Tree growth and management in Ugandan agroforestry systems: effects of root pruning on tree growth and crop yield
Tellie-Nelson Wajja-Musukwe (1), Julia Wilson (2, 3), Janet I. Sprent (4), Chin K. Ong (5), J. Douglas Deans (2) and John Okorio (1)
1. Forestry Resources Research Institute, P.O. Box 1752, Kampala, Uganda / 2. Centre for Ecology and Hydrology, Bush Estate, Penicuik, Midlothian EH26 0QB, U.K. / 3. Corresponding author () / 4. School of Life Sciences, University of Dundee, Nethergate, Dundee DD6 8PW, U.K. / 5. World Agroforestry Centre, P.O. Box 30677, Nairobi, Kenya / Received October 27, 2006; accepted March 12, 2007; published online December 3, 2007
Summary
Tree root pruning is a potential tool for managing belowground competition when trees and crops are grown together in agroforestry
systems. We investigated the effects of tree root pruning on shoot growth and root distribution of Alnus acuminata (H.B. & K.), Casuarina equisetifolia L., Grevillea robusta A. Cunn. ex R. Br., Maesopsis eminii Engl. and Markhamia lutea (Benth.) K. Schum. and on yield of adjacent crops in sub-humid Uganda. The trees were 3 years old at the commencement of
the study, and most species were competing strongly with crops. Tree roots were pruned 41 months after planting by cutting
and back-filling a trench to a depth of 0.3 m, at a distance of 0.3 m from the trees, on one side of the tree row. The trench
was reopened and roots recut at 50 and 62 months after planting. We assessed the effects on tree growth and root distribution
over a 3 year period, and crop yield after the third root pruning at 62 months. Overall, root pruning had only a slight effect
on aboveground tree growth: height growth was unaffected and diameter growth was reduced by only 4%. A substantial amount
of root regrowth was observed by 11 months after pruning. Tree species varied in the number and distribution of roots, and
C. equisetifolia and M. lutea had considerably more roots per unit of trunk volume than the other species, especially in the surface soil layers. Casuarina equisetifolia and M. eminii were the tree species most competitive with crops and G. robusta and M. lutea the least competitive. Crop yield data provided strong evidence of the redistribution of root activity following root pruning,
with competition increasing on the unpruned side of tree rows. Thus, one-sided root pruning will be useful in only a few circumstances.
