Biophysical interactions in perennial biomass alley cropping systems

Understanding the nature and degree of competition between trees and co-planted crops in agroforestry systems can inform management decisions, future species selection, and system design. We measured variation in herbaceous biomass yield, photosynthetically active radiation (PAR), soil water potential, and residual soil NO₃ in alley cropping systems consisting of prairie cordgrass (Spartina pectinata) or an 11-species native grass-forb-legume polyculture planted between rows of shrub willow (Salix purpurea L. ‘Fish Creek’) at two Minnesota sites. At Empire, biomass yield increased with distance from the tree row for both alley crops, as did PAR and NO₃ availability. At Granada, no spatial pattern in crop yield was evident, despite reduced PAR and NO₃ availability adjacent to tree rows. At both sites, patterns in soil water potential suggested that trees competed with crops for soil water within 2.4 m of tree rows, but had a facilitative effect on crop water use at 4.8 m. Alleys had differing cardinal orientations at the two sites, and light availability was lower in a north–south alley (Empire) than in west–east and northwest-southeast orientations (Granada). At Empire, mixed effects analysis indicated that competition for light and soil water were responsible for reductions in crop yield. For every 100 μmol m⁻² s⁻¹ increase in PAR, model estimated herbaceous crop yield increased by 623 kg DM ha⁻¹. For every 20 kPa increase in average daily water potential, crop yield increased by 1038 kg DM ha⁻¹. The relative impact of competition on biomass yield was similar for both crops, though prairie cordgrass had higher average yields at Empire. Similar alley systems should avoid north–south alley orientations to minimize competition for light.