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

Abstract

Summary1 Efforts to understand and forecast long-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 shadetolerant 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 sizemortality 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.

A Demographic Approach to Evaluating Tree Population Sustainability

“…This is also the case for the northern hardwood permanent plot records, spanning 54 years, in CANOPY's calibration data set [18,36]. These U-shaped mortality-size trends are consistent with the observed rotated sigmoid structure in old-growth stands.…”

“…Depending on time demands of a particular simulation design, either 500 or 1000 year time frames were examined, a period about two to four times the average age at time of death for canopy trees in this ecosystem [36]. The simulations in this paper were designed to examine the long-term consequences of a particular demographic structure under current environmental conditions, not to predict future consequences of global environmental change.…”

Community Composition and Tree Population Structure in Undisturbed and Disturbed Tropical Semi-Evergreen Forest Stands of North-East India

“…However, the lower density of lower diameter class (10-20 cm dbh) as compared to intermediate girth class (21-30 dbh) give the appearance of a positively skewed distribution curve, in both stands and can be explained as the result of selective felling of lower as well as higher girth classes in the disturbed stand, while in the undisturbed stand it can be interpreted as due to low tree mortality and lower removal rate across the intermediate diameter classes [51,66] and lower recruitment of component tree species. The lower density of the higher dbh classes of trees in the undisturbed stand, as compared to intermediate or lower dbh classes, can be attributed to the relatively high mortality of large canopy trees [20,35]. The above characteristics of girth classes also suggests that the forest is still growing and yet to reach to its climax stage as it is only of about 25 years old forest.…”

Late-Successional and Old-Growth Forests in the Northeastern United States: Structure, Dynamics, and Prospects for Restoration

“…At first glance, the study by Filip et al [35] might appear to be an exception, but the use of "old growth" in that study does not conform to modern definitions. Thus, in general, there is a need for studies that link structure to dynamics and underlying mechanisms [33], and analyses of data from remeasured plots (e.g., [36][37][38][39]) are rare and geographically sparse. Moreover, in the context of restoration, there is a need for more specific silvicultural targets and guidelines that reflect the likely dynamics and changes in these forests over time, including responses to disturbance.…”