Evaluating short‐ and long‐term impacts of fuels treatments and simulated wildfire on an old‐forest species

Douglas J Tempel,R J Gutiérrez,Danny L Fry,Sheila A Whitmore,Matthew J Reetz,Maggi Kelly,M Zachariah Peery,Scott L Stephens,Gavin M Jones,William J Berigan,Qinghua Guo,Brandon M Collins,Yanjun Su,John J Battles

doi:10.1890/es15-00234.1

Abstract

Fuels‐reduction treatments are commonly implemented in the western U.S. to reduce the risk of high‐severity fire, but they may have negative short‐term impacts on species associated with older forests. Therefore, we modeled the effects of a completed fuels‐reduction project on fire behavior and California Spotted Owl (Strix occidentalis occidentalis) habitat and demography in the Sierra Nevada to assess the potential short‐ and long‐term trade‐offs. We combined field‐collected vegetation data and LiDAR data to develop detailed maps of forest structure needed to parameterize our fire and forest‐growth models. We simulated wildfires under extreme weather conditions (both with and without fuels treatments), then simulated forest growth 30 years into the future under four combinations of treatment and fire: treated with fire, untreated with fire, treated without fire, and untreated without fire. We compared spotted owl habitat and population parameters under the four scenarios using a habitat suitability index developed from canopy cover and large‐tree measurements at nest sites and from previously derived statistical relationships between forest structure and fitness (λ) and equilibrium occupancy at the territory scale. Treatments had a positive effect on owl nesting habitat and demographic rates up to 30 years after simulated fire, but they had a persistently negative effect throughout the 30‐year period in the absence of fire. We conclude that fuels‐reduction treatments in the Sierra Nevada may provide long‐term benefits to spotted owls if fire occurs under extreme weather conditions, but can have long‐term negative effects on owls if fire does not occur. However, we only simulated one fire under the treated and untreated scenarios and therefore had no measures of variation and uncertainty. In addition, the net benefits of fuels treatments on spotted owl habitat and demography depends on the future probability that fire will occur under similar weather and ignition conditions, and such probabilities remain difficult to quantify. Therefore, we recommend a landscape approach that restricts timber harvest within territory core areas of use (~125 ha in size) that contain critical owl nesting and roosting habitat and locates fuels treatments in the surrounding areas to reduce the potential for high‐severity fire in territory core areas.

Highlights

The management of fire-adapted forests in the western U.S is increasingly challenged by the need to consider the ecological impacts of wildfire and fuels-reduction treatments intended to modify wildfire behavior (Stephens et al 2013)
We found that values for percent canopy cover and large tree density were similar for field plots and their associated map polygon, on average the field plot values were slightly lower than the map values
Several studies have investigated the shortterm and long-term impacts of fuel treatments on habitat availability for old-forest species using various treatment simulations (Lee and Irwin 2005, Ager et al 2007, Thompson et al 2011, Roloff et al 2012), but ours was unique in several respects

Summary

Introduction

The management of fire-adapted forests in the western U.S is increasingly challenged by the need to consider the ecological impacts of wildfire and fuels-reduction treatments intended to modify wildfire behavior (Stephens et al 2013). Historic fire regimes in many of these forests were characterized by fires burning at intervals of less than 20 years and having primarily low- and moderate-severity fire effects but interspersed with some areas of high-severity effects (Agee 1993, Skinner and Chang 1996), and some high-severity fire apparently occurred with regularity (Collins and Stephens 2010, Hanson and Odion 2014). This type of fire regime resulted in highly heterogeneous landscapes in which vegetation conditions were governed by complex interactions between topography, site productivity, and disturbance (Collins et al 2015, Stephens et al 2015). Old-forest species with large home ranges are typically rare and preventing their populations from reaching critically small sizes is widely regarded as an important policy objective (e.g., National Forest Management Act of 1976), in part because meeting the habitat needs of such species might protect broader old-forest communities (Temple 1997)

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Conclusion