Farmers across Kenya have long planted Grevillea trees alongside maize, hoping the tall, fast-growing species would add timber, shade and income without hurting the crop underneath. A long-term field trial in semi-arid Kenya put that idea to the test over 4.5 years and nine growing seasons, tracking maize and cowpea grown either alone or alongside grevillea trees. The findings come from a study called Long-term productivity of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya: II. Crop growth and system performance , published in the journal Forest Ecology and Management, based on a trial at Machakos in eastern Kenya. For the first four seasons, crop yields in the tree and maize plots matched those grown without trees. But as the grevillea matured, yields collapsed in most later seasons, falling close to zero in three of the final four, and reaching as much as half the tree-free yield only in the one season when rainfall was unusually high, pointing to a problem hiding underground rather than up in the canopy. This comprehensive long-term study serves as a crucial reminder that agroforestry benefits can diminish significantly over time, highlighting the importance of understanding evolving root-level competition before committing to permanent farming systems.
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