The optimal size of a Paulownia-crop agroforestry system for maximal economic return in North China Plain

Intercropping system has been widely adopted in China to reduce soil erosion, disease and pest damage, to increase biodiversity and land productivity over the last four decades. However there is no clear guideline on what is the optimal size of an intercropping system for maximizing the economic return in the region. In this study, we measured the effects of a two to five-year old Paulownia tree belt on winter wheat and summer maize yield in a 160 m by 320 m Paulownia-crop intercropping system in North China Plain. Field measurements showed that tree shading reduced the incoming photosynthetically active radiation (PAR) at crop by up to 26% for winter wheat and up to 38% for summer maize at the location near the tree belt over their respective growing season, and the effect of tree shading became insignificant at a distance of more than 15 m from the tree belt. The effect of tree shading on crop yield was proportional to the effect on incoming PAR at a rate of 0.32 g mol(-1) for summer maize and 0.11 g mol(-1) for winter wheat at the study site. A model of PAR transmission was also validated using field measurements and was then used to estimate the reduction of crop yield as a result of shading by the tree belt at different tree age or size for a range of plot size of a Paulownia-crop intercropping system. Together with the estimated cost and benefit, we found that the optimal size of a typical Paulownia-crop intercropping system was 26 ha with two rows of tree at 4 m spacing with pruning at the study site. The optimal size increases with grain price and decreases with timber price. The approached we developed here can be used to guide the future design of Paulownia-crop intercropping systems in North China Plain.