What it is
Forests are a major carbon sink on land, and greater tree diversity in the overstory is often associated with more carbon accumulating in trees and soils. Yet the contribution of the understory, the most species-rich layer of a forest, to carbon accumulation in trees and soils has remained largely unknown. Using Canada's National Forest Inventory database, the authors asked whether understory diversity, the richness and evenness of shrubs, herbs and bryophytes, is associated with carbon accumulation in overstory trees and in soils across gradients of air temperature, climate moisture and nitrogen deposition. The species richness of herbs and of bryophytes was positively associated with tree carbon accumulation over a decade, with the strongest effects at warmer, drier and nitrogen-limited sites, while understory diversity showed no consistent association with soil carbon accumulation. On a decadal scale, raising herb and bryophyte species richness from their minimum to their median values corresponded to increases in tree carbon accumulation of 15% and 6% respectively. The authors present this as large-scale observational evidence that diversity in the lower vegetation layers is positively associated with overstory carbon sequestration, strongly modulated by the environment, and suggest that species-rich herb and bryophyte communities be considered in strategies for sustaining forest carbon sinks, especially under warmer, drier and nutrient-limited conditions.
Why it matters
In Canada's forest inventory, more herb and bryophyte species underfoot went with faster tree carbon accumulation over a decade, most at warm, dry, nitrogen-limited sites. Raising herb richness from its minimum to its median value corresponded to 15% more tree carbon accumulation, and bryophyte richness to 6%. The understory is the most species-rich layer of a forest, yet its part in the forest carbon sink had been largely unknown, and the authors suggest that strategies for sustaining forest carbon sinks take these communities into account.
Underlined numbers link to their source. Every metric and quoted figure is listed under Sources and data below.
Filed underForest ecology and management, Ecology and Vegetation Dynamics Studies, Soil Carbon and Nitrogen Dynamics
