Post-fire fuel succession in a rare California, USA, closed-cone conifer

Bret A Mcnamara,David F Greene,Jeffrey M Kane

doi:10.1186/s42408-019-0059-3

Abstract

BackgroundIncreasing frequency and size of wildfires over the past few decades have prompted concerns that populations of obligate seeding species may be vulnerable to repeat, short-interval fires that occur prior to these species reaching maturity. The susceptibility of populations to this risk is partially dependent on the amount and characteristics of fuel loading over time and their influence on fire behavior and effects. This study characterized fuel dynamics and modeled fire behavior across a time-since-fire chronosequence in stands of the rare, serotinous conifer, Baker cypress (Hesperocyparis bakeri [Jeps.] Bartel), ranging in age between 3 and 147 years post fire.ResultsLitter and fine woody fuel loading (1- to 100-hour) were highest in the 10-year-old and 147-year-old stands, while coarse fuel loading (1000-hour) peaked in the 10-year-old stand and subsequently decreased with time since fire. Duff loading consistently increased with time since fire. Cone production had not occurred in the first 10 yr of stand development. Foliar moisture content in Baker cypress was inversely correlated with stand age, and older foliage had lower moisture content than younger foliage. Modeled surface fire behavior was highest in the 10-year-old and 107-year-old stands in accordance with higher litter, fine woody fuel, or shrub fuel accumulation. While foliar moisture content was higher in younger stands and influenced the critical fireline intensity, we did not observe changes in fire type.ConclusionsFine-fuel loading in Baker cypress stands followed a U-shaped pattern over time (first decreasing, then stable, then increasing), consistent with findings in other forests with stand-replacing fire regimes. Our results indicated that early-successional stages of Baker cypress forests have sufficient fuels to allow for the spread of wildfire and 10-year-old stands could burn with substantive fire behavior prior to cone production. Whenever possible, we recommend suppressing wildfire in stands less than 20 yr old to avoid substantial decreases or local extirpation of these rare Baker cypress populations. Our results highlight the importance of knowing the cone production patterns, fuel dynamics, and corresponding fire behavior over the development of obligate-seeder species to assess the risk of population loss due to short-interval fires.

Highlights

The quantity, quality, and arrangement of live and dead fuels in fire-prone forests are important drivers of fire behavior and effects
Herbaceous fuel loading was persistently low across all stands, but 333% higher in the 3-yearold stand compared to the 10-year-old stand, which had the lowest herbaceous fuel loading
Recent projections of annual fire probability for 2026 to 2050, based on a model of fire occurrence developed for California (Mann et al 2016), suggest that the probability of fire occurring for most of the Baker cypress range is < 1% per year (CalFire 2019)

Summary

Introduction

The quantity, quality, and arrangement of live and dead fuels in fire-prone forests are important drivers of fire behavior and effects. Fuel succession patterns in fire-prone forests are reflective of fire behavior and effects and the subsequent ability of fuels to accumulate over time. Few studies have examined longer-term fuel succession patterns following fire, but previous research has highlighted substantial variation within and among forest types (Brown and See 1981; Lotan et al 1985). In forests that experience a high-severity fire, post-fire fuel succession typically has pulses of fine fuel accumulating during the early and late stages of stand development (Fahnestock 1976; Agee and Huff 1987; Kashian et al 2013). The slower accumulation of fuels over time following a high-severity fire in these ecosystem types is likely due to greater consumption of fine fuels in the fire-killed trees or slower vegetative regrowth that occurs in cooler, higher-latitude climates. This study characterized fuel dynamics and modeled fire behavior across a time-since-fire chronosequence in stands of the rare, serotinous conifer, Baker cypress (Hesperocyparis bakeri [Jeps.] Bartel), ranging in age between 3 and 147 years post fire

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