title: Wood Specific Gravity Variations and Biomass of Central African Tree Species: The Simple Choice of the Outer Wood.
abstract: Wood specific gravity is a key element in tropical forest ecology. It integrates many aspects of tree mechanical properties and functioning and is an important predictor of tree biomass. Wood specific gravity varies widely among and within species and also within individual trees. Notably, contrasted patterns of radial variation of wood specific gravity have been demonstrated and related to regeneration guilds (light demanding vs. shade-bearing). However, although being repeatedly invoked as a potential source of error when estimating the biomass of trees, both intraspecific and radial variations remain little studied. In this study we characterized detailed pith-to-bark wood specific gravity profiles among contrasted species prominently contributing to the biomass of the forest, i.e., the dominant species, and we quantified the consequences of such variations on the biomass.
  
  
  
  Radial profiles of wood density at 8% moisture content were compiled for 14 dominant species in the Democratic Republic of Congo, adapting a unique 3D X-ray scanning technique at very high spatial resolution on core samples. Mean wood density estimates were validated by water displacement measurements. Wood density profiles were converted to wood specific gravity and linear mixed models were used to decompose the radial variance. Potential errors in biomass estimation were assessed by comparing the biomass estimated from the wood specific gravity measured from pith-to-bark profiles, from global repositories, and from partial information (outer wood or inner wood).
  
  
  
  Wood specific gravity profiles from pith-to-bark presented positive, neutral and negative trends. Positive trends mainly characterized light-demanding species, increasing up to 1.8 g.cm-3 per meter for Piptadeniastrum africanum, and negative trends characterized shade-bearing species, decreasing up to 1 g.cm-3 per meter for Strombosia pustulata. The linear mixed model showed the greater part of wood specific gravity variance was explained by species only (45%) followed by a redundant part between species and regeneration guilds (36%). Despite substantial variation in wood specific gravity profiles among species and regeneration guilds, we found that values from the outer wood were strongly correlated to values from the whole profile, without any significant bias. In addition, we found that wood specific gravity from the DRYAD global repository may strongly differ depending on the species (up to 40% for Dialium pachyphyllum).
  
  
  
  Therefore, when estimating forest biomass in specific sites, we recommend the systematic collection of outer wood samples on dominant species. This should prevent the main errors in biomass estimations resulting from wood specific gravity and allow for the collection of new information to explore the intraspecific variation of mechanical properties of trees.
authors.affiliation: Landscape Ecology and Plant Production Systems Unit, Université libre de Bruxelles, CP264-2, B-1050 Bruxelles, Belgium.|BIOSE Department, Gembloux Agro-Bio Tech, Université de Liège, B-5030 Gembloux, Belgium.|Ecole Régionale post-universitaire d'Aménagement et de gestion Intégrés des Forêts et Territoires tropicaux, Kinshasa, DR Congo.
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Bastin
authors.affiliation: BIOSE Department, Gembloux Agro-Bio Tech, Université de Liège, B-5030 Gembloux, Belgium.
authors.forename: Adeline
authors.initials: A
authors.lastname: Fayolle
authors.affiliation: Landscape Ecology and Plant Production Systems Unit, Université libre de Bruxelles, CP264-2, B-1050 Bruxelles, Belgium.
authors.forename: Yegor
authors.initials: Y
authors.lastname: Tarelkin
authors.affiliation: UGCT, University Ghent Centre for X-ray Tomography, Proeftuinstraat 86, 9000 Ghent, Belgium.
authors.forename: Jan
authors.initials: J
authors.lastname: Van den Bulcke
authors.affiliation: BIOSE Department, Gembloux Agro-Bio Tech, Université de Liège, B-5030 Gembloux, Belgium.|Laboratory for Wood Biology and Xylarium, Royal Museum for Central Africa, Tervuren, Belgium.
authors.forename: Thales
authors.initials: T
authors.lastname: de Haulleville
authors.affiliation: UPR BSEF, CIRAD, Campus International de Baillarguet, F-34398 Montpellier, France.
authors.forename: Frederic
authors.initials: F
authors.lastname: Mortier
authors.affiliation: Laboratory for Wood Biology and Xylarium, Royal Museum for Central Africa, Tervuren, Belgium.
authors.forename: Hans
authors.initials: H
authors.lastname: Beeckman
authors.affiliation: UGCT, University Ghent Centre for X-ray Tomography, Proeftuinstraat 86, 9000 Ghent, Belgium.
authors.forename: Joris
authors.initials: J
authors.lastname: Van Acker
authors.affiliation: Ecole Régionale post-universitaire d'Aménagement et de gestion Intégrés des Forêts et Territoires tropicaux, Kinshasa, DR Congo.|Behavioural Biology Unit, University of Liege, Liege, Belgium.|Conservation Biology Unit, Royal Belgian Institute of Natural Sciences, Brussels, Belgium.
authors.forename: Adeline
authors.initials: A
authors.lastname: Serckx
authors.affiliation: BIOSE Department, Gembloux Agro-Bio Tech, Université de Liège, B-5030 Gembloux, Belgium.
authors.forename: Jan
authors.initials: J
authors.lastname: Bogaert
authors.affiliation: Landscape Ecology and Plant Production Systems Unit, Université libre de Bruxelles, CP264-2, B-1050 Bruxelles, Belgium.
authors.forename: Charles
authors.initials: C
authors.lastname: De Cannière
mesh_terms: D000350:Africa, Central; D018533:Biomass; D013046:Specific Gravity; D014197:Trees; D014934:Wood