Graduate Thesis Or Dissertation
 

Coarse woody detritus dynamics, variable decay rates and their contribution to wildland fuel succession following high-severity fire disturbance in dry-mixed conifer forests of Oregon's eastern Cascades

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  • Reducing future fire severity is a proposed ecological benefit of salvage logging following wildfire disturbance. Considerable debate continues over the ability of such management practices to achieve this objective given limited understanding of coarse woody detritus (CWD) dynamics, fuel bed alterations, and post-fire vegetative growth. The objective of this study was to estimate the dynamics of snags and logs in conjunction with surface fuel accumulation following high-severity fire disturbance in dry-mixed conifer forests of Oregon’s eastern Cascades. Snag dynamics (fall and breakage rates) were estimated for Abies sp., Pinus ponderosa and Pinus contorta in three DBH classes of <23 cm (small), 23-41 cm (medium) and >41 cm (large). A total of 5,103 snags in thirty 0.25-ha plots were sampled at seven different fire sites, covering a 24 year chronosequence following high-severity fire disturbance. Pinus ponderosa and Pinus contorta snags had the quickest fall rates with estimated half-lives of 7-8 and 12-13 years for small and medium sized snags, respectively. Large Pinus ponderosa snags had an estimated half-life of 17-18 years. Abies sp. snags fall rates were slower, with half-life estimates of 8-9, 14-15 and 20-21 years for small, medium and large snags respectively. Breakage rates were variable but correlated with wood strength, crown and stem weight and crown position (exposure to wind). Decomposition loss rate-constants were obtained from the same fire sites, up to seven years post-fire, by removing three cross-sections from each of sixty fire-killed Abies sp. snags, sixty Pinus ponderosa snags, and forty Pinus ponderosa logs. Abies sp. snags exhibited significant decay with an estimated decomposition loss rate-constant of k = 0.0149 yr⁻¹. Pinus ponderosa snags did not exhibit significant decay, but logs did. Sapwood and heartwood decomposition loss rate-constants equaled k = 0.0362 yr⁻¹ and k = 0.0164 yr⁻¹, respectively. These values confirm hypothesized differences in decay rates among species and between snags and logs in dry forest environments. An empirical model was developed to link snag fall and breakage with snag and log decomposition during succession in order to estimate the contribution of fire killed biological legacies to fine and coarse woody detritus accumulation. Legacy CWD is responsible for the largest total accumulation of surface fuel as snags break and fall, but primarily in 100- and 1000-hr fuel classes. Decomposition rates increase as CWD moves from standing to downed material, reducing total CWD biomass by 30-50% in 24 years. Fine fuels are primarily derived from post-fire vegetation and steadily increase over the 24-year period. Herbaceous fuel loads peak within 2-4 years but decrease rapidly as Ceanothus velutinus and Arctostaphylos patula shrubs establish quickly and steadily increase in total biomass over 24 years. Spread rates and flame lengths in post-fire environments are primarily driven by fuels generated from new growth. The dynamic process of snag fall and breakage, and decomposition of snags and logs, limits CWD's effect on fire spread and intensity if reburning occurs, although soil heating and total heat release can be exacerbated by the combustion of decayed logs. Salvage logging significantly reduces CWD fuels but has limited impacts on other fuel bed components. Results of this study suggest post-fire management decisions consider vegetation dynamics as well as dead wood dynamics if reducing fire hazard is a primary objective.
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