Plant growth patterns in a tripartite strip relay intercrop are shaped by asymmetric aboveground competition

Abstract

Intercropping is a promising model for ecological intensification of modern agriculture. Little information is available on how species growth patterns are affected by size-asymmetric above- and belowground competitive interactions, especially in intercrops with more than two species. We studied plant growth and competitive interactions in a novel intercropping system with three species: wheat, watermelon and maize. Wheat and maize are grown sequentially (as a double cropping system) in narrow strips while watermelon is grown between the cereal strips, with partial overlap in growing period with the two cereals. Growth patterns were monitored over two years and described with logistic growth curves. Root barriers were used to study the effect of belowground interactions. Wheat produced 31% greater yield per plant in the intercrop than in the sole crop but 24% lower yield per unit total (inter)crop area. Wheat yield increase per plant was associated with faster growth and substantial overyielding in the outer rows of wheat strips. Watermelon did not competitively affect wheat. Watermelon biomass was substantially reduced at the time of wheat harvest. However, compensatory growth after wheat harvest and greater allocation to fruits resulted in a good yield of intercropped watermelon, 92% of monoculture yields, at final harvest. Intercropped maize produced 32% lower grain yield per plant and per unit area than sole maize, as a consequence of later sowing and a changed plant configuration in the intercrop as compared to the sole crop, and competitive effects of watermelon, as shown by comparison with a skip-row maize system without watermelon. Root barriers did not affect yield of any of the species, indicating that aboveground competitive interactions in this case played a more important role in shaping the observed growth responses than belowground interactions. Plant interactions in this tripartite intercrop system are consistent with the hypothesis of size-asymmetric competition for light.