Crop functional redundancy is as important as crop functional diversity in weed suppression during early agroforest succession

Abstract A trait‐based ecology approach can be used to assess different components of biodiversity, that is, functional diversity (FD, diversity of functional traits) and functional redundancy (FRed, diversity of species performing similar functions in the ecosystem). In order to test the relationship of crop and weed functional structure (i.e., FD and FRed) in weed suppression we used data from an agroforestry experiment, established in late 2016 in humid subtropical Southern Brazil. We quantified traits of crops and weeds and used Structural Equation Modelling to test a causal model that explains how crop FD and crop FRed affect (1) weed FD, (2) weed FRed, (3) soil cover by crops and (4) soil cover by weeds. We found that the most important traits driving the patterns of FD and FRed in the crop community are maximum plant vegetative height (H) and leaf nitrogen concentration (LNC). Crop FRed negatively affected weed cover, directly and indirectly by decreasing weed FRed, which was positively related to weed cover. High crop FD increases soil cover by crops. Crop cover and weed cover are negatively related, and these two variables were used as a surrogate for weed suppression. On the one hand, weeds can reduce crop cover through resource competition, but on the other hand, crop resource pre‐emption is a key mechanism that drives weed suppression. Agroecosystem properties like soil cover by crops, that require high crop FD, can be achieved by mixing crop species that both vary in H and LNC (high FD), as well as other crops with similar values of these traits (high FRed). Therefore, it is possible to achieve a sustainable weed management in the critical early phase of agroforestry establishment by enhancing both components of planned agrobiodiversity (FD and FRed).