Vegetation structure and fire weather influence variation in burn severity and fuel consumption during peatland wildfires

Abstract

Abstract. Temperate peatland wildfires are of significant environmental concern but information on their environmental effects is lacking. We assessed variation in burn severity and fuel consumption within and between wildfires that burnt British moorlands in 2011 and 2012. We adapted the Composite Burn Index (pCBI) to provide semi-quantitative estimates of burn severity. Pre- and post-fire surface (shrubs and graminoids) and ground (litter, moss, duff) fuel loads associated with large wildfires were assessed using destructive sampling and analysed using a Generalised Linear Mixed Model (GLMM). Consumption during wildfires was compared with published estimates of consumption during prescribed burns. Burn severity and fuel consumption were related to fire weather, assessed using the Canadian Fire Weather Index System (FWI System), and pre-fire fuel structure. pCBI varied 1.6 fold between, and up to 1.7 fold within, wildfires. pCBI was higher where moisture codes of the FWI System indicated drier fuels. Spatial variation in pre- and post-fire fuel load accounted for a substantial proportion of the variance in fuel loads. Average surface fuel consumption was a linear function of pre-fire fuel load. Average ground fuel combustion completeness could be predicted by the Buildup Index. Carbon release ranged between 0.36 and 1.00 kg C m−2. The flammability of ground fuel layers may explain the higher C release-rates seen for wildfires in comparison to prescribed burns. Drier moorland community types appear to be at greater risk of severe burns than blanket-bog communities.

A Critical Review of the IUCN UK Peatland Programme’s “Burning and Peatlands” Position Statement

“…Consequently, rewetted bogs may have a higher fuel load over extensive areas (i.e. not broken up into a mosaic of heather ages and such biomass load), which will lead to higher fire temperatures if a wildfire does manage to ignite (Hobbs and Gimingham 1984;Davies et al 2016a;Noble et al 2019a). Finally, contrary to the claims made in the literature (Baird et al 2019), there are indications that higher water tables may not inhibit the horizontal and downward spread of smouldering peatland wildfires (Huang and Rein 2017).…”

“…Moving on, we agree that: "greater fuel load ≈ greater heat intensity ≈ prolonged fire ≈ potential for greater damage to vegetation and ignition of the underlying peat soil". Indeed, this concept is well established (Davies et al 2016a;Davies et al 2016b;Davies et al 2010b;Noble et al 2019a), as is the fact that prescribed burns can be used to reduce fuel loads on UK peatlands (Lee et al 2013a;Alday et al 2015;Milligan et al 2018;Whitehead and Baines 2018;Grau-Andrés et al 2019a;Heinemeyer et al 2019c;Marrs et al 2019a). However, we disagree with the unverified assertion that rewetting peatlands is crucial for mitigating wildfire risk.…”

Effects of canopy midstory management and fuel moisture on wildfire behavior

“…Contreras et al 50 used light detection and ranging (LiDAR) mapped forest structure data to characterize the role of vegetation connectivity and thinning operations (that reduce connectivity) in the context of crown fire potential. White et al 51 and Davies et al 52 determined that the flammability of surface fuels are also important in governing fire severity. The type of ignition is another factor that sets up the initial condition for fire propagation.…”