Pollarding and root pruning as management options for tree-crop competition and firewood production
Thesis (MScFor) (Forest and Wood Science)--University of Stellenbosch, 2003.
Planting of upperstorey trees along boundaries has been introduced in KabaleUganda with good reception from local farmers. Trees have been planted along agricultural fields, but both Alnus acuminata and Grew/lea robusta out-compete food crops. Managing competition between trees and crops for water, light, and nutrients to the benefit of farmers is a determinant of successful agroforestry. The scarcity and fragmentation of farmland coupled with the hilly nature of Kabale, highlights the need to address the question of tree-crop competition for resources if the technology of on-farm tree planting is to be widely disseminated and adopted in its different guises. Five-year old trees of A acuminata and G. robusta were subjected to treatments of pollarding, or a combination of pollarding and one side root pruning and compared with unpruned controls. The objectives were to assess their potential in reducing competition with food crops and providing firewood to farmers as well as their effects on tree growth. Pollarding has many benefits to farmers because it provides firewood and stakes for climbing beans, it reduces competition for resources between trees and crops and enables continued tree planting on-farm. Continued on-farm tree planting alleviates problems associated with limited land and contributes to environmental resilience. To ensure this, effect of pollarding and root pruning of upperstorey boundary trees of A acuminata 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 root pruned one side. In general, pooled data from 12 sites over 5 m away from trees indicated that a combination of pollarding and root pruning increased bean yield by 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 breast height (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 a light crown. A. acuminata also had more branches per tree (34) compared to G. robusta with only 25. These factors influence water uptake, light penetration through the canopy and transpiration rates, and thus affect tree-food crop competition. It is concluded that pollarding and root pruning have a great potential to reduce tree-crop competition, thereby paving the way for continued on-farm tree planting. The effect of pollarding on timber quality, moisture seepage into timber through the cut surface, if any, and the extent of its damage are areas for further research. The rate of root recovery is also to be followed closely to determine an appropriate frequency for cutting back of roots to recommend to farmers how often they need to prune their trees. It is also suggested that a thorough study be conducted on the amount of water uptake from the soil by each of the species Alnus acuminata and Grevillea robusta. This will help further explain the differences in competition between the two species.