Tree crown pruning as a management tool to enhance the productivity of parklands in West Africa

The effects of crown pruning of Vitellaria paradoxa C.F. Gaertn (karité) and Parkia biglobosa Gacq.) Benth. (néré) on above and below-ground interactions with associated crops of Pennisetum glaucum (L.) (millet) and Sorghum bicolor (L.) Moench (sorghum) were investigated as well as the use of the pruned material as mulch on soil fertility and millet production in an agroforestry parkland system in Burkina Faso. Three treatments of crown pruning (total-pruning, half-pruning and no-pruning) were applied to karité and néré. The area under each tree was divided into four concentric tree influence zones before pruning the trees (Zones A : up to 2 m from the tree trunk, B: up to half of the radius of the tree crown, C: up to the edge of the tree crown and D: up to 2 m away from the edge of the tree crown). Millet production under these zones and outside was assessed over two years' cropping seasons and sorghum over one cropping season. Crop production was analysed in relation to tree transpiration, light transmission, root distribution and soil fertility. Investigation on the use of the pruned material as mulch involved assessment of its decomposition and nutrient release patterns, its effects on soil properties and crop production and the existence of allelopathy in it. The results showed that tree crown pruning had significant effect on associated crop production. Total-pruned trees gave the highest crop production due to the reduction by crown pruning of the effects of large tree crowns on PAR transmission below crowns and rates of transpiration by trees. Root niche differentiation also occurred in which fine root length density of trees was reduced and crop roots dominated the upper soil layer due to crown pruning and as a consequence crop production of up to eight fold higher was achieved under total-pruned trees than under unpruned trees. Soil was more fertile closer to the tree trunks than outside treee crowns and therefore crop overall performed better under Zone B than outside tree crowns. The higher production of crop under Zone B than under Zone A, the zone closer to the tree trunk, may be due to lower light intensity and more intense competition for water and nutrients between trees and crops under Zone A. Pruned leaf litter of néré decomposed and released nutrients faster than karité. However, the slow rate of decomposition of karité leaves was beneficial to crops perhaps in terms of protecting the soil against excessive heat, surface runoff and erosion and due to the achievement of good synchrony between nutrient release from decomposing leaves of karité and crop nutrient demand. As a consequence of these, millet grain yield was increased by 120% and total dry matter by 43% by applying leaf litter of karité as mulch. The results of the laboratory allelopathy experiments were, however, contradictory to the field experiment. Leachate of karité showed more depressive effect on crop seed germination compared with néré. It was argued that this may be due to the fact that allelopathic effects under natural conditions are mediated by environmental conditions and decomposers community and therefore can show different results from the laboratory. Due to the results of the pruning and mulching experiments it was concluded that karité was a more suitable tree species than néré for semi-arid agroforestry parkland systems. Since the period of three years of the present study was too short to evaluate the effect of pruning on the overall productivity of the system it was finally recommended that the monitoring is continued over a longer period to determine if complete recovery of fruit production or an optimum compromise between fruit arld crop production could be achieved before conclusions are made whether pruning is beneficial or not.