Restoration of functionally diverse forests has been proposed as an effective nature-based solution to mitigating climate change and maintaining ecosystem functioning. Numerous experimental studies indicate that tree functional diversity usually increases forest productivity. In most forest experiments, herbaceous plants are typically weeded to avoid the competition between herbaceous plants and tree individuals. However, little is known about how herbaceous plants influence the relationship between tree functional diversity and productivity, and therefore contribute to the total forest community productivity. We conducted a 4-year non-weeded tree diversity experiment composed of eight woody species differing widely in plant economic strategies to test the role of herbaceous plants on the relationship between tree functional diversity and early-stage forest productivity. The positive relationship between tree functional diversity and tree aboveground productivity was robust under both weeded and non-weeded treatments, whereas herbaceous plants weakened the relationship between tree community-weighted mean (CWM) of acquisitive traits and aboveground productivity. There were weak negative effects of tree functional diversity on herbaceous aboveground productivity, but the positive effects on community total productivity strengthened through time. Tree functional diversity increased the dominance of herbaceous species with functional traits typical of low-light conditions (lower CWM of maximum plant height, but larger CWM of specific leaf area), and promoted shifts from herb-dominated to tree-dominated communities through time. This study highlights the significance of the natural community processes between target and non-target species in deeply understanding the effects of tree diversity on forest ecosystem functioning and forest restoration.
