Tree-Ring Indicators of Fire in Two Old-Growth Coast Redwood Forests

Abstract

Fires that burn through forests cause changes in wood anatomy and growth that can be used to reconstruct fire histories. Fire is important in Sequoia sempervirens (D. Don) Endl. (coast redwood) forests, but fire histories are limited due to difficulties crossdating annual rings of this species. Here we investigated three fires (1985, 1999, 2008) in two old-growth forests (Montgomery Woods State Natural Reserve and Landels-Hill Big Creek Reserve, California, USA) to quantify these responses via crossdated increment cores from lower trunks of 53 trees, including 10 that were climbed and cored at 10 m height intervals. Redwoods frequently responded to fire by producing anomalous growth during the fire year; 100 of 240 lower trunk cores recorded at least one anatomical indicator (i.e., intra-annual density fluctuation, faint latewood, resin, or scar). Following fire, radial growth decreased by 29% to 43% compared to the fire year. After accounting for climatic influences, radial growth was 27% to 32% lower than expected in the post-fire year and declined to as low as 46 % after three years. Growth suppression persisted for up to seven years after fire, followed by up to 40% higher than expected radial growth. Several of the climbed trees expressed disruption of incremental growth along the height gradient following fire. The 1985 event consistently generated stronger growth and anatomical responses than the 1999 and 2008 events, and showed a co-occurrence between faint latewood during the fire year and subsequent narrow or missing rings. We used post-fire low growth relative to drought combined with anatomical indicators to detect past fires, identifying five additional events at Landels-Hill Big Creek Reserve dating back to 1634. Although other disturbances could have initiated these responses, our detection method enhances current capabilities for the spatiotemporal resolution of redwood fire histories via non-scar indicators on increment cores from living redwoods.