Herbaceous competition does not affect positive tree diversity effects on seedling crown complementarity

Naturally regenerated herbaceous species play a significant role in shaping community structure and stand dynamics, particularly through their interactions with tree seedlings in early-successional stands. The seedling and grass competition can affect seedling crown complementarity, a key canopy process that enhances the productivity of tree mixtures compared to monocultures by optimizing light capture and resource use. However, little is known about how their competition advantages shift from “grass to seedling” or “seedling—grass—seedling” or just grass. We conducted a four-year tree diversity experiment involving weeded versus unweeded treatments, with eight functionally divergent woody species comprised of 17 functional combinations. We examined how crown complementarity, crown volume, and stem basal area of tree seedlings varied between weeded and unweeded treatments along the gradients of tree species richness and functional diversity over time. Crown complementarity and crown volume of tree seedlings were positively affected by tree species richness and functional diversity, and the positive tree diversity effects on seedling crown complementarity strengthened over time under both weeded and unweeded treatments. On average, seedling crown complementarity was greater in unweeded than weeded treatments. Herbaceous plants increased crown volume but decreased stem basal area after three years, and the tree diversity effects on stem basal area were not significant in both weeded and unweeded treatments. Overall, herbaceous plants did not change the tree diversity effects on seedling crown complementarity, but mediated biomass allocation trade-offs between crown expansion and horizontal growth of stem in tree seedlings during the early stage of stand development. Our results suggest that herbaceous plants can boost seedling crown complementarity but their competitive effects are not large enough for affecting the positive tree diversity effects on seedling crown complementarity in the early stage of stand development. This study provides mechanistic insight into the role of herbaceous plants in shaping tree diversity effects on canopy processes and stand structures through resource competition during the early successional period of forest communities.