Positive Diversity Effect on Woody Biomass Production by Promoting Cell Number and Cell Wall Thickness

While biodiversity has been widely shown to promote tree growth in forests, the effects of diversity at the cellular level remain unclear. This hinders a deeper understanding of how diversity influences wood production. Here, we investigated the influence of neighbourhood composition on wood formation processes at the cellular level. We found that neighbourhood diversity significantly increased both the number and wall thickness of xylem fibre cells generated during the growing season, resulting in enhanced biomass growth. Compared with monocultures, woody biomass production at the highest neighbourhood species richness (NSR = 4) increased by an average of 19%–29% across the studied species. Neighbourhood diversity also led to a shift in cambium phenology. An earlier start of cambium activity and an extended duration of wall thickening were observed under higher tree species richness. Furthermore, neighbourhood competition decreased vessel area, which was closely associated with cell wall thickness. Our study offers a microscopic perspective on the effects of diversity and competition on wood formation and woody biomass production. We highlight that the contribution of diversity to woody biomass accumulation may be underestimated when variation in cell wall thickness is ignored.