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. © 2020 Elsevier GmbH
Tag: growth rate
Variation in Seedling Germinationand Growth in Five Populations Ofvitellariaparadoxa C.F. Gaertn.subsp. Nilotica: A Threatenedusefulfruit Tree Species in Uganda
We studied seedlings germination and growth performance in an economically and socially important fruit tree species of Vitellaria paradoxaC.F. Gaertn.subsp. nilotica (Kotschy) in Uganda. The study aimed at determining variability in germination of the five shea tree seed lots and seedlings growth performance based on their growth traits. Five populations were considered from four agroecological zones of Uganda and 180 candidates plus trees (16 ethnovarieties) selected as seed trees based on their traits for faster growth and high oil yield. A total of 1204 biological seeds were collected and sown in a general tree nursery at Ngetta Zonal Agricultural Research and Development Institute (NgeZARDI)during the month of June 2018 in a randomized incomplete block design with three replications. Significant variation (χ = 526; p < 0.01) was observed in seedling phenotypic traits within and between populations. Regression equation for height growth and leaf size index were given asy = 0.3787 + 12.671x and y = 0.6483 + 15.413x respectively. Root collar diameter was more correlated to leaf size index (0.425) than to height growth (0.30). Clustering of shea tree seedlings based on phenotypic growth traits revealed one aggregated cluster indicating that most of the seedlings from the five populations were similar (Jaccard index 0.92, p < 0.01). However, clustering based on SNP markers revealed three different populations. We recorded higher growth (χ = 708; p< 0.001) in Arua shea population (19.69 cm)compared to the population average (19.04 cm). Red seeded; thin pulped and hairy fruited shea tree ethnovarieties assumed faster growth than the rest. The phenotypic analysis of shea tree seedlings revealed some fast-growing genotypes from the five studied populations. Thesegenotypes can be selected for faster growth for shea tree breeding in Uganda.
Competition influences tree growth, but not mortality, across environmental gradients in Amazonia and tropical Africa
Competition among trees is an important driver of community structure and dynamics in tropical forests. Neighboring trees may impact an individual tree’s growth rate and probability of mortality, but large-scale geographic and environmental variation in these competitive effects has yet to be evaluated across the tropical forest biome. We quantified effects of competition on tree-level basal area growth and mortality for trees = 10 cm diameter across 151 ~1-ha plots in mature tropical forests in Amazonia and tropical Africa by developing non-linear models that accounted for wood density, tree size and neighborhood crowding. Using these models, we assessed how water availability (i.e., climatic water deficit) and soil fertility influenced the predicted plot-level strength of competition (i.e., the extent to which growth is reduced, or mortality is increased, by competition across all individual trees). On both continents, tree basal area growth decreased with wood density, and increased with tree size. Growth decreased with neighborhood crowding, which suggests that competition is important. Tree mortality decreased with wood density and generally increased with tree size, but was apparently unaffected by neighborhood crowding. Across plots, variation in the plot-level strength of competition was most strongly related to plot basal area (i.e., the sum of the basal area of all trees in a plot), with greater reductions in growth occurring in forests with high basal area, but in Amazonia the strength of competition also varied with plot-level wood density. In Amazonia, the strength of competition increased with water availability because of the greater basal area of wetter forests, but was only weakly related to soil fertility. In Africa, competition was weakly related to soil fertility, and invariant across the shorter water availability gradient. Overall, our results suggest that competition influences the structure and dynamics of tropical forests primarily through effects on individual tree growth rather than mortality, and that the strength of competition largely depends on environment-mediated variation in basal area.