Simulated Fire Behavior and Fine-Scale Forest Structure Following Conifer Removal in Aspen-Conifer Forests in the Lake Tahoe Basin, USA

Justin P Ziegler,Jonathan W Long,Brandon M Collins,Christa M Dagley,Chad M Hoffman,William Mell

doi:10.3390/fire3030051

Abstract

Quaking aspen is found in western forests of the United States and is currently at risk of loss due to conifer competition at within-stand scales. Wildfires in these forests are impactful owing to conifer infilling during prolonged fire suppression post-Euro-American settlement. Here, restoration cuttings seek to impact wildfire behavior and aspen growing conditions. In this study, we explored how actual and hypothetical cuttings with a range of conifer removal intensity altered surface fuel and overstory structure at stand and fine scales. We then simulated wildfires, examining fire behavior and effects on post-fire forest structures around aspen trees. We found that conifer removal constrained by lower upper diameter limits (<56 cm) had marginal effects on surface fuel and overstory structure, likely failing to enhance resource conditions sufficiently to sustain aspen. Increasing the diameter limit also led to a higher likelihood of fire spread and a higher rate of spread, owing to greater within-canopy wind speed, though crown fire activity decreased. Our simulations suggest heavier treatments could facilitate reintroduction of fire while also dampening the effects of wildfires on forest structure. Cutting specifications that relax diameter limits and remove a substantial portion of conifer overstory could better promote aspen restoration and mitigate fire hazard.

Highlights

Land management activities for over a century have led to significant changes in forest structure and ecosystem processes in mixed conifer-quaking aspen (Populus tremuloides Michx.) forests where wildland fire once regulated forest developmental pathways [1,2,3,4]
The Lake Tahoe Basin lies in the Sierra Nevada of California and Nevada, USA (39◦ 050 N latitude, 120◦ 020 W longitude), and is approximately 134,000 ha with an elevation range between 1900 to 3050 m
Our results demonstrate how a significant removal of moderate to large conifer trees may promote restoration of aspen by alleviating conifer crowding around aspen and altering fuel and fire

Summary

Introduction

Land management activities for over a century have led to significant changes in forest structure and ecosystem processes in mixed conifer-quaking aspen (Populus tremuloides Michx.) forests where wildland fire once regulated forest developmental pathways [1,2,3,4] In these forests, low- and mixed-severity fires produced openings sufficiently large to modify microclimates, yielding fine-scale diversity of growing environments and regeneration niches [3,5,6]. Fire 2020, 3, 51 stands as a critical component in an otherwise conifer-dominated landscape due to their relative scarcity, high biological diversity, influence on hydrological processes, esthetics, and impact on landscape scale fire behavior and severity [7,11] Studies of these forests across the greater Sierra Nevada (including the Lake Tahoe Basin) have suggested that up to 70% of stands with aspen are at moderate or high risk of being wholly replaced by shade-tolerant conifers (e.g., Abies spp.) [7]

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Conclusion