Yield Potential of Tree Species in Organic and Conventional Short-Rotation Agroforestry Systems in Southern Germany

The increasing demand for bioenergy and the combination of agricultural production with conservation has made short-rotation agroforestry systems (SRAFS) a sustainable land-management option. Aboveground woody biomass is a decisive factor in economic and ecological assessment of those systems. To study the yields of organic and conventional SRAFS, the tree species black alder, black locust, poplar clone Max 3, poplar clone Androscoggin, willow clone Inger, and a mixture of different native species were established in an alley-cropping configuration in 2009 and coppiced in 2012. Biomass was determined by harvesting the inner rows of the tree strips and, to investigate row differences within a strip, by an allometric model which estimates tree biomass from stem diameter. Significant variation was observed between species. For inner rows and at the conventional system, highest harvested average annual yield was observed for poplar Androscoggin (10.5 todt ha−1 year−1), followed by black locust (9.7 todt ha−1 year−1), poplar Max 3 (8.6 todt ha−1 year−1), black alder (7.6 todt ha−1 year−1), the native mix (4.9 todt ha−1 year−1), and willow (3.9 todt ha−1 year−1). At the organic system, highest yields were observed for poplar Max 3 (Androscoggin not planted) (10.9 todt ha−1 year−1), followed by black locust (8.1 todt ha−1 year−1), black alder (7.4 todt ha−1 year−1), willow (6.4 todt ha−1 year−1), and the native mix (4.7 todt ha−1 year−1). Farming system differences were only significant for willow and poplar Max 3; however, the higher yields of the organic system seemed to be a result of varying small-scale site properties rather than a management effect. Border rows showed 18–111 % more yield than inner rows because of greater tree diameters or heights and higher number of stems. This edge effect was emphasized in the conventional systems, possibly indicating that trees benefit from fertilizers applied at adjacent crop fields.