Geological Substrates Shape Tree Species and Trait Distributions in African Moist Forests

Abstract

BackgroundUnderstanding the factors that shape the distribution of tropical tree species at large scales is a central issue in ecology, conservation and forest management. The aims of this study were to (i) assess the importance of environmental factors relative to historical factors for tree species distributions in the semi-evergreen forests of the northern Congo basin; and to (ii) identify potential mechanisms explaining distribution patterns through a trait-based approach.Methodology/Principal FindingsWe analyzed the distribution patterns of 31 common tree species in an area of more than 700,000 km2 spanning the borders of Cameroon, the Central African Republic, and the Republic of Congo using forest inventory data from 56,445 0.5-ha plots. Spatial variation of environmental (climate, topography and geology) and historical factors (human disturbance) were quantified from maps and satellite records. Four key functional traits (leaf phenology, shade tolerance, wood density, and maximum growth rate) were extracted from the literature. The geological substrate was of major importance for the distribution of the focal species, while climate and past human disturbances had a significant but lesser impact. Species distribution patterns were significantly related to functional traits. Species associated with sandy soils typical of sandstone and alluvium were characterized by slow growth rates, shade tolerance, evergreen leaves, and high wood density, traits allowing persistence on resource-poor soils. In contrast, fast-growing pioneer species rarely occurred on sandy soils, except for Lophira alata.Conclusions/SignificanceThe results indicate strong environmental filtering due to differential soil resource availability across geological substrates. Additionally, long-term human disturbances in resource-rich areas may have accentuated the observed patterns of species and trait distributions. Trait differences across geological substrates imply pronounced differences in population and ecosystem processes, and call for different conservation and management strategies.

Specific and generic stem biomass and volume models of tree species in a West African tropical semi-deciduous forest

“…In general, fast-growing trees are light demanding and produce low-density wood as found by Rueda and Williamson (1992), van Gelder et al (2006) and Henry et al (2010). However other studies have shown that some pioneer light demanding tree species can, unexpectedly, have high wood density (e.g Lophira alata, Fayolle et al 2012) and this may be explained by wood composition and structure (Martínez-Cabrera et al 2009;Poorter et al 2010;Zanne et al 2010).…”

Legacy of logging roads in the Congo Basin: How persistent are the scars in forest cover?

“…However, this has only recently arisen as a concern in the Congo Basin (Gourlet-Fleury et al 2011) where it has not yet been taken into account in forest management practices (N. Bayol personal communication). Large parts of the southern CAR and the northern Republic of Congo are characterized by ''Carnot'' sandstone, producing resource-poor soils associated with dense, slow-growing forest types (Fayolle et al 2012; Fig. 1).…”

“…A homogenized geological map (Fayolle et al 2012) based on three national maps (Gazel 1956, Orstom 1967, Boulvert 1996 was used to identify areas of contrasting ''substrate fertility''. Geological types were characterized analogous to Gourlet-Fleury et al (2011).…”

“…For the northwestern Congo Basin it has been shown that the functional diversity of tree communities is heavily influenced by geological substrate (Fayolle et al 2012). The range of substrates includes those that are sandy and of low fertility which are hypothesised to support slower recovery from deforestation and degradation (Chazdon 2003, Gourlet-Fleury et al 2011.…”

Biodiversity and the functioning of tropical forests

“…4.1). For example, in African forests, sandy soils, compared to clayey soils, had a higher abundance of species with high wood density that are more drought tolerant and better survive on sandy and resourcelimited soils (Fayolle et al 2012). In our study forest, disturbance due to logging treatments changed the trait composition of demographic groups towards more acquisitive trait values (e.g., high specific leaf area and low wood density) that are typical of pioneer species that benefit from higher light levels (Carreño-Rocabado et al 2012).…”