Modelling the scaling of stem growth with crown size and optimum stocking densities for systematic grid plantation of Faidherbia albida

摘要

Faidherbia (Faidherbia albida) is being promoted widely in interventions for combating desertification, re-greening of the Sahel, carbon offset and various agroforestry projects. However, there is a dearth of information on its growth and canopy development. There are also no guidelines for optimum stocking densities for practitioners to follow. Therefore, the objective of this work was to evaluate scaling relationships between its growth in height, stem diameter and crown size and based on these relationships define stocking densities. In order to achieve this we: (1) modelled its growth in relation to plant density; (2) identified appropriate models for scaling stem height and diameter with crown size; (3) using information from step 2, we derived stocking densities under different scenarios. Crown diameter (CD) was found to scale with stem diameter (D) isometrically, while stem height scaled with CD allometrically. The scenarios derived using the CD-D scaling indicated that densities > 50 plants ha(-1) are untenable when DBH exceeds 40 cm. High initial densities (> 625 plants ha(-1)) appear to lead to rapid self-thinning. Starting with low initial densities (< 100 plants ha(-1)) was also expected to result in sub-optimal use of site resources and delayed net ecosystem production. As a compromise, we recommend establishment of stands at initial densities of about 625 trees ha(-1) (or 4 m x 4 m spacing) and progressive thinning as stem diameter increases. The focus of this analysis has been on monoculture plantations of Faidherbia and the spacing may not be directly applicable where crops are integrated with trees. Therefore, we propose a follow-up study including modelling tree behaviour in mixed stands in order to refine recommendations