Forest Age Distribution under Mixed- Severity Fire Regimes – a Simulation- Based Analysis for Middle Boreal

• Main Author: Juho Pennanen
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2002
• Subjects: disturbance; forest fi re; modeling; landscape; Fennoscandia; Fennoscandian
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.603.1752; http://www.metla.fi/silvafennica/full/sf36/sf361213.pdf

A simulation model was used to study the age structure of unmanaged forest landscapes under different fi re regimes. Stand age was defi ned as the age of the oldest tree cohort in a stand. When most fi res are not stand-replacing, the theoretical equilibrium stand age distribution is either bell-shaped or bimodal and dominated by old age-classes. Old-growth forests (oldest cohort> 150 y) dominate the landscape unless fi res are both frequent and severe. Simulation results and analytical calculations show that if a regime of frequent fi res (about every 50 y) maintains landscapes dominated by old-growth forests, then old-growth dominance persists when the number of fi res is decreased, despite the associated increase in fi re severity. Simulation results were applied to Pinus sylvestris-dominated landscapes of middle boreal Fennoscandia, which according to empirical results were dominated by old-growth forests when fi res were frequent during the 19th century. Since the changes in the fi re regime can be plausibly explained by changes in the number of human-caused ignitions, old-growth forests have evidently also dominated the landscapes earlier when fi res were less frequent. The simulation model is used to produce plausible age distributions of middle boreal Fennoscandian forest landscapes under different historical fi re regimes. In summary, the frequency of large-scale disturbance alone predicts forest landscape dynamics poorly, and the roles played by fi re severity and residual stands need to be considered carefully. Maintaining and restoring old-growth structures is essential to regaining the natural variability of Fennoscandian forest landscapes.