Pollarding and root pruning as management options for tree-crop competition and firewood production

[摘要] Planting of upperstorey trees along boundaries has been introduced in KabaleUgandawith good reception from local farmers. Trees have been planted alongagricultural fields, but both Alnus acuminata and Grew/lea robusta out-compete foodcrops. Managing competition between trees and crops for water, light, andnutrients to the benefit of farmers is a determinant of successful agroforestry.The scarcity and fragmentation of farmland coupled with the hilly nature ofKabale, highlights the need to address the question of tree-crop competition forresources if the technology of on-farm tree planting is to be widely disseminatedand adopted in its different guises.Five-year old trees of A acuminata and G. robusta were subjected totreatments of pollarding, or a combination of pollarding and one side rootpruning and compared with unpruned controls. The objectives were to assesstheir potential in reducing competition with food crops and providing firewood tofarmers as well as their effects on tree growth. Pollarding has many benefits tofarmers because it provides firewood and stakes for climbing beans, it reducescompetition for resources between trees and crops and enables continued treeplanting on-farm. Continued on-farm tree planting alleviates problems associatedwith limited land and contributes to environmental resilience. To ensure this,effect of pollarding and root pruning of upperstorey boundary trees of Aacuminata and G. robusta was tested on 12 farmers' fields in Kabale.Food crops (beans and maize) grown in the sequence beans-maize-beans,grew very well at less than 50 em from trees that had been pollarded and rootpruned one side. In general, pooled data from 12 sites over 5 m away from treesindicated that a combination of pollarding and root pruning increased bean yieldby 240% and maize by 154%, while pollarding alone increased bean yield by 181%and maize yield was increased by 123% in comparison to non-pruned trees.However, pollarding and root pruning treatments reduced tree growth rates.Notable was more competition with crops by A. acuminata than by G.robusta. This was attributed to differences in root architecture, diameter at breastheight (dbh) sizes, crown spread and crown density between the two species.Five-year-old A. acuminata had bigger dbh (12.40 cm), wider crown spread (6 m)and a dense crown, while G. robusta had dbh 10.82 em, 3 m crown spread and alight crown. A. acuminata also had more branches per tree (34) compared to G.robusta with only 25. These factors influence water uptake, light penetrationthrough the canopy and transpiration rates, and thus affect tree-food cropcompetition.It is concluded that pollarding and root pruning have a great potential toreduce tree-crop competition, thereby paving the way for continued on-farm treeplanting. The effect of pollarding on timber quality, moisture seepage into timberthrough the cut surface, if any, and the extent of its damage are areas for furtherresearch. The rate of root recovery is also to be followed closely to determine anappropriate frequency for cutting back of roots to recommend to farmers howoften they need to prune their trees. It is also suggested that a thorough study beconducted on the amount of water uptake from the soil by each of the speciesAlnus acuminata and Grevillea robusta. This will help further explain the differencesin competition between the two species.

[发布机构] Stellenbosch University