Tree growth and management in Ugandan agroforestry systems: effects of root pruning on tree growth and crop yield

Abstract

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.

Root distribution of trees and crops: competition and/or complementarity.

“…ring-porous trees with large diameter xylem vessels), removal of branches will directly affect the associated roots, and vice versa (Perry, 1989). Studies of the growth of intercrops in conjunction with Alnus acuminata, Casuarina equisetifolia, Grevillea robusta, Maesopsis eminii and Markhamia lutea indicate that partial pruning of some surface roots may increase the activity of the remaining surface roots, thus providing no overall improvement in crop yield, but have little effect on tree growth (Wajja-Musukwe et al, 2008).…”

Tree species and pruning regime affect crop yield on bench terraces in SW Uganda

“…Some reports suggest there is little competition on bench terraces due to spatial or temporal separation of the trees and crops (Cooper et al 1996), although farmers have reported that trees may compete with adjacent crops (Wajja-Musukwe et al 1997;Sun et al 2008). This is important as crop production on the lower terrace is vital for food security during the first 2-3 years after planting while farmers await the benefits of trees grown on the upper terrace.…”

Agroforestry: Hydrological Impacts

“…Thus, trees usually compete with crops during the rainy season as their preexisting rooting framework provides a competitive advantage over the emerging roots of germinating crops. Though tree species vary in rooting architecture and root biomass, this is not necessarily reflected by their growth or competitiveness (Wajja-Musukwe et al, 2008). An evaluation of the ratio of root number in the top 1.8 m of soil to trunk volume has shown that Casuarina equisetifolia and Markhamia lutea had significantly higher ratios than A. acuminata, Maesopsis eminii, and Grevillea robusta; Casuarina and Maesopsis were most competitive with crops, whereas Markhamia and Grevillea were least competitive.…”

“…Pruning offers a direct and adaptable method of controlling competition and has been the focus of several studies in East Africa (Tefera, 2003;Rao et al, 2004;Wajja-Musukwe et al, 2008;Namirembe et al, 2009;Siriri et al, 2010Siriri et al, , 2012. This practice enables farmers to grow the tree species they prefer by managing their growth.…”