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.

Natural tree regeneration and coarse woody debris dynamics after a forest fire in the western Cascade Range

“…The speed of reforestation in our seedling results contrasts with some retrospective studies, which portrayed natural reforestation of P. menziesii stands as slow or sparse (Franklin and Hemstrom 1981, Huff 1995, Tappeiner et al 1997a). However, none of our results are unanticipated when compared to the handful of quantified For example, our mean abundance of 13,900 P. menziesii per ha at 14 years postfire (table 6) fits into the range established by Hofmann's (1917) map, 0 to >37,000 per hectare.…”

“…The results in figure 1 span a number of climate zones and topographical conditions, from moist sites in the Olympic Mountains (Huff 1995) to dryer and hotter areas in the Klamath and Siskyou Ranges (Donato et al 2009, Shatford et al 2007). …”

“…Some "retrospective studies" have echoed this idea of slow or sparse regeneration after fire. These works examine current-day stands and project their structure backward through time, suggesting that at some sites, establishment of P. menziesii can span a century or more (Franklin and Hemstrom 1981, Huff 1995, Tappeiner et al 1997a, with trees growing in the open at low densities (e.g. about 100 trees/ha, Tappeiner et al 1997a).…”

Long-Term Fire Regime Estimated from Soil Charcoal in Coastal Temperate Rainforests

“…For instance, throughout its range in the American Pacific Northwest and the adjacent south coast of British Columbia, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) is an indicator of stands where summer drought is a significant factor shaping plant communities and where fire has been a significant factor in forest dynamics, either as a result of high severity fires at longer intervals (e.g., 200-600 years) or more frequent fires of varying severity (Waring and Franklin 1979, Agee 1993, Huff 1995, Long et al 1998). …”

Variability of Stand Structures and Development in Old-Growth Forests in the Pacific Northwest, USA

“…Old-growth forests have been characterized as having structural characteristics of large sized trees and snags, large down woody debris, and complex canopy structures and DBH distributions [8,13,46]. Many of these characteristics are rarely found together in a stand and others are commonly found together.…”