Development and wildfire dynamics of dry coastal temperate forests, BC, Canada

Abstract

The vegetation and fire history at the Coastal Douglas-fir (CDF) and Coastal Western Hemlock (CWH) boundary on east-central Vancouver Island was reconstructed. A basal non-arboreal assemblage at the most inland site likely represents an open parkland community during the cold early late-glacial interval. Widespread Pinus woodland followed under cold, dry climate, yielding to closed-canopy mixed conifer forest as climate moistened. While fire disturbance was initially rare, it increased in the late-glacial mixed conifer forest. In the early-Holocene, dry coastal temperate forest replaced the mixed-conifer assemblage. Fire disturbance was widespread, characterised by frequent, possibly lower severity burns. These CDF-like forests expanded northward and westward under warm, dry conditions, attaining maximum extent 9500-10,500 cal yr BP, retreating thereafter towards the modern CDF-CWH boundary, which established ~6000 years ago as climate moistened. Quercus garryana communities were scattered along the coast at this time, maintained by surface fire. Throughout the remainder of the record, the forest canopy closed as modern forests developed and a mixed-severity fire regime developed. The extended early Holocene range of CDF-like forest suggests that the existing CWHxm biogeoclimatic subzone may be replaced by CDF stands with climate change.

Holocene fire regimes, fire‐related plant functional types, and climate in south‐coastal British Columbia forests

“…At ~7300 cal yr BP, there is a notable increase in Q. garryana pollen at Roe Lake and by 6500 cal yr BP, Q. garryana accounts for 16% of the pollen assemblages (Figure 5). Nearby pollen records on Vancouver Island (e.g., Brown et al, 2022, 2019; Pellatt et al, 2001) show similar increases in Quercus , reflecting the establishment of Garry oak savanna communities throughout the region. At this time, fires continue to be frequent on Pender Island with fires occurring, on average, every 101 ± 29 years between 7300 and 6000 cal yr BP.…”

“…With future warming (Wang et al, 2016), the range of P. menziesii is likely to expand northward and upslope on a regional scale (Brown et al, 2022; Halofsky et al, 2018; Lacourse & Adeleye, 2022; MacKenzie & Mahony, 2021). Elevated temperatures and more intense summer drought will decrease fuel moisture content in the abundant fuel conditions that have been created by fire exclusion and suppression (Abatzoglou & Williams, 2016; Hagmann et al, 2021).…”

“…The mean FRI lengthens to 176 ± 54 years, but return intervals vary considerably, reaching 350 years between 3000 and 2500 cal yr BP before fire frequencies increase over the last 2000 years. The shift toward less frequent fire in the mid-Holocene is typical of many sites in the Pacific Northwest, especially those with abundant P. menziesii (e.g., Brown et al, 2022Brown et al, , 2019Long et al, 2007;Walsh et al, 2008). Sites to the north of Pender Island such as Somenos Lake on Vancouver Island ("F" in Figure 1; Murphy et al, 2019) and to the south in western Oregon, such as Lake Oswego ("Z" in Figure 1; Walsh et al, 2018) and Lost and Little lakes ("X" and "AB" in Figure 1; Long et al, 2007), also show a late Holocene increase in fire frequency similar to Roe Lake.…”

“…Previous work has shown that fire was uncommon throughout the region at the start of the Holocene, around 12,000 calendar years before present (cal yr BP), but increased over time as temperatures increased in the early Holocene (Walsh et al, 2015). By 10,000 cal yr BP, fire frequency was high at many sites in the Pacific Northwest (e.g., Brown et al, 2022Brown et al, , 2019Prichard et al, 2009;Walsh et al, 2015Walsh et al, , 2010Walsh et al, , 2008White et al, 2015). Warmer and drier climate in the early Holocene coupled with increased seasonality would have facilitated this period of more frequent fire.…”

“…Map of the Pacific Northwest showing Roe Lake (star) on Pender Island, British Columbia, Canada and other macroscopic charcoal studies (circles) in the region: (A) Langley Lake (Brown et al, 2022), (B) Clayoquot Lake/Valley (Gavin et al, 2003a, 2003b), (C) Enos and Boomerang lakes (Brown & Hebda, 2002a), (D) Shingle Point (Derr, 2014), (E) Porphyry Lake (Brown & Hebda, 2003), (F) Somenos Lake (Murphy et al, 2019), (G) Whyac Lake (Brown & Hebda, 2002b), (H) Pixie Lake (Brown & Hebda, 2002b), (I) Walker Lake (Brown & Hebda, 2003), (J) Begbie Lake (Brown et al, 2019), (K) East Sooke Fen (Brown & Hebda, 2002b), (L) Moran State Park (Sugimura et al, 2008), (M) Frozen Lake (Hallett et al, 2003), (N) Mount Barr Cirque Lake (Hallett et al, 2003), (O) Chadsey Lake (Murphy et al, 2019), (P) Panther Potholes (Prichard et al, 2009), (Q) Kirk Lake (Cwynar, 1987), (R) Fish Lake (Walsh et al, 2018), (S) Moose Lake (Gavin et al, 2001), (T) Martins Lake (Gavin et al, 2001), (U) Yahoo Lake (Gavin et al, 2013), (V) Sunrise and Shadow lakes (Walsh et al, 2017), (W) Taylor Lake (Long et al, 2007), (X) Lost Lake (Long et al, 2007), (Y) Battle Ground Lake (Walsh et al, 2008), (Z) Lake Oswego (Walsh et al, 2018), (AA) Beaver Lake (Walsh et al, 2010), (AB) Little Lake (Long et al, 2007), and (AC) Tumalo Lake (Long et al, 2011). …”