Neighbour effects on plant biomass and its allocation for forbs growing in heterogeneous soils

Focal plants are considerably affected by their neighbouring plants, especially when growing in heterogeneous soils. A previous study on grasses demonstrated that soil heterogeneity and species composition affected plant biomass and above- and belowground allocation patterns. We now tested whether these findings were similar for forbs. Three forb species (i.e. Spartina anglica, Limonium bicolor and Suaeda glauca) were grown in pots with three levels of soil heterogeneity, created by alternatively filling resource-rich and resource-poor substrates using small, medium or large patch sizes. Species compositions were created by growing these forbs either in monocultures or in mixtures. Results showed that patch size × species composition significantly impacted shoot biomass, root biomass and total biomass of forbs at different scales. Specifically, at the pot scale, shoot biomass, root biomass and total biomass increased with increasing patch size. At the substrate scale, shoot biomass and total biomass was higher at the large patch size than at the medium patch size, both in resource-rich and resource-poor substrates. Finally, at the community scale, monocultures had more shoot biomass, root biomass and total biomass than those in the two-species mixtures or three-species mixture. These results differ from earlier findings on the responses of grasses, where shoot biomass and total biomass decreased with patch size, and more shoot biomass and total biomass were found in resource-rich than resource-poor substrate. To further elucidate effects of soil heterogeneity on the interactions between neighbour plants, we advise to conduct longer-term experiments featuring a variety of functional groups.