Trends in gap dynamics among pole, mature, and old-growth northern hardwood stands were investigated on eight sites in the Porcupine Mountains of western upper Michigan. Recent gaps (created between 1981 and 1992) were identified using permanent plot records of tree mortality, while older gaps (1940–1981) were identified using stand reconstruction techniques. Although canopy gaps were somewhat more numerous in pole and mature stands, gaps were <25% as large as those in old-growth stands because of smaller gap-maker size, and the proportion of stand area turned over in gaps was only about half as large. Gap makers in younger stands generally had mean relative diameters (ratio of gap-maker DBH to mean DBH of canopy trees) <1.0 and were disproportionately from minor species such as eastern hophornbeam (Ostryavirginiana (Mill.) K. Koch). Gap makers in old-growth stands had mean relative diameters >1.5 and were predominantly from the dominant canopy species. Even in old-growth forests, most gaps were small (mean 44 m2) and created by single trees. Based on the identity of the tallest gap tree in each gap, nearly all shade-tolerant and midtolerant species have been successful in capturing gaps, but gap capture rates for some species were significantly different from their relative density in the upper canopy. The tallest gap trees of shade-tolerant species were often formerly overtopped trees, averaging more than 60% of the mean canopy height and having mean ages of 65–149 years. Canopy turnover times, based on gap formation rates over a 50-year period, were estimated to average 128 years for old-growth stands dominated by sugar maple (Acersaccharum Marsh.) and 192 years for old-growth stands dominated by hemlock (Tsugacanadensis (L.) Carrière). While these estimates of turnover time are substantially shorter than maximum tree ages observed on these sites, they agree closely with independent data on mean canopy residence time for trees that die at the average gap-maker size of 51 cm DBH. The data support previous hypothetical explanations of the apparent discrepancy between canopy turnover times of <130 years for hardwood species and the frequent occurrence of trees exceeding 250 years of age.

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.

1 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.

No abstract

No abstract