Tree-ring analysis of an African long-lived pioneer species as a tool for sustainable forest management

Abstract

Improved forest management in the tropics is hampered by the limited availability of quantitative data, especially in tropical Africa. Important management parameters such as the minimum logging diameter (MLD), the associated biological rotation age (BRA) and the timing of silvicultural treatments are too often based on merchantable dimensions and state regulations instead of ecological data and tree growth. This study combines inventory data and bootstrapped tree-ring curves of natural and planted trees of the long-lived pioneer species Terminalia superba Engl. & Diels. A growth-oriented MLD was calculated, and the need and timing of silvicultural treatments was estimated based on the analysis of growth releases and suppressions. Study sites were situated in the Congolese Mayombe forest and western Ivory Coast. Tree rings from 41 natural forest trees (stem discs) and 29 plantation trees (increment cores) were measured, along with diameter and height measurements. Planted and natural forests showed considerable differences in mean growth rate and growth curves. More than 50% of the trees nevertheless reached the canopy without growth releases or suppressions, confirming that T. superba does not require intensive management. The growth-oriented MLD not only differs considerably between sites but bootstrapping revealed large differences within forest regions/types. Furthermore, volume-based MLD and BRA are on average larger than basal area-based calculations. The modified monocyclic management system is suggested, especially for planted forests with light-demanding, fast-growing tree species. One small-scale thinning during the juvenile phase is recommended before a final harvest that includes all trees above the growth-oriented MLD. The introduction of sustainable management for T. superba therefore primarily depends on forest type and cannot be generalized at the species level.