Long-Term Effects of Fuels Treatments, Overstory Structure, and Wildfire on Tree Regeneration in Dry Forests of Central Washington

Allison K Rossman,Charles B Halpern,Jonathan D Bakker,David W Peterson

doi:10.3390/f11080888

Allison K Rossman, Charles B Halpern + Show 2 more

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https://doi.org/10.3390/f11080888

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Abstract

The long-term effectiveness of dry-forest fuels treatments (restoration thinning and prescribed burning) depends, in part, on the pace at which trees regenerate and recruit into the overstory. Knowledge of the factors that shape post-treatment regeneration and growth is limited by the short timeframes and simple disturbance histories of past research. Here, we present results of a 15-year fuels-reduction experiment in central Washington, including responses to planned and unplanned disturbances. We explore the changing patterns of Douglas-fir regeneration in 72 permanent plots (0.1 ha) varying in overstory abundance (a function of density and basal area) and disturbance history—the latter including thinning, prescribed burning, and/or wildfire. Plots were measured before treatment (2000/2001), soon afterwards (2004/2005), and more than a decade later (2015). Thinning combined with burning enhanced sapling recruitment (ingrowth) into the overstory, although rates of ingrowth were consistently low and greatly exceeded by mortality. Relationships between seedling frequency (proportion of quadrats within a plot) and overstory abundance shifted from weakly negative before treatment to positive after thinning, to neutral in the longer term. However, these relationships were overshadowed by more recent, higher-severity prescribed fire and wildfire that stimulated seedling establishment while killing advanced regeneration and overstory trees. Our results highlight the dependence of regeneration responses on the history of, and time since, fuels treatment and subsequent disturbance. Managers must be aware of this spatial and temporal complexity and plan for future disturbances that are inevitable but unpredictable in timing and severity.

Highlights

Large areas of dry coniferous forests in western North America require restoration to reduce the risk of high severity wildfire and to restore the structures and functions that characterized these forests historically [1,2,3,4,5]
We initially considered a mixed-model approach but rejected it because lack of replication at the unit scale precluded testing of all disturbance histories
R2 is the percentage of variance explained, and is calculated as 1 minus the ratio of the residual sum of squares to the total sum of squares

Summary

Introduction

Large areas of dry coniferous forests in western North America require restoration to reduce the risk of high severity wildfire and to restore the structures and functions that characterized these forests historically [1,2,3,4,5]. Restoration treatments, including mechanical thinning and prescribed burning, have proven effective for achieving these objectives in the short term by reducing surface and ladder fuels, increasing canopy base height, and reducing canopy bulk density [6,7,8,9,10,11,12]. The longevity of these effects varies considerably within and among landscapes [13], and the factors influencing this variability are not well understood. A key factor contributing to fuels-treatment longevity is the rate at which trees re-establish and grow following treatment. Knowledge of the factors that regulate regeneration and growth is critical to the design of fuels-treatment prescriptions, including the need for, or timing of, re-entry to maintain fuels or structural objectives

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