Contrasting early- and late-Holocene vegetation and wildfire regimes in a high-value drinking water supply area, Canada

Paleoenvironmental reconstructions of past ecosystems and fire regimes can strengthen interpretations of modelled future fire environments. In this study, sediment cores from four lakes in a high-value municipal water supply area are used to compare climate, vegetation, and fire along a regional east-west precipitation gradient, contrasting warm-dry early- and cool-moist late-Holocene conditions. Results indicate that inferred precipitation was lower in the past, with more open-canopy xeric Pseudotsuga menziesii (Douglas-fir) forests compared to present-day closed-canopy mesic Tsuga heterophylla (western hemlock) and Thuja (cedar) forests. Overall, the wettest and western-most site experienced the greatest change through time, with more frequent early-Holocene fires yielding to longer fire return intervals in the late-Holocene. This implies that northern coastal temperate rain shadow forests, currently experiencing little fire, may become more vulnerable to increased fire disturbance in the future. It also highlights susceptibility to fire regime shifts consistent with regional observations and models suggesting current and future increases in extreme fire disturbance.