Fire as a catalyst for rapid ecological change in the Puget Lowlands over the Holocene

Background/Question/Methods

Disturbance can be an agent for rapid ecological change and understanding where, when, and how this effect may occur is a research priority. We asked whether rapid ecological change in Puget Lowland forests of western Washington state, U.S.A., was significantly associated with fire over the Holocene. We studied forest history with sediment cores from two small hollows in the Marckworth State Forest, located 1 km apart in the Tsuga heterophylla Zone. Chronologies were based on accelerator mass spectrometer radiocarbon ages of terrestrial macrofossils, pollen/spore concentrates, and the Mazama ash. For charcoal analysis, 3-cm³ subsamples were taken at intervals of 0.25-0.50 cm, disaggregated, and sieved to 500 and 150 microns. For pollen analysis, 1-cm³ sediment subsamples were taken at intervals of 0.25-1.00 cm and prepared following standard techniques. We identified peaks in the CHAR series that likely represented high-severity fires burning within 0-50 m from the hollow’s edge. We quantified the nature and timing of vegetation change from centennial through millennial time scales, highlighting both persistent vegetation assemblages and rapid change following fire events. We used four methods to accomplish these goals: stratigraphically-constrained clustering, nonparametric multidimensional scaling (NMS), rate-of-change analysis, and a superposed epoch analysis.

Results/Conclusions

Both cores showed similar dynamics. The NMS explained 95% of the variation in pollen/spore assemblages (80% on axis 1). The median rate of change in pollen/spore assemblages was similar between the two sites (14% and 12%; p = 0.145). Vegetation rates of change immediately following CHAR peaks was significantly higher than series-wide medians at both sites (28-38%, p < 0.003), showing that fire was a catalyst for rapid ecological change throughout the Holocene. Rapid fire-driven change was characterized by (1) large-scale vegetation state changes during times of high climate variability and (2) successional changes during times of low climate variability. Large-scale changes include (1) Pseudotsuga arrival at the Younger Dryas terminus, (2) Cupressaceae arrival ~8,000 years ago, and (3) logging. Post-fire successional changes characterize rapid change after ~8,000 years ago and include Tsuga and Alnus increases, and Cupressaceae decreases. Abrupt climate change can interact with fire and redirect successional pathways to cause state changes in vegetation. In contrast, fires during times of low climate variability did not alter successional pathways and the dominant vegetation remained resilient. These patterns suggest that fire can be an important catalyst for rapid state changes, accelerating vegetation shifts in response to large-scale climate change.