Productivity of poplar short rotation coppice in an alley-cropping agroforestry system

In temperate regions, short rotation woody crops cultivated as tree strips in alley-cropping agroforestry systems (ACS) can provide economic benefits by producing an additional commodity, and environmental benefits, e.g., soil erosion control, protection against evaporation and increased biodiversity. Moreover, compared to agrisivilcutural systems incorporating noble trees, ACS with short rotation coppice (SRC)-strips provide periodically high energy outputs and could help to answer environment and energetic political objectives in Germany. However, limited data are available in such systems concerning biomass production of different strip designs. An ACS incorporating poplar SRC has been established near Braunschweig in Lower Saxony (Germany) in 2008. Two harvest cycles (a coppiced 3-year rotation cycle and a un-coppiced 6-year rotation cycle) and two strip designs (“SRC”: 6 poplar rows; “Combined”: 4 poplar rows and 1 aspen row in the centre) were compared. Diameters at breast height, tree heights, shoot numbers and mortality rates were measured to describe growth and estimate yield of outer and middle poplar rows within a tree strip. Concerning the 3-year rotation cycle, higher numbers of shoots per tree as well as higher biomass yields compared to the control field were measured in outer rows, both leeward and windward. With the 6-year rotation cycle, all leeward rows and the middle rows of the combined design showed larger diameters and higher biomass yields. Middle rows of the SRC design, in both rotation cycles, revealed a quicker height growth than outer rows, but a reduced biomass production. Both rotation cycles showed similar yearly biomass production. The results can contribute to improve the design of poplar SRC-strips in ACS in order to optimize biomass production. We recommend reducing the number of rows within SRC-strips, while increasing their total length. Further research is however needed to determine effects of increased biomass in outer tree rows on adjacent crop fields, which influences the whole system productivity.