Rate of spread of fires in Calluna vulgaris‐dominated moorlands

Abstract

1. Calluna-dominated heaths occur throughout Europe but are in decline across their range. There is growing interest in using prescribed burning for their management, but environmental and social change will impact future fire regimes. Understanding fire behaviour is vital for the sustainable use of fire, but no robust models exist to inform management. 2. Shrub fuels display complex fire behaviour. This is particularly true in UK moorlands which are unusual in their fuel structure and moisture regime, being dominated by live fuel and an oceanic climate. 3. We burnt 27 experimental fires in the Scottish uplands during the legal burning season using a replicated experimental design. Plots were assigned to one of three commonly identified growth phases. We estimated a range of prefire fuel characteristics, including heterogeneity in fuel structure. We recorded wind speed and direction and estimated rate of spread (RoS). 4. Redundancy analysis was used to investigate the relationship between fire behaviour parameters as a whole and control variables. Fuel structure and heterogeneity, wind speed and canopy fuel moisture content were strongly related to variation in fire behaviour. 5. Best subsets regression was used to generate models of fire spread based on wind speed, vegetation height, canopy fuel moisture and an index of fuel heterogeneity. RoS was determined largely by wind speed, but this interacted strongly with vegetation structure. Changes in fuel horizontal continuity and vertical structure reduced rates of spread in low wind speeds. 6. Synthesis and applications. Live fuel moisture and fuel heterogeneity play an important role in dampening fire behaviour, aspects of shrub fuels that have previously not been examined in detail. Careful use of fire for moorland management increases habitat diversity and creates fire-safe landscapes. Escaped prescribed fires burn large areas, homogenize landscapes and have severe impacts on ecosystem services. The complex relationship between fuel structure and wind speed implies that changes in behaviour can be rapid and unexpected. Models can be used to assess fire hazard prior to prescribed burning and to choose fuels that can be burnt safely under prevailing or forecast conditions.

Fuel Moisture Thresholds in the Flammability of Calluna vulgaris

“…Fuel moisture content will affect all three aspects of flammability. Previous fire behaviour research on Calluna has failed to adequately identify the role of FMC on fire behaviour [14,17], and though Davies et al [18] advanced our knowledge somewhat, the relative importance of dead and live fuels and their respective moisture contents are still not well understood. The fact that Calluna-dominated fuels mostly comprise a live/green component that can display significant seasonal, intra-seasonal and diurnal variation [19][20][21] makes it particularly difficult to predict changes in their flammability.…”

“…Fires were small (2 m 9 2 m) ignition tests as these allowed rapid set-up and monitoring in response to changing weather conditions. Such small tests also allowed us to burn safely in a wider range of conditions than would be possible with larger fires typical of accidental wildfires or those used by managers [18]. Our objectives focused on quantifying variation in fire sustainability.…”

“…The fact that Calluna-dominated fuels mostly comprise a live/green component that can display significant seasonal, intra-seasonal and diurnal variation [19][20][21] makes it particularly difficult to predict changes in their flammability. Management-scale experimental fires [18] are one way to try and discern the relationships between environmental variables and fire behaviour, but are labour and time intensive to set up and monitor and, by necessity, have focused on a restricted range of conditions when fires were likely to be self-sustaining. It has been demonstrated that the relationship between fire behaviour and FMC in shrub fuels is non-linear such that changes in moisture content can function as an ''on/off'' switch at the lower end of the fire behaviour spectrum [22].…”

“…All fires were carried out on flat ground. The vegetation at both sites was classified as belonging to a ''Late-building'' type fuel load (15-20 years old, with a somewhat open but generally uniform canopy, few stems >2 mm in diameter, continuous bryophyte layer) [18] though it was generally taller and sparser at Crubenmore than at Castlelaw Hill. This fuel is that typically burnt by managers.…”

Empirical Support for the Use of Prescribed Burning as a Fuel Treatment

“…Recent empirically based models for fire spread rate account for the effect of fuel structure through fuel depth or height, namely in semiarid shrubland [25], oceanic-climate shrubland [26], shrubland in general [27] and Mediterranean pine forest [28]. For the latter, the effect of fuel recovery (translated as time since PB) on simulated fire behaviour indicates that it is fireline intensity rather than rate of spread that reflects treatment persistence (Fig.…”