Effects of defoliation on growth characteristics and N, P, K content in an alder/maize agroforestry system

The effects of defoliation on the growth characteristics and N, P, K content of potted alder (Alnus acuminata) and companion maize (Zea mays) grown outdoors in sole and mixed cropping were examined. Alnus seedlings were subjected to 0, 25, 50 and 75% repeated artificial defoliation. Defoliation reduced the height and diameter, and stem, leaf, root and nodule biomass in Alnus. Alnus showed higher, albeit mostly not significant (P > 0.05), growth performance in mixed culture than in sole culture. However, the biomass of Alnus nodules was significantly lower in mixed compared to sole culture. Defoliation increased Alnus leaf N, P, K and nodule P and K concentrations, but decreased nodule N concentration and also significantly reduced both leaf and nodule total N, P, K content. In contrast, defoliation of Alnus increased the height, biomass and total N, P, K content in companion maize crop. The most severe defoliation treatment (75%) increased maize height by 57%, total biomass by 209%, and shoot total N, P, K by 209%, 238% and 208%, respectively compared to maize grown in association with non-defoliated trees. However, maize grown in sole culture showed significantly higher height, biomass and total shoot N, P, K content than did maize intercropped with Alnus. Results of this study indicate that defoliation of Alnus may not only influence the performance of the species, but it may also have significant effects on the performance of associated plants with important bearing in nutrient cycling in agroforestry.