Yet, seed dispersal appeared much more fat-tailed in the latter species (15%-25% seeds dispersing >500 m), putatively due to storm winds (papery pods). The reproductive success was correlated to trunk diameter in E. suaveolens and crown dominance in D. benthamianus. Contrary to D. benthamianus, E. suaveolens underwent significant assortative mating, increasing further the already high inbreeding of its juveniles due to selfing, which seems offset by strong inbreeding depression. To achieve sustainable exploitation, seed and pollen dispersal distances did not appear limiting, but the natural regeneration of E. suaveolens might become insufficient if all trees above the minimum legal cutting diameter were exploited. This highlights the importance of assessing the diameter structure of reproductive trees for logged species.

Patterns of Effective Pollen Dispersal in Larch: Linking Levels of Background Pollination with Pollen Dispersal Kernels

“…The sampling limitations of large continuous populations are problematic because the precision of parameters of fat-tailed dispersal kernels depend mainly on the number of distant dispersal events [23,26,38,56], and limiting the area of sampling of potential paternal individuals (truncating the maximum dispersal distance within the plot) has negative impacts on the precision of kernel parameter estimates. Consequently, in many studies, the estimate of mean kernel dispersal distance is associated with wide confidence intervals, which is particularly evident for fat-tailed dispersal kernels [23,32,38]. The complexity of dispersal events (spatial or temporal variance) and the influence on probabilities of dispersal from other factors (tree density, topography, the variance of fecundities, etc.)…”

“…This is because local pollen dispersal (at the scale of tens to hundreds of meters) is governed by gradient processes, but long-distance pollen dispersal at the mesoscale is a more complex and highly unpredictable event due to specific multidimensional atmospheric processes affecting air movements [29,30]. Currently available methods for studying pollen dispersal generally fail to connect background pollination levels to patterns of local pollen dispersal as described by dispersal kernels, with some exceptions [31,32].…”

Contrasting dates of rainforest fragmentation in Africa inferred from trees with different dispersal abilities

“…It is an indicator of disturbed rainforests. Although most of the D. benthamianus seeds disperse over short distances (c. 70 m), 30% of seed immigration was detected (15). This indicates that the distribution of dispersal distances is very flat tailed, indicative of long-distance dispersal events.…”

“…Based on their ecology and dispersal biology, P. elata and D. benthamianus are the most adapted to long-distance colonization. They are both light-demanding and wind-dispersed, with significant long-distance dispersal events (15). The two Erythrophleum species are light-demanding and besides its primary ballistic dispersal they exhibit secondary dispersal of their seeds by animals, but no evidence of long-distance dispersal has been detected in direct measurements with molecular markers (15).…”

“…They are both light-demanding and wind-dispersed, with significant long-distance dispersal events (15). The two Erythrophleum species are light-demanding and besides its primary ballistic dispersal they exhibit secondary dispersal of their seeds by animals, but no evidence of long-distance dispersal has been detected in direct measurements with molecular markers (15). S. zenkeri , a strict shade-tolerant species with seed dispersal non-assisted by wind or animals, exhibits the most limited colonising capacity.…”