Multiple mechanisms drive biodiversity-ecosystem service multifunctionality but the dominant one depends on the level of multifunctionality for natural forests in northeast China

Biodiversity plays a key role in maintaining and regulating ecosystem multifunctionality (EMF). However, there remains a knowledge gap on the ecological mechanisms behind the relationship between biodiversity and EMF (BEMF), and how these mechanisms vary with EMF gradients, especially at large scale. We used the dataset of 2224 permanent sample plots in Jilin Province, northeast China to test three BEMF mechanisms and their var-iations with EMF, including mass ratio, ecological niche complementarity and the green soup hypotheses. Biodiversity metrics were taxonomic, structural, phylogenetic, functional diversity and functional identity. EMF was composed of five services: water retention, soil conservation, carbon storage, stand productivity and soil fertility maintenance. EMF was measured using both the averaging (as the average of the standardized values of all services) and threshold (as the number of services passed a threshold) approach. Totally 23 piecewise structural equation models (pSEMs) were developed for EMF with the threshold level ranging from 5% to 95% by an interval of 5%. Results showed that stand biomass had strong positive effect on EMF. Stand structural diversity had significant direct and indirect effects on EMF, but taxonomic, functional diversity and functional identity only had significant direct and indirect effects on EMF at low thresholds. Stand age had significant direct effects on EMF and indirectly enhanced EMF through stand structural diversity and biomass. Our results provided strong evidence that EMF was driven simultaneously by mass ratio, niche complementarity and green soup hypotheses, but the dominant mechanism was dependent on EMF level. Generally, the green soup hypothesis played more important role in explaining EMF. Recommendations for maintaining high functional and structural diversity and large biomass for maximizing EMF were discussed for forest management.