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Plant Community Dynamics Following Fuel Treatments and

Mega-Wildfire in a Warm, Dry Mixed-Conifer Forest

By Judith D. Springer, Sr. Research Specialist

Introduction

The heightened risk of unnaturally severe mega-fires in warm, dry mixed-conifer forests has led to ecological restoration and hazardous fuel reduction efforts across the western US. Restoration treatments typically in-clude tree thinning, prescribed burning, and/or managed fire to restore forest structure and reduce hazardous fuels. Wildfires that burn over treated areas provide important opportunities to examine effects of pre-fire treatments on ecosystem responses. In 2004, a landscape-scale stewardship project was implemented to reduce hazardous fuels and to protect human communities from severe fire in the White Mountains of eastern Arizo-na at the wildland-urban interface (WUI). Subsequent to the project, the human-caused Wallow Fire in 2011 burned more than 450,000 acres of mixed-conifer and ponderosa pine forest and set the stage for an opportun-istic examination of longer-term understory plant community responses to pre-fire fuel reduction treatments. Our main research questions included the following: 1) Were there differences in native and non-native spe-cies cover, richness, and diversity between treated and untreated areas five years after wildfire? 2) Was there a convergence in species composition between treated and untreated areas? 3) What environmental factors (e.g., slope, aspect, tree mortality), affected species composition? 4) Were there shifts in composition toward drought and/or heat-tolerant species over time?

Research Findings

Five years post-fire, total plant cover in untreated units had increased to nearly the same levels as the treated units. However, total cover was signifi-cantly greater in treated units at 22.3 percent com-pared with untreated units at 19.6 percent.

The overall mean for non-native cover was not significantly different between treated and untreat-ed units. Non-native cover increased between 2012 and 2016 in both treated and untreated units.

Species cover was negatively correlated with total tree basal area and canopy cover in both treated and untreated units, and positively correlated with tree mortality and basal area mortality in the un-treated units (Figure 1).

Species richness showed positive correlations with tree mortality in the untreated units, and negative correlations with topographic position, insolation, and elevation in both treated and untreated units.

Multivariate analysis using plant community data indicated that insolation, elevation, aspect, and slope were primary drivers of the plant community over time in treated and untreated areas.

January 2019

Figure 1. Average native and non-native aerial species cover

within treated and untreated units. Bars represent one standard error of the mean (N=9). Significant differences are indicated

by asterisks.

Conclusions

Warmer, drier climatic conditions are likely to drive increases in large mega-fires in mixed-conifer forests of the Southwest. Treatments that reduce overstory density and utilize prescribed burning are likely to lead to more resilient plant communities that are more tolerant of drought and warm temperatures. Additional long-term studies that elucidate interactions of wildfire, ecological restoration, and hazardous fuel treatments will be valuable for improving conserva-tion of forest ecosystems in a changing climate.

Fuel reduction treatments led to increased cover of native species immediately following the fire and this increased cover was still evident five years later. These results lend weight to the usefulness of hazardous fuel reduction treatments in promoting native plant cover.

High-severity fire effects led to more open conditions across much of the fire’s footprint in both types of units. Non-native species such as Bromus tectorum (cheatgrass), Carduus nu-tans (musk thistle), Cirsium vulgare (bull thistle), Poa compressa (Canada bluegrass), Poa pratensis (Kentucky bluegrass) and Tragopogon dubius (yellow salsify) increased in frequency across the entire study area in both treated and untreated units.

Our results suggest that community composition is driven by increased insolation following loss of tree cano-py. Species that benefit from the in-creased light and temperatures, such as some shrubs, wind-dispersed forbs, and perennial graminoids, have in-creased in cover and frequency fol-lowing hazardous fuel treatments and wildfire (Figure 2).

This fact sheet summarizes information from the following publication:

Springer, J.D., D.W. Huffman, M.T. Stoddard, A.J. Sánchez Meador, and A.E.M. Waltz. 2018. Plant community dynamics following hazardous fuel treatments and mega-wildfire in a warm-dry mixed-conifer forest of the southwestern USA. Forest Ecology and Management 429:278-286.

Further Reading

Hurteau, S. 2016. White Mountains Stewardship Project: Evaluating the Impacts of the Nation’s First Long-Term Stewardship Contract. Final Report. Integrated Biological Solutions, LLC.

Sitko, S., and S. Hurteau. 2010. Evaluating the impacts of forest treatments: The first five years of the White Mountain Stewardship Project. The Nature Conservancy. Phoenix, AZ.

Waltz, A.E.M., Stoddard, M.T., Kalies, E.L., Springer, J.D., Huffman, D.W., Sánchez-Meador, A., 2014. Effectiveness of fuel reduction treatments: assessing metrics of forest resiliency and wild-fire severity after the Wallow Fire, AZ. Forest Ecology and Management, 334, 43-52.

Contact: Judith D. Springer, Judy.Springer@nau.edu

NAU is an equal opportunity provider.

This research was funded by a grant from the USDA Forest Service.

Figure 2. South-facing slope with high tree mortality. It is domi-nated by wind-dispersed heat and sun-tolerant species, such as

native perennial grasses and Senecio wootonii (Wooton’s rag-wort).