Exploratory Dendrochronological And Dendroisotopic Analyzes Of Riparian Forests Along The Milo River (Guinea)

The Milo River in Guinea is one of the tributaries of Upper Niger. Its basin has a semi-arid climate with alternating dry and wet seasons in a biogeographic context of savannah. The West African monsoon brings rain between June and September with a long dry period affecting river levels and vegetation. The rainfall and flow of the Milo River have declined of about 20 % since the 1970s. Trees that concentrate mainly along river margins are likely to be affected by this change in climate and water resource. Little data is available on the age, growth rate and origin of water used by riparian trees in Guinea. These data are nevertheless essential for measuring the impact of environmental changes on the functioning and sustainability of riparian forests. A dendrochronological and dendroisotopic study was carried out on trees bordering the Milo River in order to test the effectiveness of these techniques for obtaining such data. A difficulty with the use of dendrochronology in tropical environments is that few species form annual rings. Of thirteen tree species studied half form more or less distinct ring boundaries but three species showed clearly visible ring boundaries. These species are Pericopsis laxiflora, Tamarindus indica and Tectona grandis. This study shows that it is possible to estimate the age (15 to 102 years) and radial growth rates (0.26 to 1.39 cm/year) for species with discernible rings. However, the poor anatomical definition of ring boundaries on most of the other species, especially towards the pith, highlights all the difficulty of conducting a long-term, robust, dendroclimatic study in the semi-arid African context. A dendroisotopic test analysis (delta C-13,delta O-18) on a single tree of Pericopsis laxiflora, Tamarindus indica and Tectona grandis shows that this riparian species is able to regulate its water acquisition during hydrological variations. This is marked by a physical trace in the rings related to the use of the more perennial alluvial water during the dry period.