Long-term mortality rates and spatial patterns in an old-growth Pinus resinosa forest
Understanding natural mortality patterns and processes of forest tree species is increasingly important given projected changes in mortality owing to global change. With this need in mind, the rate and spatial pattern of mortality was assessed over an 89-year period in a natural-origin Pinus resinosa (Aiton)-dominated system to assess these processes through advanced stages of stand development (stand age 120–209 years). Average annual mortality rates fluctuated through time, yet were within the range reported in other studies (0.60%–3.88% depending on species and sampling interval). Tree mortality was attributed to multiple agents, including the senescence of the short-lived Pinus banksiana Lamb., windthrow, root-rot fungi (Armillaria ostoyae (Romagn.) Herink), and perhaps infrequent droughts. Despite the often contagious nature of many disturbance agents, the overall spatial pattern of mortality events (the arrangement of dead trees within the fixed initial population of live trees) was random at all scales tested. Similarly, the current spatial pattern of dead trees was predominantly random, despite clustering at small scales (2–4 m). These findings underscore the importance of studying mortality rates, agents, and spatial patterns over long time periods to avoid misinterpreting stochastic mortality events, and their influence on longer term stand structure and development.