Tropical tree assembly depends on the interactions between successional and soil filtering processes

Abstract

Aim

Successional and soil filtering processes are key drivers of the assembly of tropical tree communities, yet little is known about how they interact. Herein, we determine whether successional pathways depend on soil type, how the soil filtering effect varies during forest succession and whether succession is accompanied by changes in trait composition.

Location

South-western Central African Republic.

Methods

We used inventory data on 90 dominant tree species (72% of the stems ≥ 30 cm d.b.h.) in 15,420 plots (each 0.5 ha) distributed over an 8300-km2 mosaic of primary and secondary forests on contrasting clay and sandy soils. We gathered data on six traits: dispersal mode, nitrogen fixation, deciduousness, leaf area, wood density and maximum diameter. After validation with historical information, we used a successional index based on pioneer proportion to assess variations in trait composition, and in α- and β-diversity, in forest succession, and within and between soil types.

Results

Taxonomic and functional dissimilarities between clay and sandy soils decreased continuously during forest succession. Within soil types, early-successional communities had a low taxonomic but relatively high functional α-diversity. At the landscape scale, β-diversity was higher among earlier successional stages, except in rich soils where taxonomic β-diversity was high throughout the succession. Mean values for all traits, except leaf area, showed marked variations during forest succession in both soil types.

Main conclusions

The effect of soil type on community composition declines during succession, suggesting that the relative importance of neutral processes is higher in mature forests. Successional pathways were fairly similar in both soil types: disturbances reduced taxonomic diversity locally and enhanced β-diversity between sites, probably because the disturbances varied in type. We also demonstrated how easy-to-collect traits help improve predictions of ecological patterns and deepen our understanding of species assembly processes.