Rapid Assessment of Postfire Plant Invasions in Coniferous Forests of the Western United States

Abstract

Fire is a natural part of most forest ecosystems in the western United States, but its effects on nonnative plant invasion have only recently been studied. Also, forest managers are engaging in fuel reduction projects to lessen fire severity, often without considering potential negative ecological consequences such as nonnative plant species introductions. Increased availability of light, nutrients, and bare ground have all been associated with high-severity fires and fuel treatments and are known to aid in the establishment of nonnative plant species. We use vegetation and environmental data collected after wildfires at seven sites in coniferous forests in the western United States to study responses of nonnative plants to wildfire. We compared burned vs. unburned plots and plots treated with mechanical thinning and/or prescribed burning vs. untreated plots for nonnative plant species richness and cover and used correlation analyses to infer the effect of abiotic site conditions on invasibility. Wildfire was responsible for significant increases in nonnative species richness and cover, and a significant decrease in native cover. Mechanical thinning and prescribed fire fuel treatments were associated with significant changes in plant species composition at some sites. Treatment effects across sites were minimal and inconclusive due to significant site and site x treatment interaction effects caused by variation between sites including differences in treatment and fire severities and initial conditions (e.g., nonnative species sources). We used canonical correspondence analysis (CCA) to determine what combinations of environmental variables best explained patterns of nonnative plant species richness and cover. Variables related to fire severity, soil nutrients, and elevation explained most of the variation in species composition. Nonnative species were generally associated with sites with higher fire severity, elevation, percentage of bare ground, and lower soil nutrient levels and lower canopy cover. Early assessments of postfire stand conditions can guide rapid responses to nonnative plant invasions.

Impacts of mixed severity wildfire on exotic plants in a Colorado ponderosa pine–Douglas-fir forest

“…[Note that the cover of Canadian horseweed (Conyza canadensis), which is considered native by the authority we used (USDA 2008), equals 27% and 18% in moderately and severely burned stands, respectively.] Freeman et al (2007) found more moderate influences of fire severity; in their study of seven wildfires, exotic cover ranged from *1% in unburned stands to *2% in lightly burned stands and *3% in severely burned stands, which is comparable to the findings presented here. Mechanisms to explain exotic response to fire have been repeatedly linked to the postfire availability of resources such as light and bare soil, which are critical for exotic establishment and growth (Freeman et al 2007;Keeley et al 2003).…”

“…Exotics repeatedly have been found to be least stimulated in lightly burned forests where overstory tree mortality and forest floor consumption are minimal, and most stimulated in severely burned forests where tree crowns and forest floor are largely consumed (Crawford et al 2001;Freeman et al 2007;Hunter et al 2006). The response of exotic plants after fire also often varies with time since fire (Klinger et al 2006;Koniak 1985;Nelson et al 2008).…”

Effects of fuel treatments on California mixed-conifer forests

“…This is consistent with findings from other regions (Freeman et al 2007;Schwilk et al 2009;Stephens et al 2012). However, non-native species richness in untreated forests that burn at high severity has been shown to exceed non-native species richness in treated forests, both in California (Stevens et al 2015) and nationally (Freeman et al 2007). These findings indicate that low-and moderate-severity fires may reduce the risk of non-native plant invasion compared with high-severity fires in some cases.…”

Implementation of National Fire Plan treatments near the wildland–urban interface in the western United States

“…Although prescribed fire and wildlandfire use (the management of naturally ignited fires to achieve resource benefits) are economical means of reducing fuel loads (24), fire was implemented only half as much within the WUI compared to across the West, reflecting the challenge of burning near communities. Overall, there is an urgent need for broadscale monitoring both of fire-mitigation efficacy (directly within stands and with respect to nearby communities) and of nontarget ecological effects of such treatments, such as increased mortality of presettlement trees (25,26) and increased abundance of nonnative plants (27,28).…”