The importance of large-diameter trees to the creation of snag and deadwood biomass

Background Baseline levels of tree mortality can, over time, contribute to high snag densities and high levels of deadwood (down woody debris) if fire is infrequent and decomposition is slow. Deadwood can be important for tree recruitment, and it plays a major role in terrestrial carbon cycling, but deadwood is rarely examined in a spatially explicit context. Methods Between 2011 and 2019, we annually tracked all trees and snags >= 1 cm in diameter and mapped all pieces of deadwood >= 10 cm diameter and >= 1 m in length in 25.6 ha of Tsuga heterophylla / Pseudotsuga menziesii forest. We analyzed the amount, biomass, and spatial distribution of deadwood, and we assessed how various causes of mortality that contributed uniquely to deadwood creation. Results Compared to aboveground woody live biomass of 481 Mg ha(-1) (from trees >= 10 cm diameter), snag biomass was 74 Mg ha(-1) and deadwood biomass was 109 Mg ha(-1) (from boles >= 10 cm diameter). Biomass from large-diameter trees (>= 60 cm) accounted for 85%, 88%, and 58%, of trees, snags, and deadwood, respectively. Total aboveground woody live and dead biomass was 668 Mg ha(-1). The annual production of downed wood (>= 10 cm diameter) from tree boles averaged 4 Mg ha(-1) yr(-1). Woody debris was spatially heterogeneous, varying more than two orders of magnitude from 4 to 587 Mg ha(-1) at the scale of 20 m x 20 m quadrats. Almost all causes of deadwood creation varied in importance between large-diameter trees and small-diameter trees. Biomass of standing stems and deadwood had weak inverse distributions, reflecting the long period of time required for trees to reach large diameters following antecedent tree mortalities and the centennial scale time required for deadwood decomposition. Conclusion Old-growth forests contain large stores of biomass in living trees, as well as in snag and deadwood biomass pools that are stable long after tree death. Ignoring biomass (or carbon) in deadwood pools can lead to substantial underestimations of sequestration and stability.