Abstract: Trait-based functional diversity (FD) is an important predictor of tree species ecological multifunctionality (TS-EMF), but its relationship may be mediated by environmental factors. Currently, the study of threshold-dependent relationship between FD and TS-EMF along urban and suburban gradients and their environmental regulatory mechanisms is still quite limited. In this study, 12 typical tree species from urban and suburban forests in Shenyang were used to calculate TS-EMF by combining the multithreshold and averaging method, and to assess community FD, aiming to reveal the role of FD on TS-EMF and how environmental factors regulate TS-EMF through FD and community-weighted mean (CWM) traits. The results showed that urban TS-EMF was generally higher than suburban (P < 0.05). There were differences in the driving mechanisms of TS-EMF at different threshold levels, with air humidity (total effect: 0.435) and CWM P_n (net photosynthetic rate, relative importance: 24.42%) being the key drivers at high threshold levels. At low threshold levels, functional evenness (FEve) played a dominant role, but the extent to which influenced TS-EMF depended on the type and number of tree species within the TS-EMF threshold range. Notably, the effects of CWM P_n and FEve on TS-EMF showed threshold dependence, with thresholds of 61.18% and 64.47%, respectively. Additionally, the urban–suburban gradient could significantly influence the driving mechanism: the direct effect of environmental factors and CWM traits prevailed in urban forests, while suburban forests showed a multifactorial cascade effect. The study showed that the formation of TS-EMF in urban forests is the result of multifactorial coupling of traits, FD and environmental factors, and this finding provides a new theoretical perspective for understanding the ecosystem service drivers of urban forests.

Key words: urban forest, ecological multifunctionality, functional diversity, urbanization, environmental change