Given the legacy of fire suppression, human land-use, and potential fire hazards linked with climatic change, understanding the historical range of natural variability of mixed-severity fire regimes in montane conifer forests is a research imperative. Using dendroecology and field measurements, we quantify stand level processes (e.g. tree establishment, mortality, regeneration, and survival) in relation to past disturbance by fire and mountain pine beetle (Dendroctonus ponderosae Hopkins) in southeastern British Columbia. We determine whether these dynamics differed among forests at mid- and high-elevations over the past 300 years. We expect that variability in fire frequency and severity (including the absence of fire) influenced the species composition, density and age structures of the forests. Field plots used to reconstruct stand dynamics were located within 20 randomly selected stands across a 15,000 ha domestic watershed using a forest cover GIS. Live and dead canopy trees (n=30) and a subset of subcanopy trees (n=10) from each site were dated to determine year of establishment (and death). Fire disturbance event dates at each site were identified using direct evidence from cambial fire scars, and substantiated using ages of post-fire cohorts. We measured tree ring patterns of suppression and release to help identify other disturbance events. Disturbance severity was inferred using relative survivorship from each event.
Results/Conclusions
Fire scar evidence suggests that between 1700 and 1910, at least 19 fires burned at mid-elevations, while only 4 occurred at high-elevations. However, over the last 100 years, only 5 small fire events were recorded, suggesting fire suppression has been effective. Historically, half of the fires were widespread (occurring at more than one site), and preliminary forest structure assessments show that spatially fire severity varied within a single fire event. Canopy tree ages suggest that relative survivorship was greatest at mid-elevations. Yet, even at the highest elevation sites, scattered old veteran snags predate even-aged live canopy trees and suggest discontinuity in high-severity fires in this stratum. Ages of subcanopy trees (<25 cm and >5 cm dbh) are between 90 and 130 years old, and established within 15 years of the last widespread fire events, rather than as a response to fire suppression. Our findings will help guide restoration objectives and fuel- management efforts for communities living in the urban-wildland interface.
