Cambial activity, growth ring analysis and wood density variability of planted teak (Tectona grandis L.f. (Lamiaceae)) in Ivory Coast

Although they currently absorb more carbon than they release, tropical forests suffer disturbances generated by human activities and amplified by global warming. Therefore, forest plantation, based on fast-growth fuelwood and timber species such as teak, becomes an option to counteract the risk of loss of these natural biomes. For its various advantages (ecological plasticity, good behaviour in plantation, fast-growth, high natural durability and wood density), teak (Tectona grandis) is introduced under different climatic conditions along the inter-tropical belt, occupying 75% of the world tropical timber plantations. However, this species remains highly sensitive to climate, suggesting that disturbances in the rainfall rate and intensity are expected to negatively affect tree growth and, consequently, induce variations in the derived wood properties. Therefore, to be successful, in response to the high demand for high quality wood, teak based reforestation programs should be underpinned with reliable and accurate information based on field data on tree growth rate and wood density dynamics. Because growth rate (reflected in ring widths) and wood density represent the main two functional traits that portray tree sensitivity/vylnerability to environment (climate). Information on the dynamics of growth and wood density remains, therefore, crucial to control and predict planted stands productivity, biomass estimate and the wood quality in regions marked by ongoing climatic variability like in Ivory Coast. Our study addressed the following questions: (i) What are the determining factors of wood formation and its fluctuations in teak planted in Ivory Coast? (ii) How does the precipitation variability influence managed and non managed tree growth? Is there a large-scale climatic signal in the tree growth rate (ring width), in addition to local the precipitation signal? (iii) How to assess the degree of variability of wood density in teak planted in Ivory Coast? Does climate variability affect teakwood density? To address these research questions, monthly cambial sampling was carried out in an evergreen forest and tree-ring analysis was performed in trees from 2 stands: a managed plantation located in an evergreen forest close to the Atlantic coast and an unmanaged semi-deciduous forest situated in the Central West Ivory Coast. Microdensity measurements were implemented on trees also from the evergreen forest, in order to assess the variations in wood density. Cambial activity showed that a 3 months dry season induced growth interruption and resulted in the formation of an annual xylem growth ring with 2 phloem rings over the same vegetative season. Anatomical quantification of the structural changes occurring in the cambial zone was closely correlated to monthly precipitations. Growth anomalies, induced by climatic and phenological variations during the growing period, were visualized through the formation of intra-annual xylem growth zones. Tree-ring analysis revealed higher growth rate in the managed trees than in the non-managed forest. In the deciduous forest, teak growth was only sensitive to the wet season precipitation while trees reacted to both wet and annual precipitation in the semi-deciduous forest. Anomalies in the sea surface temperature of the Gulf of Guinea also influenced trees growth. Moreover, growth patterns reflected the effect of three major El Niño events in the southern forest. Overall, climatic influence was more pronounced (slightly high correlation) in the non-managed teak plantation compared to the managed one as a consequence of the lack of sylvicultural intervention in the non-managed stand. This indicates the role of sylviculture in enabling trees to cope with a water deficit resulting from climate disturbances. The results reveal clear and direct effect of the broad-scale climate on tree growth, in addition to local precipitation recordings. Microdensitometrical profiling revealed that wood density was more variable within the tree than between trees. Intra-tree density variations reflected fluctuations occurring in the intra-ring xylem anatomy. Direct and significant correlations were found between October rainfall and the total ring wood density (r= 0.37; p<0.05) and latewood density (r= 0.41; p<0.05) which also correlated with the post-wet season rainfall (r=0.47; p< 0.05). The intra-ring anatomy related density changes led to the occurrence of the alternating pattern of intra-ring denstiy variation which best supported the correlation with the bimodal monthly precipitation. Inter and intra-annual density variations were high and persistent in the mature wood portion, indicating that the cambial age was not the main driving factor of the density variability. This research work, founded on original field data and unique for this region, provides highly useful results for teak plantations management in West-Africa. This scientific data evidences the sensitivity of teak growth to climate at both local and global scales. As a result of this climate sensitivity, (1) growth slowed down during years of low precipitation and (2) in response to intra-annual precipitations disturbances, trees formed intra-ring anatomy fluctuations which, in turn, result in the intra-ring density variations, the main source of the mean tree density variability. As a perspective to future research, a weekly cambial sampling could test the hypothesis on the intra-annual xylem growth zones formation which affected tree-ring density and, consequently, mean tree density. Monitoring of thinnings and pruning operations, combined with tree-ring analysis, and the monitoring of leaf area index, evapotranspiration and soil water content, could be a key to unravel the mechanism through which the managed stand trees adapt to water deficits. Heritability study could allow to establish the degree of the genetic control on wood density namely the earlywood features which could contribute to stabilize the mean tree wood density