Characteristics of Burns Conducted under Modified Prescriptions to Mitigate Limited Fuels in a Semi-Arid Grassland

David J Augustine,David P Smith,Justin D Derner

doi:10.4996/fireecology.1002036

Abstract

AbstractIn semi-arid grasslands of the North American Great Plains, fire has traditionally been viewed as having few management applications, and quantitative measurements of fire behavior in the low fuel loads characteristic of this region are lacking. More recently, land managers have recognized potential applications of prescribed fire to control undesirable plant species and to manage habitat for wildlife in this region. Working in the shortgrass steppe of northeastern Colorado over a 7-year period, we quantified peak temperatures, heating duration, and heat dosage produced near ground level during prescribed burns conducted under a wide range of fuel loads and weather conditions. We use an information theoretic approach to develop models that predict peak temperature and heat dosage as a function of weather parameters and fuel loads. Under the weather conditions that we examined, successful burns (>80 % of target area burnt) occurred with fuel loads varying from 350 kg ha−1 to 1175 kg ha−1, while burns with fuel loads <350 kg ha−1 generally failed to spread and burned less than 60 % of target areas. Peak temperatures, heat duration, and heat dosage during shortgrass burns: 1) were lower than reported for mixed grass prairies, 2) increased linearly with increasing fuel loads, and 3) were secondarily influenced by wind speed, ambient air temperature, and relative humidity. Compared to desert grassland, heat doses near the ground surface were similar, but peak temperatures were lower and heat duration longer in shortgrass steppe burns. Our findings provide quantitative predictions for heat production from fires in shortgrass steppe near the ground surface, where most plant meristems are located. Based on these relationships, we provide suggestions for burn prescriptions to achieve goals such as reducing abundance of undesirable plant species and providing habitat for native grassland birds.

Highlights

Fire is widely recognized as an important ecological disturbance process and management tool in grasslands of the North American Great Plains, but most research and management applications related to fire have focused on the more mesic and productive grasslands in the region (Wright and Bailey 1982, Anderson 2006, Fuhlendorf et al 2009)
One set of burns was conducted with mean fuel loads of 350 kg ha-1 and relatively ideal weather conditions that resulted in flames carrying successfully across the plots; we visually estimated that 85 % to 90 % of the plots were burned, and 83 % of thermocouples burned
We returned to these sites two weeks later when relative humidity was lower (17 % to 24 %), winds were moderate (6 m sec-1 to 8 m sec-1), and ambient temperatures greater (15 °C to 18 °C), and found that patches of grassland that had not burned previously were able to carry the fire

Summary

Introduction

Fire is widely recognized as an important ecological disturbance process and management tool in grasslands of the North American Great Plains, but most research and management applications related to fire have focused on the more mesic and productive grasslands in the region (Wright and Bailey 1982, Anderson 2006, Fuhlendorf et al 2009). Based on work in desert grasslands of central New Mexico, USA, McDaniel et al (1997, 2000) recommended modifying burn prescriptions in desert grasslands to occur with air temperatures of 22 °C to 28 °C, relative humidity of only 10 % to 20 %, winds of 3 m sec-1 to 8 m sec-1, and fuel loads >500 kg ha-1 Prescribed burns under these conditions achieved heat dosages (calculated as area under the time-temperature curve using a baseline of 0 °C) on the order of 10 000 °Csec to 16 000 °Csec, which in turn induced mortality of undesirable subshrubs (McDaniel et al 1997). Studies are needed to examine burn characteristics under modified prescriptions that include lower relative humidity and higher temperatures than are typical of burns in mesic grasslands

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