Optimizing crop types in temperate alley-cropping agroforestry depends on tree age

Temperate alley-cropping agroforestry with short-rotation coppice is gaining interest due to its multifunctionality in agroecosystems; however, yield uncertainties from potential tree-crop competition and unknown wood production limit its adoption. To our knowledge, this is the first study to compare crop yields between agroforestry and open cropland while quantifying wood production across tree ages and rotations. Our objectives were to (1) quantify crop yields and wood production of poplar ( Populus nigra × × Populus maximowiczii ) aged 1–9 years across the first four rotations and (2) determine how tree age affects spring and winter crop yields in agroforestry relative to open cropland across sites and years. The study was conducted over six years (2018–2023) at four sites on different soils in Germany. Here we show that, compared to open cropland, spring crops in agroforestry had 10–35% lower yields when trees were ≥ 5 years old, attributed to yield reductions at distances of 1–4 m from the tree rows due to light limitation by tree shade. There were no significant differences in soil water and plant-available nutrients between agroforestry and open cropland. Winter crops had comparable yields between agroforestry and open cropland across 1- to 9-year-old trees. Annual wood production increased with tree age and was three times as high in the third and fourth tree rotations as in the first and second tree rotations. To optimize crop yields and wood production in agroforestry, we recommend both spring and winter crops when trees are ≤ 3 years old and only winter crops when trees are > 4 years old. Alternatively, shade-tolerant crops can be cultivated with winter crops near the trees (≤ 4 m) in agroforestry as tree age increases (> 4 years).