Environmental filtering of dense-wooded species controls above-ground biomass stored in African moist forests

Abstract

Summary 1.Regional above-ground biomass estimates for tropical moist forests remain highly inaccurate mostly because they are based on extrapolations from a few plots scattered across a limited range of soils and other environmental conditions. When such conditions impact biomass, the estimation is biased. The effect of soil types on biomass has especially yielded controversial results. 2. We investigated the relationship between above-ground biomass and soil type in undisturbed moist forests in the Central African Republic. We tested the effects of soil texture, as a surrogate for soil resources availability and physical constraints (soil depth and hydromorphy) on biomass. Forest inventory data were collected for trees ‡20 cm stem diameter in 2754 0.5 ha plots scattered over 4888 km 2 . The plots contained 224 taxons, of which 209 were identified to species. Soil types were characterized from a 1:1 000 000 scale soil map. Species-specific values for wood density were extracted from the CIRAD's data base of wood technological properties. 3. We found that basal area and biomass differ in their responses to soil type, ranging from 17.8 m 2 ha )1 (217.5 t ha ). While shallow and hydromorphic soils support forests with both low stem basal area and low biomass, forests on deep resource-poor soils are typically low in basal area but as high in biomass as forests on deep resource-rich soils. We demonstrated that the environmental filtering of slow growing dense-wooded species on resource-poor soils compensates for the low basal area, and we discuss whether this filtering effect is due to low fertility or to low water reserve. 4. Synthesis. We showed that soil physical conditions constrained the amount of biomass stored in tropical moist forests. Contrary to previous reports, our results suggest that biomass is similar on resource-poor and resource-rich soils. This finding highlights both the importance of taking into account soil characteristics and species wood density when trying to predict regional patterns of biomass. Our findings have implications for the evaluation of biomass stocks in tropical forests, in the context of the international negotiations on climate change.

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

“…The non-destructive sampling applied was based on the assumption that wood density does not vary significantly from pith to bark along the stem. Given that tree species studied were in the same edaphoclimatic conditions, the large differences observed between wood densities of the eighteen tree species could not be directly attributable to environmental conditions, but more to differences between species (Nogueira et al 2007;Gourlet-Fleury et al 2011). These differences could be explained by the intrinsic characteristics of species (Chave et al 2006;Zanne et al 2010;Zeidler, 2012).…”

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

“…Slow regeneration of tropical forests on soils of poor quality is a wellknown general issue (Finegan 1996, Crk et al 2009). 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.…”

“…Geological types were characterized analogous to Gourlet-Fleury et al (2011). Sandstone and ironstone-capped plateaux and terraces provide a base for nutrient-poor substrates, and alluviums for intermediate substrates.…”

“…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. However, none of the forest management plans that are currently active in this region take account of these site conditions when assigning annual cut zones and potentially vulnerable areas (N. Bayol, personal communication).…”

Recommendations for the use of tree models to estimate national forest biomass and assess their uncertainty

“…Another solution would be sampling along gradients where allometric variation is known, e.g., altitudinal gradients having strong influence on soil types, climate, vegetation forms, and biomass (Girardin et al 2010). Soil physical conditions influence the floristic composition (Infante Mata et al 2011) and constrain the amount of biomass stored in tropical forests, highlighting the need to consider the importance of taking into account soil characteristics and species wood density when assessing national forest biomass (Gourlet-Fleury et al 2011). Unless for homogenous ecosystems, such as plantations or mangroves, it is not easy to take into consideration the floristic composition in the sampling scheme for the tropics because the number of tree species can be up to 300 per hectare (Gibbs et al 2007).…”