Les cernes dans les bois tropicaux africains : nature et périodicité. Peuvent-ils révéler l'âge des arbres?

Abstract

To draw up logging plans for African for- ests, it is essential to know at what age the trees can be felled, whether for timber from dense humid forests or for fuelwood from savannah areas. However, attempts are rarely made to monitor tree growth over a number of years. Could growth rings be used to provide at least an ap- proximation of the age of trees marked for felling? This methodological article published nearly 40 years ago, and now translated into English, first gives an over- view of the nature and anatomy of growth rings in African timber, explaining how, even when they are hard to discern with a magnifying glass, they can be counted in- dividually by applying a mechanical treat- ment to the surface, followed by X-rays. The second part of the article discusses ways of determining the periodicity of growth ring formation, with a close look at the main difficulty that arises for tropical species, which is that growth rates in one and the same species do not necessarily match the seasonal cycle. Various meth- ods have been used since the 1920s to analyse the dynamics of wood formation in tropical tree species. Taking periodical samples of the cambium around the cir- cumference of the trunk produces good results but the method is destructive. Incisions made into the bark leave scars from which different points in the wood can be dated. Another method involves making an incision each year and placing dendrometric tape around the tree. Regu- lar monthly readings then show the exact periods when wood is being formed. Two years of observation are enough to pro- duce good results.