Rates and causes of western redcedar and yellowcedar mortality in old-growth forests of mount rainer national park

We examined the rates and causes of mortality for Thuja plicata Donn ex. D. Don (western redcedar) and Chamaecyparis nootkatensis (D. Don) Spach (yellow cedar), and in contrast with co-occurring Pinaceae, using a three decade record of forest dynamics in old-growth forests of Mount Rainer National Park, WA, USA. All trees ≥15 cm dbh were tagged within 12, 1.00 ha permanent plots; trees were added to the study when they grew past the lower sampling size limit. Plots were censused every 4 to 10 years. Detailed observations of position, condition and causes of mortality were recorded for newly dead trees. The fates of 625 C. nootkatensis and 226 T. plicata were followed: a total of 58 C. nootkatensis and 13 T. plicata stems died during the study. Mortality rates of T. plicata and C. nootkatensis were exceptionally low, with average annual rates of 0.1% to 0.2% and 0.2% to 0.3%, respectively. Mechanical processes--uprooting, stem breakage and crushing by falling macrolitter--accounted for 69.2% of T. plicata mortalities; these same agents were responsible for 53.4% of C. nootkatensis mortalities. In contrast, a smaller proportion (43.4%) of co-occurring Pinaceae species were killed by mechanical processes, suggesting a family-level tradeoff characterized by lower susceptibility to biological mortality agents and greater tolerance of competitive stress for the Cupressaceae, and relatively greater resistance to mechanical mortality agents by Pinaceae in old-growth forests of the Pacific Northwest.