Long-Term Effects of Burn Season and Frequency on Ponderosa Pine Forest Fuels and Seedlings

Douglas J Westlind,Becky K Kerns

doi:10.4996/fireecology.130304261

Abstract

Prescribed fire is widely applied in western US forests to limit future fire severity by reducing tree density, fuels, and excessive seedlings. Repeated prescribed burning attempts to simulate historical fire regimes in frequent-fire forests, yet there is limited long-term information regarding optimal burn season and frequency. In addition, burns are operationally feasible only in the spring and late fall, largely outside the historical wildfire season. This study quantifies the effect of seasonal reburns on woody surface fuels, forest floor fuels, and understory tree regeneration abundance in six previously thinned ponderosa pine (Pinus ponderosa Lawson & C. Lawson) stands in the southern Blue Mountain Ecoregion of Oregon, USA. Each stand consisted of an unburned control, and four season by reburn treatments: spring 5 yr, spring 15 yr, fall 5 yr, and fall 15 yr. We evaluated season and frequency of reburn for woody surface fuel (1 hr to 1000 hr), forest floor fuel (litter and duff, undifferentiated), and understory conifer seedling density (regeneration). Burning was initiated in the fall of 1997 and spring of 1998; we present results in this paper through 2014 for fuel, and 2015 for seedlings. All reburn treatments reduced the forest floor depth compared to those areas not burned (controls). Fall burning, regardless of frequency, generated 1000 hr fuel primarily from overstory mortality resulting from the initial entry burns and subsequent snag and branch fall. But, for the other woody fuel types, reburning had minimal impact, regardless of season or frequency. All reburn treatments reduced regeneration survival, but 5 yr fall reburning was most effective in reducing excessive conifer seedlings. Repeated spring or fall reburns following thinning will reduce forest floor depth but, to achieve low woody fuel loads and control excessive conifer regeneration, it may be necessary to conduct reburns using different timing, such as during drier periods when wildfire ignitions by lightning occurred historically.

Highlights

Prescribed fire is a widely applied tool for lowering future wildfire severity in western US ponderosa pine
Ponderosa pine forests are widely distributed in North America (Mirov 1967), occurring primarily in low to mid elevations that historically developed in conjunction with frequent low and mixed severity wildfires (Pyne 1982, Hessburg and Agee 2003)
The objectives of this study are twofold: 1) compare the effect of reburn season and frequency (5 yr and 15 yr) on fuel in the ponderosa pine forests of the southern Blue Mountain Ecoregion of Oregon, and 2) describe the fuel dynamics of 5 yr seasonal reburning in these same forests

Summary

Introduction

Prescribed fire is a widely applied tool for lowering future wildfire severity in western US ponderosa pine Ponderosa pine forests are widely distributed in North America (Mirov 1967), occurring primarily in low to mid elevations that historically developed in conjunction with frequent low and mixed severity wildfires (Pyne 1982, Hessburg and Agee 2003). European settlement and related land management practices such as fire suppression and overgrazing interrupted the historical fire regime, advancing secondary succession and the creation of denser stands, accumulation of woody and forest floor fuel, and greater fuel connectivity, increasing the potential for high-severity stand-replacing wildfire (Mutch et al 1993; Covington and Moore 1994; Agee 1996a, b; Belsky and Blumenthal 1997; Kolb et al 1998; Hessburg et al 1999; Hessburg et al 2000; Hessburg et al 2005; Marlon et al.2012). In addition to greater risk of high-severity fire, dense stand conditions and water deficits increase forest susceptibility to disease and insect attack (Castello et al 1995, Covington et al 1997, Fettig et al 2007)

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