Understory diversity is positively associated with overstory carbon sequestration

Forests are a major terrestrial carbon sink, and higher overstory diversity is often associated with greater carbon accrual in trees and soils. Yet the contribution of the understory vegetation, the most species-rich forest stratum, to tree and soil carbon accumulation remains largely unknown. Here, we analyzed Canada's National Forest Inventory database to explore whether understory diversity (richness and evenness of shrubs, herbs, and bryophytes) is associated with carbon accumulation in overstory trees and soils across environmental gradients of air temperature, climate moisture, and nitrogen deposition. We found that herb and bryophyte species richness were positively associated with decadal tree carbon accumulation, with the strongest effects at warmer, drier, and nitrogen-limited sites. In contrast, understory diversity showed no consistent association with soil carbon accumulation. On a decadal scale, increasing herb and bryophyte species richness from their minimum to median values corresponded to 15% and 6% increases in tree carbon accumulation, respectively. These results provide large-scale observational evidence that diversity in lower vegetation layers is positively associated with overstory carbon sequestration and reveal strong environmental modulation of these multistratal biodiversity-ecosystem functioning relationships. These findings suggest that species-rich herb and bryophyte communities should be considered in strategies for sustaining forest carbon sinks, especially under warmer, drier conditions and nutrient limitation.