Tree mortality rates and longevity in mature and old-growth hemlock-hardwood forests

1 Efforts to understand and forecast tong-term forest dynamics are often hindered by limited data on mortality rates and longevity of trees in older stands. In this study, mortality data were analysed from 11-year permanent plot records in 10 tracts of hemlock-hardwood forest with little past human disturbance in the upper Great Lakes region, USA. 2 We compared tree size-mortality trends in mature stands (canopy trees mostly 100-170 years old) with those in true old-growth stands (canopy trees mostly 200-350 years old). Stem sections were also cut from 71 recently fallen trees to determine average and maximum longevity for canopy trees of each species. 3 All five mature stands had descending monotonic size-mortality trends for shade-tolerant species, with low rates of mortality (generally much less than 1% per year) for trees > 18 cm d.b.h. In contrast, all five old-growth stands had U-shaped mortality functions, with annual mortality rates of 1.5-2.9% for trees > 66 cm d.b.h. These size-mortality trends are biologically consistent with 'rotated sigmoid' size distributions observed for major species, although mortality rates were nearly size-invariant for trees between 18 and 66 cm d.b.h. 4 Both late-successional and gap-phase species had fairly high mortality rates for saplings (2-10 cm d.b.h.) of 25-34% over the 11-year period, which suggests that size distributions with moderately steep negative slopes are needed to compensate for mortality and ensure population sustainability. 5 Average age at time of death for canopy trees was 216 years for sugar maple (Acer saccharum) and 301 years for hemlock (Tsuga canadensis), suggesting that the biological transition from mature to old-growth forest probably doesn't occur on these sites until a stand age of about 180-250 years.