A meta-analysis on the impact of trees on yield of intercrops in alley-cropping systems of temperate climates

CONTEXT: The environmental benefits of agroforestry have been highlighted worldwide, although improved intercrop productivity has been clearly demonstrated only in the tropics. OBJECTIVE AND METHODS: This meta-analysis aimed at summarizing knowledge from 18 trials on grain yield of arable intercrops in alley-cropping systems of temperate climates, within a mixed-effect model framework. RESULTS AND CONCLUSIONS: A general negative impact of trees on crop grain yield was documented, with an average reduction by 30 % compared to full sun, across the whole inter-row of wheat, barley, soybean and maize. Key findings included: (i) distance from trees is the major driver of the grain yield response, with increasing impact in the vicinity of the tree row, (ii) significance of crop phenology and species choice, with lower impact on winter vs. summer crops; (iii) tree age is the only relevant variable of the woody component, with increasing impact with aging; and (iv) available rainfall and potential evapotranspiration are key moderators, with a less detrimental or positive impact of trees under low rainfall and high evapotranspiration. This study also describes the implications of some tree design and management practices: (i) branchless tree rows allows for halving the alley width where crop yield is negatively affected by the tree line compared to a hedgerow, (ii) crop yield recovers at distances (D) from the tree row at which D/tree heigh approaches 1, this suggesting the interrow should be at least twice the maximum height of trees, (iii) intercrop should be varied across the tree cycle, by cultivating high shade-tolerant species/varieties when the tree age is >8 years. SIGNIFICANCE: This meta-analysis underscores the need for further empirical studies on other intercrops and within climatic zones where limited data is currently available.