Mediterranean alley-cropping agroforestry with walnut and medicinal and aromatic plants: 14-year insight in Sagàs, northeastern Spain

Abstract

Agroforestry is increasingly promoted in Mediterranean regions as a strategy to enhance system resilience and multifunctionality, although evidence remains limited for modern alley cropping systems. We report findings from a 14-year alley-cropping experiment conducted in Sagàs (northeastern Spain) combining hybrid walnut (Juglans × intermedia Mj209 x Ra) with sage (Salvia officinalis, years 1–7) and lemon balm (Melissa officinalis, years 8–14), compared with respective monoculture systems. Linear mixed-effects models, accounting for repeated measurements, were used to assess crop and tree performance and micro-site modifications over time. During the first crop phase, sage productivity was driven primarily by interannual climate variability, with no significant treatment differences. In contrast, progressive tree canopy development during the second phase reduced understory light availability (up to ~ 50% in mid-summer) and buffered soil temperature extremes by ~ 2–3 °C. Under these conditions, lemon balm plant and leaf dry biomass declined substantially under agroforestry (41% and 42% lower than monoculture averages, respectively), indicating an increasingly detrimental tree shading effect as the system matured. Similarly, plant physiological indicators (NDVI) and quality (rosmarinic acid content) were negatively affected by tree shadowing. Hybrid walnut maintained high survival and stable physiological status in both agroforestry and monocropping systems. Agroforestry trees showed slightly higher diameter and height growth than monocropping trees, especially in the first years. Because treatments were not replicated at the system level, results should be interpreted as evidence from a long-term field experiment under operational conditions, rather than as outcomes from a fully replicated experimental trial. Overall, the study highlights that tree-crop interactions in Mediterranean alley-cropping systems are dynamic over time and species-dependent, underscoring the importance of adaptive canopy management and crop selection aligned with system development.