The tree mortality regime in temperate old-growth coniferous forests: the role of physical damage

Aspects of the tree mortality regime were characterized for old-growth conifer forests in Mount Rainier National Park, Washington, USA, using individual tree (stems >= 5 cm diameter at breast height (dbh)) records from a network of permanent forest research plots. Average annual forest-wide mortality rates of trees >= 15 cm dbh never exceeded 1% on a stem-density or basal-area basis; mortality was slightly higher for stems <15 cm dbh. Physical agents of mortality (uprooting, stem breakage, and crushing by falling debris) accounted for approximately 40% and 45% of mortality events in trees <15 and >= 15 cm dbh, respectively. These physical processes were chronic sources of mortality: they were not associated with a single or few disturbance events. Preexisting decay fungi were associated with trees that died proximately due to stem breakage (41%) and uprooting (22%), consistent with a predisposing role of decay fungi in trees that die due to mechanical damage. Given the importance of physical processes in the tree mortality regime, we suggest that a richer mechanistic understanding of the causes and consequences of tree mortality in natural forests will be achieved with models that consider the physical, as well as the physiological, attributes of trees and forests.