Creating Hotter Fires in the Sonoran Desert: Buffelgrass Produces Copious Fuels and High Fire Temperatures

Christopher J Mcdonald,Guy R Mcpherson

doi:10.4996/fireecology.0902026

Abstract

AbstractBuffelgrass (Pennisetum ciliare [L.] Link) can create a grass-fire cycle in many parts of the world because it is a highly competitive, fire-tolerant grass and can replace less fire-tolerant native plants. Fuel loads, loss of buffelgrass biomass after herbicide treatments, and allometric data of buffelgrass growth were measured across sites in southern Arizona, USA. Prescribed fires also were conducted in buffelgrass-dominated fields to measure fire temperatures and quantify relationships between temperature and fuel load. We directly recorded temperatures up to 871 °C and indirectly recorded temperatures of 900 °C. There was no relationship between fuel load and temperature, likely because increased fuel was insufficient to contribute to additional fire intensity beyond minimum fuel loads. Compared to previously described buffelgrass stands and across different desert ecosystems, buffelgrass fuel loads were higher than reported in most other studies, and inter-annual variation in buffelgrass biomass is much lower than that of other invasive grasses, including annual and perennial grasses. As a result, buffelgrass creates a more consistent year-to-year fire hazard than annual grasses. Managers have used herbicide to reduce buffelgrass biomass and we found that, after three years of decomposition, stands of dead buffelgrass were unlikely to support fire spread. Allometric relationships can provide an accurate estimate of buffelgrass biomass of individual plants, but not fuel loads. Buffelgrass produces nonnative grasslands at relatively low elevations of the Sonoran Desert, with more biomass than comparable grasslands in more mesic environments.

Highlights

Nonnative invasive species (NIS) are altering fire regimes in many parts of the world (D’Antonio and Vitousek 1992, Brooks et al 2004)
Our objectives were to: 1) measure fire behaviors, fire temperature, to determine if fires fueled by buffelgrass can create intense temperatures that are both lethal to surrounding plants and higher than other North American deserts; 2) determine if the allometry of buffelgrass can be used as a simple and accurate correlate of fuel loads; and 3) determine if the dead fuel decomposes and reduces fire threats within a 3-year time period when buffelgrass fuels are managed with herbicides
Average biomass in Avra Valley was significantly higher than in SNP (P < 0.001), but fuel load did not differ between some individual plots in SNP and Avra Valley (Table 1), indicating that potential fires in SNP could exhibit the extreme fire behaviors observed in Avra Valley

Summary

Introduction

Nonnative invasive species (NIS) are altering fire regimes in many parts of the world (D’Antonio and Vitousek 1992, Brooks et al 2004). In the Sonoran Desert, the NIS buffelgrass (Pennisetum ciliare [L.] Link = Cenchrus ciliaris L.) is filling normally barren gaps between native grasses, shrubs, trees, and charismatic succulents, and further spreads to displace native vegetation (Olsson et al 2012). These bare areas formerly acted as fire breaks in low-elevation desert sites (Thomas 1991), but they currently form continuous grass stands that create fire hazards in a landscape where fires were historically rare (Wright and Bailey 1982). Cacti have numerous physiological adaptations that help them thrive in the desert, but some of these characteristics ensure low post-fire survival (Thomas 1991, 2006)

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