Knowledge of tree growth and mortality: a prerequisite for sustainable forestry

A  research project on tree growth and mortality in Central Africa is published in the journal Forest Ecology and Management. Eleven co-authors from different institutions in the North and South have constructed a substantial database to estimate the growth rate and mortalitw rate of 42 commercially important tree species. The use of these values will improve the sustainability of forest management in Central Africa.

In the tropical rainforests of Central Africa, industrial timber production is based on the periodic (25-30 year) harvesting of a few commercially valuable species and trees that exceed a specific diameter threshold.

In order to ensure the viability of this exploitation system, it is therefore essential that the volume of exploitable trees be reconstituted between two cuts. "On average, one to two trees per hectare are cut every 25 to 30 years. This is very little if we compare this intensity of harvesting with that practiced in Belgium," notes Gauthier Ligot, the principal author of this study. Nevertheless, these removals are concentrated on a small number of species scattered among many other trees, less commercially interesting. In some cases, despite compliance with national regulations and certification standards, it is feared that the numbers of certain tree species are dwindling to the point of threatening economic activity in the medium term.

Tree growth and mortality are two central processes for predicting the evolution of a forest between successive harvests. These processes depend on many factors that are sometimes difficult to quantify. "The species studied, the age and size of the trees, the nature of the soil, the climate, the spacing between trees, the effect of disturbance are all factors that influence the growth and mortality of trees", says Gauthier Ligot.

In order to predict the evolution of harvested forests while taking these multiple factors into account, the strategy of this study is to acquire a large number of data on annual growth in diameter and mortality rate for different species and in different sites. Thus, more than 20,000 trees belonging to 42 species were monitored annually in 8 sites in Central Africa for a period ranging from 1 to 7 years. This monitoring continues in parallel with the publication of this research work.

"It is a grueling and repetitive job. Camping in the forest, finding marked trees to measure them. Yet, this is what allows us to guarantee that what is taken from these forests is equal to what the forest produces". It is the challenge of a sustainable society that is outlined in these figures.

The study identifies the effect of certain key factors and proposes reference values to be used in forest modelling. These values are accessible via this page. In particular, tables with growth and mortality values by species and diameter can be found here.