OOS 15-3 - Intercropping for urban agriculture: plant interactions for sustainable crop production on green roofs.

With the rising interest in urban agriculture and local food, extensive green roofs are an untapped potential for both crops and citizen engagement to achieve food security, access, and resiliency. Yet, facilitative interactions between plants are vital for improving plant survival on green roofs, which are often water limited. Intercropping Sedum (Crassulaceae), the dominant plant genus of extensive green roofs, with crops, may provide beneficial effects on crop performance in these constrained systems. Our objective is to determine the facilitative effects of Sedum on crop performance in extensive green roofs. We used 48 extensive green roof modules with bean (Phaseolus vulgaris) monocropped or intercropped with Sedum, plus an artificial Sedum treatment and a high crop diversity (Brassica napus va pabularia; and Petroselinum crispum va neapolitanum) treatment. We implemented two watering levels (low: 12 – 35% moisture; high: above 35% moisture). We measured bean leaf economic traits (photosynthesis (Asat, μmol CO2 m-2 s-1); Specific Leaf Area (SLA, mm2 mg-1), leaf hydraulic traits (Instantaneous water use efficiency (WUE, mmol CO2 mol-1 H2O); chlorophyll fluorescence (Fv / Fm) crop performance traits (biomass, g; height, cm) and root traits (root diameter, mm; Specific Root Length, SRL, m g-1, Specific Root Tip Density, tips g-1). We hypothesized that beans intercropped with sedum will outperform beans in monoculture under water limitations. We also expect that bean roots will express higher acquisitive traits when intercropped with sedum. As expected, reduced watering had a large effect on performance traits, with higher available water prevailing over the effects of intercropping. Yet the hydraulic trait, WUE, was lower in beans when grown with Sedum. Bean root diameter decreased when grown with sedum with higher watering while beans grown with Sedum expressed larger root diameter with reduced watering. Our study is the first to provide evidence on the role of intercropping on extensive green roofs, generating new hypotheses for environmental and design effects on transitioning extensive green roofs into urban agriculture systems.