Dendrochronologie, densité du bois et dendrométrie de quelques espèces forestières exploitées au Cameroun

Abstract

Congo Basin is a focal point for conservation of biodiversity (flora and fauna). Sustainable management of these forest resources depends on the use of available and accessible qualitative and quantitative data. However, these tools are still lacking in Central Africa. This is the case with knowledge of the exact age, the rates of tree increments, the height of a standing tree, and the density of wood. To enrich knowledge on these parameters, a study was conducted in FMUs 10 025 (August 2013) and 10 052 (September-November 2015) of SFIL-GDC. It aimed at gaining knowledge of the biology and wood technology of some species exploited in Cameroon. Specifically, this involved: (1) studying growth rings (identifying growth ring boundaries and their identification anomalies; estimating tree ages and annual tree increments; (2) determining radial variation in wood density; and (3) establishing height-diameter correlation of trees. Legal destructive method was used on 9 species (1 individual/species) of Annual Cutting Block (ACB) 3-3 of FMU 10.025 and three species of ACB 1-3 of FMU 10.052. In ACB 3–3, before a tree was felled, DBH was taken. Once the tree was felled, big end diameters was measured, and some stem discs were taken from the base of log: Afzelia bipindensis (2); Cylicodiscus gabunensis (2); Entandrophragma cylindricum (2); Entandophragma utile (1); Erythrophleum suaveolens (2); Mansonia altissima (2); Milicia excelsa (3); Pterocarpus soyauxii (3) and Triplochiton scleroxylon (3). These 20 stem discs were processed and analyzed to highlight the existence or not of tree rings and to determine radial variation of wood density using Archimedes principle. In ACB 1-3, before felling a tree, DBH was measured. After felling, total height (TH) of tree was measured, as well big, small end diameters (BED and SED), and height of the logs on 227 trees: E. cylindricum (45), E. suaveolens (100), and T. scleroxylon (82). 41 stem discs were collected (1 stem discs per tree): E. cylindricum (11); E. suaveolens (15); and T. scleroxylon (15). These 41 stem discs have been machined and sanded. Tree rings were observed/marked, and anatomical sections were performed and treated, then classified according to their limits. Ring boundaries were measured to an accuracy of 0.01 mm by Image J/SOP DHXCT, 2013/2015, corresponding to the respective years of formation of the last tree rings. On the basis of growth ring width series obtained by transect, the diametric increments and the basal surfaces per ring were calculated. Growth rings of T. scleroxylon, E. cylindricum, and E. suaveolens are clearly visible. Those of T. scleroxylon are wider (5.08 ± 1.42 mm) than those of E. suaveolens (3.51 ± 0.81 mm) and E. cylindricum (2.97 ± 0.24 mm). Anomalies in terms of missing rings, false rings, and discontinuous rings represent, respectively 13.56 %, 10.88 % and 6.48 % in E. cylindricum, E. suaveolens and T. scleroxylon. Mean cambial age to reach the minimum logging diameters (MLDs) in E. cylindricum, E. suaveolens and T. scleroxylon is 155, 69 and 68 years respectively, and the biological rotation age (BRA) is 112, 133 and 79 years. Average annual diameter increase is 0.63 ± 0.32, 0.73 ± 0.29 and 1.06 ± 0.26 cm.yr-1 respectively, and that of the basal area increments is 2.10-4 ± 0.0002, 2.10-4 ± 0.0003 and 3.10-4 ± 0.0003 m2.yr-1 respectively. The radial variation in wood density (bark to pith) is A. bipendensis (0.65 - 0.75), C. gabunensis (0.65 - 0.93), E. cylindricum (0.64 - 0.74), E. utile (0.51 - 0.58), E. suaveolens (0.78 - 0.84), M. altissima (0.61 - 0.65), M. excelsa (0.45 - 0.73), P. soyauxii (0.59 - 0.69) and T. scleroxylon (0.37 - 0.6 g.cm-3). Density averages are respectively: 0.70 ± 0.05; 0.79 ± 0.14; 0.69 ± 0.05; 0.55 ± 0.03; 0.81 ± 0.03; 0.63 ± 0.02; 0.65 ± 0.08; 0.64 ± 0.05 and 0.44 ± 0.07 g.cm-3. Compared to those of GWDD, these densities are either lower (A. bipendensis and P. soyauxii), equal (C. gabunensis) or higher (E. cylindricum; E. utilite; E. suaveolens; M. altissima; M. excelsa and T. scleroxylon). The height-diameter relationship is a linear correlation whose equation and coefficient of determination are: TH (m) = 17.79 + 20.18*BED (m) with R2 = 0.47 in E. cylindricum; TH (m) = 26.08 + 9.74*BED (m) with R2 = 0.12 in E. suaveolens and TH (m) = 12.21 + 27.40*BED (m) with R2 = 0.25 in T. scleroxylon. These models are verified by more than 90 %, the remaining 10% being due to effects other than the explanatory variables (H and D) which vary simultaneously. This work confirms the huge potential of tree-ring analysis, detailed wood density and height-diameter correlation data for long-term planning of sustainable forest management and carbon stocks. Both methods can be used complementary to inventories, combining the diversity of inventory variables (height, recruitment, mortality) with tree rings that extend decades to centuries into time. A larger sample size, including older trees, is recommended to lengthen regional tree-ring chronologies for future dendroclimatological and dendroclimatical studies. It would also enable a better detection of sampling biases, more reliable estimates of the culmination of growth variables and serve as a solid base for bootstrapping. Apart from the tree level and the inter annual level, the obtained high-resolution wood density profiles offer also possibilities to study intra-annual density variations and their possible relation with climate and wood anatomy.