Biometric relationships and growth of pruned and non-pruned Acacia saligna under runoff irrigation in northern Kenya

Abstract

The objective of this study was to compare allometric equations for pruned and non-pruned Acacia saligna (Labill.) H.L. Wendl. and to evaluate their possible use in the prediction of optimum pruning intervals. The necessary data were obtained from a field experiment carried out in the Turkana District of Kenya where the objective was to examine the production potential of runoff agroforestry systems. Trees were planted at different densities (2500 and 833 trees ha −1), pruned or not, and intercropped or grown in sole stands. Measurements of cross sectional area (CSA) at the trunk base were taken at monthly intervals. Intermediate biomass yields were obtained twice during the experimental period by complete pruning of the trees at 1.5 m height and by harvesting of whole trees at the end of the experiment. Linear regressions of CSA versus biomass yield were established for whole trees, individual plant organs (leaves, branches and trunks) for both pruned and non-pruned trees. Similar slopes were found for the relationships between CSA and total above ground biomass of non-pruned trees and between the increase in CSA and the total above ground biomass of trees after pruning. These results suggest that the regression coefficients obtained for non-pruned Acacia saligna trees can be used to estimate the regrowth of pruned trees. The allometric equations were used to calculate average daily increments of plant growth (ADI) and relative growth rates (RGR). During the trial, ADI's were higher for trees planted at low density. RGR's clearly responded to the pruning treatments. Five months after the first pruning was carried out, the RGR of pruned trees were higher than those of non-pruned trees and increasing, as opposed to those of non-pruned trees which were decreasing. RGR is a simple tool that allows the determination of optimal pruning intervals. We used the allometric equations to compute RGR and suggest that this approach is appropriate to determine pruning intervals in agroforestry systems. In our study we found that the imposed pruning intervals were too short and suggest an optimal range of 6–8 months.