Characterisation of the fuel and fire environment in southern Ontario’s tallgrass prairie

Abstract

Prescribed burning can be an integral part of tallgrass prairie restoration and management. Understanding fire behaviour in this fuel is critical to conducting safe and effective prescribed burns. Our goal was to quantify important physical characteristics of southern Ontario’s tallgrass fuel complex prior to and during prescribed burns and synthesise our findings into useful applications for the prescribed fire community. We found that the average fuel load in tallgrass communities was 0.70 kg m–2. Fuel loads varied from 0.38 to 0.96 kg m–2. Average heat of combustion did not vary by species and was 17 334 kJ kg–1. A moisture content model was developed for fully cured, matted field grass, which was found to successfully predict moisture content of the surface layers of cured tallgrass in spring. We observed 25 head fires in spring-season prescribed burns with spread rates ranging from 4 to 55 m min–1. Flame front residence time averaged 27 s, varying significantly with fuel load but not fire spread rate. A grassland spread rate model from Australia showed the closest agreement with observed spread rates. These results provide prescribed-burn practitioners in Ontario better information to plan and deliver successful burns.

Forbs, grasses, and grassland fire behaviour

“…First, differences in heat released per unit of fuel burned, and the completeness with which fuel burned, are unlikely to explain why grasses increased fire intensity more strongly than did forbs per unit biomass. We assumed a constant heat yield per unit biomass because the heat of combustion of various grass and forb species had a coefficient of variation in only 5.3% in a Canadian tallgrass prairie (Kidnie, ), and 1.2% in a South American grassland (Britton, Dodd, & Weichert, ). We also assumed complete combustion; if (coarser) forbs were less completely consumed by fire than (finer) grasses then intensity would be over‐estimated for forbs more than for grasses, making our inference that grasses more strongly increase fireline intensity conservative.…”

“…To calculate fireline intensity (rate of energy release per unit length of fire front, Byram, ), we multiplied the forward rate of spread by the fuel load (approximated as biomass per unit area late the previous summer) and the approximate heat of combustion 20 MJ/kg (Williams, Gill, & Moore, ). We assumed the heat of combustion to be constant across species compositions because it is very similar across herbaceous species (Byram, ; Kidnie, ). We calculated reaction intensity per unit area by dividing fireline intensity by the flaming zone depth.…”

Season affects fire behavior in annually burned humid savanna of West Africa

“…During each fire, wind speed (m s −1 ), air temperature (°C), and relative humidity (%) were recorded every five minutes, similar to methods of Kidnie and Wotton (2015), using an in situ automatic weather station (reference number SM55PRO; Inovalley, Pontoise, France) placed at 2 m aboveground, in the fire break between two adjacent plots. The recorded values were averaged over the period of fire propagation for each plot.…”

Flammability Characteristics of Green Roofs

“…For example, for tall grasses, the reported range of values is 16.8 to 19.1 MJ/kg [53,54]. The averaged value of 17.3 MJ/kg was found by Kidnie [34] for different tall grasses. From the experimental research on several Mediterranean shrub species (tested twigs and leaves), the heating value was between 19.9 and 22.9 MJ/kg [55].…”