Stock recovery rates are not the panacea to assess timber yield sustainability: Evidence from managed Central African forests

After the change of the forestry laws in six Central African countries in the 1990s, the stock recovery rate—the ratio of the exploitable timber stock at the end of a felling cycle over the initial pre-felling stock—has become the standard index to assess timber yield sustainability. The Dimako formula, that is the legal expression of the stock recovery rate, is not necessarily the best estimate of this index. In particular, it accounts for only a limited knowledge on vital rates, it does not always increase with the diameter cutting limit, and it is negatively sensitive to the number of trees above the cutting limit, which may promote heavy initial logging when assessing forest management over several felling cycles. Here, we show that the Dimako formula can be generalised by using a more detailed model of forest dynamics, or a finer description of logging damage, and we demonstrate that an alternative formula has better properties. We used four species with contrasted initial diameter distribution, inventoried in the Dimako forest, eastern Cameroon, to illustrate the behaviour of alternative formula. If an increased level of details had to be incorporated in the Dimako formula, boosted post-logging growth should be considered at first. Because the stock recovery rate results from the combined influence of the initial dbh distribution of the species, which is a history-dependent feature of the population, and of its vital rates, which is an ecological species-specific feature of the population, its interpretation must at least take account of the autecology of the species. Particular caution must be taken when applying the formula to groups of species rather than to single species. The stock recovery rate alone gives a reductionist insight into yield sustainability.