Biophysical interactions in a short rotation willow intercropping system in southern Ontario, Canada

Recently, there has been interest in shifting to carbon-neutral sources of energy, including bioenergy from short rotation woody crops. This study looked at the growth and yields of short rotation willow in an agroforestry intercropping system compared to a conventional single variety plot system used as a control. Three willow clones (Salix dasyclados SV1, Salix miyabeana SX67 and Salix purpurea 9882-41) were established in each field setup, where in the agroforestry field willow plots were located between 15m wide rows of 20-year-old mixed tree species. Differences in photosynthetic photon flux density (PPFD), soil temperature, soil moisture and soil/foliar nitrogen between the two field setups were investigated from 2006 to 2007. Willow yields were significantly higher in the agroforestry fields during both years of the study, with 0.8 and 0.5odtha⁻¹ for the agroforestry and control fields, respectively, in 2006, and 3.0 and 1.1odtha⁻¹ respectively, in 2007. There were opposite trends in clonal yields between the two field setups in 2006, but in 2007, clonal yields were in the same order across fields with averages of 2.8, 2.2 and 1.2odtha⁻¹ for SV1, SX67 and 9882-41, respectively. Daily average photosynthetic photon flux density was 210μmolm⁻² s⁻¹ (16%) lower in the agroforestry system, and PPFD was correlated with soil temperatures that were on average 0.4°C and 2.7°C lower in the agroforestry field in 2006 and 2007, respectively (r =0.82 and 0.93). Soil temperatures were negatively correlated with soil moisture levels that were on average 1.4% and 1.9% higher in the agroforestry field in 2006 and 2007, respectively (r =-0.54 and -0.41), and soil moisture content was positively correlated with willow yields (r =0.49 and 0.72). There was less soil available nitrogen in the agroforestry field, but no difference in foliar nitrogen between fields. An experiment excluding root competition in the top 1 m of soil between intercropped trees and willows in the agroforestry field found no significant competition for soil moisture or nitrogen in the first two years of growth. Results of this study suggest that moderate shading in an intercropping setup can result in a buffering effect on microclimate conditions, where there is less variation in soil moisture content and soil temperature across a range of weather conditions.