Productivity at the tree-crop interface of a young willow-grassland alley cropping system

Alley cropping multi-rows of shrub willow hybrids and grassland is a promising temperate agroforestry practice for an environmentally sound provision of bioenergy feedstock. The effect of willows, aged 2–3 years, on two grassland mixtures (clover-grass, diversity oriented mixture) was determined at three positions along the tree-crop interface at a study site in Central Germany. Willows modified the incident light on understory along the interface. Biennial mean daily light integral at position south-west (SW) was 22 mol m⁻² w⁻¹, in the center of the alley 30 mol m⁻² w⁻¹ and at position north-east (NE) 26 mol m⁻² w⁻¹. Accordingly, soil temperature was lower at the positions SW and NE being adjacent to the willows. There was no clear pattern of the distribution of volumetric soil moisture content along the tree-crop interface in 15 cm depth, except that moisture content was highest in 35 cm depth at SW position in both years. In the early establishment phase, the diameter at breast height (DBH) of pooled inner willow rows (17 mm) was significantly different from pooled outer rows (21 mm). Direction had a significant influence on DBH in 2012, but not in 2013. The impact of willows on productivity of the two grassland mixtures was not confirmed until the third year after establishment. Dry matter yield was on par with those reported for single-cropped grassland adjacent to the agroforestry system. Sward composition of clover-grass changed along the tree-crop interface. Dry matter contribution of legumes was lower at the position SW. No remarkable impact of trees on quality parameters of grassland mixtures were found along the interface. Horizontal and vertical growth of the trees may modify the microclimate during the life-span of the alley cropping system consisting of willows and grassland. More research is needed on long-term monitoring of competitive, complementary and facilitative effects along the tree-crop interface.