Efficient temperate agroforestry systems often associate deciduous tree species with winter crops, resulting in a limited period of light competition, from tree bud burst in spring until crop harvest in summer. For a walnut wheat system, light competition occurs for a 3-month time span only, from April to June. Climate change may affect the timing of phenological stages of both trees and crops. Temperature increases may result in a slower mean rate of completion for the chilling requirement, and a higher mean rate of completion for the heat requirement of both trees and crops. Former investigations showed that a warming in the early spring (February–April) by 1°C causes an advance in the beginning of the growing season of some tree species of 7 days. Differential responses of trees’ and crops’ phenologies to climate change may lead to a significant change of tree and crop complementarity for resource capture and use, and therefore affect the total productivity of the system. Conversely, the adequate choice of tree and crop varieties with properly lagged phenologies could be a way to improve the efficiency of agroforestry systems. We used the Hi-sAFe process-based simulation model to: 1) evaluate the part of the total over-yield that may be explained by phenology lags; 2) investigate the sensitivity of the performance of the agroforestry system to the tree and crop phonologies; and 3) explore the impact of global warming on the efficiency of a temperate agroforestry system. Several criteria were used to synthesize the effect of increased temperatures, total capture of resources (light and water) by the trees and crops, and global efficiency of the system as expressed by its land equivalent ratio