| Change in temporal patterns of mortality rates, tree population structure (size and density), biomass accumulation, input of coarse woody debris, and causes of mortality of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western hemlock (Tsuga heterophylla (Raf) Sargent) were investigated in nine stands spanning the young (45–80 yr), mature (81–200 yr), and old-growth (>200 yr) stages of forest development using long-term records (12 to 84 yr) in permanent plots in the Cascade Mountains of western Washington and Oregon. High average annual mortality rates for Douglas-fir (1.1–2.9%) occurred in young stands which was driven by the mortality of small, suppressed trees (<25 cm DBH). In old growth, mortality rates were substantially lower (0.5–0.7%) and causes were from density-independent agents notably root and stem decay. In young and mature stands, the high rates of mortality of small Douglas-fir had little negative effect on the accumulation of biomass which increased steadily (∼4.8 Mg ha−1 yr−1). However, in old growth, due to minimal recruitment, slow growth, and continued mortality, Douglas-fir biomass accumulation rates were mostly negative and standing stocks decreased. Average annual input of Douglas-fir woody debris was similar among all stand ages (∼2.1 Mg ha −1 yr−1). Western hemlock average annual mortality rates were low in young (0.0%) and mature (0.1%) stands, but, in old growth, rates were greater (0.3–0.5%) due to a combination of suppression, snow loading and physical damage from falling trees. Western hemlock contribution to biomass was minimal as the majority of individuals were small. Mortality was also composed mostly of small trees and input to woody debris was light (0.2–0.8 Mg ha−1 yr −1). Many of the patterns that emerge from these records support previous assumptions about mortality, although these are among the first data to confirm these trends. The larger perspective provided by these data sets allow for some new inferences about relationships of tree mortality to forest stand development and ecosystem processes. |
