Evidence of a historical frequent, low-severity fire regime in western Washington, USA

Abstract

Fire is a common disturbance in many forests. We conducted a fire history study on 40 Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees from two sites, Kellett Bluff and Turn Point, in the San Juan Islands of Washington state, USA. In total, 146 fire scars were identified and dated, representing 34-35 fires per site. Fires occurred between 1565 and 1964. Individual trees recorded up to 10 fires. The composite mean fire interval (MFI) was 11-13 years over the entire study period and 6 years in the historical period (1780-1895). These sites were structured by frequent, low-severity fires, yet supported a tree component for centuriesthe oldest tree in this study was more than 500 years old. A program of frequent, low-severity fires may be critical for their long-term persistence. Comparisons of fire history data among these and five other local sites indicate frequent fires but little synchronicity; the MFI was 4 years, but most fires were only recorded at single sites. Although forests west of the Cascade Mountains are often described as subject to infrequent, high-severity fires, these results highlight the need for a more refined understanding of historical fire regimes in this region.

Pre-Settlement Forests around Puget Sound: Eyewitness Evidence

“…Douglas-fir's diameter histogram ( Figure 5b) reveals equal populations of small-and mid-sized trees, as well as several larger specimens, indicating either a) that younger cohorts were intimately intermingled with considerably older specimens, or b) that the landscape was a geographic mosaic of stands of diverse ages. This distribution of diameters is unlike Douglas-fir populations in other distinct areas, and it may signify that local parts of Rainshadow experienced episodic opportunities for Douglas-fir establishment, such as relatively frequent, small-scale wildfires, as recently reported by Bakker et al (2019). Diameters of the uncommon hemlock ( Figure 5c) were concentrated in the 1-5-inch class, again suggesting either that wildfire had intervened in the recent past or, alternatively, that hemlock growth was older than it seemed but was stunted by low soil moisture.…”

“…Early settlers were strongly attracted to such sites in north Whidbey, Sequim Prairie (T30/R2W), and San Juan Islands by providing immediate pasturage for livestock and prospects for grain production. For untold centuries prior to settlement, these prairies and numerous beach landings were used by competing groups of Native people, and it is presumed that anthropogenic wildfires fairly frequently swept through these dry landscapes, periodically and locally suppressing fire-sensitive tree species (Bakker et al, 2019). In some cases the absence of trees is traceable to excessively drained soil types, which exacerbate already harsh droughtiness.…”

Garry oak ecosystem stand history in Southwest British Columbia, Canada: implications of environmental change and indigenous land use for ecological restoration and population recovery

“…Fire is considered an important disturbance mechanism for Garry oak ecosystems (MacDougall et al 2004;Pellatt et al 2007;Pellatt and Gedalof 2014;Bakker et al 2019). Garry oak ecosystems were established on southern Vancouver Island and surrounding Gulf Islands approximately 8300 cal yr BP, during past periods of warmer and dryer climate (Pellatt et al 2001).…”

Garry Oak Ecosystem Stand History in Southwest British Columbia, Canada: Implications of environmental change and Indigenous land use for Ecological Restoration and Population Recovery

“…Fire is considered an important disturbance mechanism for Garry oak ecosystems (MacDougall et al 2004; Pellatt et al 2007;Pellatt and Gedalof 2014;Bakker et al 2019). Garry oak ecosystems were established on southern Vancouver Island and surrounding Gulf Islands approximately 8300 cal yr BP, during past periods of warmer and dryer climate (Pellatt et al 2001).…”