Life-span growth dynamics and xylem anatomical patterns of diffuse-porous Afzelia africana Sm. (Fabaceae) in different ecological zones in Burkina Faso

Abstract

Tropical West African savannas are exposed to high climatic variability with potential impacts on tree growth, forest dynamics and ecosystem productivity. In such context, understanding the long-term ecological responses of savanna trees to changing environmental conditions is of great relevance for taking appropriate conservation actions. We conducted the first study on tree-ring analysis and quantitative wood anatomy on Afzelia africana Sm. in Burkina Faso, to investigate the life-span growth trajectories and wood anatomical adjustment to site and to climate variations. A total of 24 stem discs was collected in four protected forests along the Sudano-sahelian and the Sudanian climatic zones. Wood samples were analyzed using standard dendrochronological methods and quantitative wood anatomy. The mean annual growth rates varied from 1.002 (± 0.249) mm. year−1 in the Sudanian zone to 1.128 (± 0.436) mm. year−1 in the Sudano-sahelian zone. Analysis of growth trajectories showed high variations within sites and between climatic zones. Wood anatomical traits significantly varied between sites. Principal Component Analysis revealed strong relationships between ring width, wood density and vessel traits, with 82.81 % of the total variance explained. Vessel size significantly increased from the pith to the bark, highlighting the ontogenetic effects on xylem anatomical variations. Inverse relationships were found between vessel size and vessel density across the driest site and the wettest site, suggesting that the higher the rainfall, the taller the tree, the larger vessel size, but the lower vessel density. By contrast, more arid conditions and high evapotranspiration lead to smaller vessel sizes and higher vessel density. Such anatomical adjustments highlight the trade-offs between water conductance efficiency and hydraulic safety, and emphasize physiological responses to climate variability. These variations on the long-term dynamics and xylem anatomical patterns underline complex interactions between ontogenetic effects and contrasting environmental factors that affect the eco-physiological functioning of A. africana throughout the Sudanian region.