Forest Age Structure and Development Following Wildfires in the Western Olympic Mountains, Washington

Abstract

Fire is an important disturbance agent influencing forest composition and structure in Pacific Northwest ecosystems. I examined the effects of a long fire‐return interval on forest development, composition, and tree age structure for a post‐fire sere on the west slope of the moist Olympic Mountains. Similar sites that burned in 1978, 1961, 1870, 1799, and circa 1465 were selected. Tree cores and size characteristics were collected from two randomly located 0.25‐ha plots at each site. Fires usually burned catastrophically, killing most overstory vegetation. Pioneer tree species were Douglas‐fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla); western hemlock assumed numerical dominance early in the sere. Forest reestablishment after fire was slow, taking ≥50 yr to complete establishment of shade‐intolerant Douglas‐fir. Understory establishment of western hemlock increased 150 yr after fire, although conditions that enhance its long‐term survival may occur ≈300 yr or more after fire. Establishment of western hemlocks existent at the 1465 fire site peaked ≈365‐424 yr after the fire; individuals establishing 150‐300 yr after the fire occurred much less frequently. Fire exclusion would shift the replacement sequence toward a wet, very‐low‐frequency fire regime, favoring western hemlock over Douglas‐fir.