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

Abstract

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.

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

“…Stem mortality rates (for trees 15 cm dbh) are within the range -but clustered at the low end -of turnover rates (average of recruitment and mortality) reported in a review of studies in 27 conifer forests (Stephenson and van Mantgem 2005). Mortality rates in the present study were similar to, but generally slightly lower than, those reported for other old-growth conifer forests (van Mantgem et al 2009) in western North American and old-growth mixed hemlock (T. canadensis) -hardwood forests in eastern North America (Lorimer et al 2001;Busing 2005).…”

“…This logic culminated in the prediction that in old-growth forests, a Ushaped curve would describe the relationship of tree mortality rate to tree diameter at breast height. Since then, several investigators have searched for the U-shaped mortality curve, with varying degrees of support (e.g., Runkle 2000;Lorimer et al 2001;Coomes et al 2003).…”

“…Mortality determines the upper bounds of tree size and age and influences the form of tree size distributions (Lorimer et al 2001). Dendroecological studies (e.g., Winter et al 2002) and manipulative experiments (e.g., Van Pelt and Franklin 1999) demonstrate that tree mortality indirectly influences subsequent forest structural dynamics: understory tree establishment, growth, and survival respond to overstory tree mortality.…”

High Mortality and Low Net Change in Live Woody Biomass of Karst Evergreen and Deciduous Broad-Leaved Mixed Forest in Southwestern China

“…The mortality rate decreases first and subsequently increases with the increased tree DBH in nonkarst forests [62][63][64][65][66][67]. Small trees are in a weak position when competing for illumination, nutrition, and spatial resources; they are also vulnerable to diseases and pests [68].…”

Individual tree mortality of silver birch (Betula pendula Roth) in Estonia

“…As a simplified approach, mortality has been generally considered either as regular, due to competition or ageing, or as irregular, caused by ecological and catastrophic events, such as storm or fire (Monserud 1976). The probability of regular mortality is believed to be U-shaped, indicating relatively high mortality rates for small and large trees due to light competition and senescence, respectively (Monserud & Sterba 1999, Lorimer et al 2001, Fraver et al 2008. The probability of irregular mortality is rather fluctuating, and thus quite difficult to predict, since it may be triggered by various biotic and abiotic factors, which change temporally and spatially, e.g., the attack of pests or diseases may cause a comparatively small mortality in one year, or eliminate the host species from the region ( Vanclay 1994).…”