Soil temperatures during autumn prescribed burning: implications for the germination of fire responsive species?

Abstract

Prescribed fire is a widely applied management tool in native forests. There have been concerns raised about the ecological impacts of prescribed fire on native flora. One aspect of the debate is the extent towhich prescribed fire heats the soil to levels reported to trigger germination in the soil seed banks.We used Thermochrons to test soil temperatures at 2 and 5 cm in prescribed burns in dry sclerophyll forests. Soil temperatures during the burns were generally low (<40•C) with less than 5% of sites being exposed to temperatures necessary for the germination of fire-dependent shrub species. High temperatures were associated with high fuel consumption and large woody debris. This information suggests that prescribed fires, carried out according to standard practices, in these forests are unlikely to trigger germination in the majority of the soil-stored seed banks. If ecological burns in these forests are aimed at promoting populations of senescing obligate seeder species, they need to be hotter than standard practice if they are to achieve their objectives, although we acknowledge that there are inherent risks associated with hotter burns. Abstract. Prescribed fire is a widely applied management tool in native forests. There have been concerns raised about the ecological impacts of prescribed fire on native flora. One aspect of the debate is the extent to which prescribed fire heats the soil to levels reported to trigger germination in the soil seed banks. We used Thermochrons to test soil temperatures at 2 and 5 cm in prescribed burns in dry sclerophyll forests. Soil temperatures during the burns were generally low (<40 • C) with less than 5% of sites being exposed to temperatures necessary for the germination of fire-dependent shrub species. High temperatures were associated with high fuel consumption and large woody debris. This information suggests that prescribed fires, carried out according to standard practices, in these forests are unlikely to trigger germination in the majority of the soil-stored seed banks. If ecological burns in these forests are aimed at promoting populations of senescing obligate seeder species, they need to be hotter than standard practice if they are to achieve their objectives, although we acknowledge that there are inherent risks associated with hotter burns.

Can a seed bank provide demographic and genetic rescue in a declining population of the endangered shrub Acacia pinguifolia?

“…Furthermore, Penman & Towerton (2008) suggest that autumn prescribed burns rarely generate soil temperatures required for germination of a range of obligate-seeding species in Australia. As our burn was conducted in autumn and over a limited spatial scale it is possible that insufficient temperatures were reached to stimulate germination of seeds in the deeper soil layers at Finniss, compared to the summer wildfire on the Eyre Peninsula which was reported to stimulate the germination of several hundred A. pinguifolia seedlings (Pobke 2007).…”

Soil phosphorus transformations under forest burning and laboratory heat treatments

“…In the southern Appalachians, Hubbard et al (2003) found that understory prescribed burning produced average soil temperatures at 1 cm and 2 cm depth of 59°C and 45°C, respectively; however, temperatures much greater than these averages were observed at some locations. Such spatial variation of soil temperature during prescribed fire was evaluated by Penman and Towerton (2008) in dry sclerophyll forests. At 2 cm depth, 57% of sensors remained below 40°C.…”

Interactive effects of climate change and fire on metapopulation viability of a forest-dependent frog in south-eastern Australia

“…During the adult phase, individuals spend >99% of their time in the forest away from breeding areas (Penman et al, 2008c) and it is these areas that are more likely to burn during a low or high intensity fire (Penman et al, 2007a;Collins et al, 2013). Low-intensity fires are likely to result in low mortality of adult frogs, due to the fossorial nature of the species, and the patchiness (Penman et al, 2007a) and low soil heating associated with low-intensity fires (Penman et al, 2006b;Penman and Towerton, 2008). Conversely, high-intensity fires tend to result in greater adult mortality (Penman et al, 2004), as such fires are more homogenous and result in higher soil heating (Humphreys and Craig, 1981;Bradstock et al, 1992).…”

“…Modelled low-intensity fires were based on observations of prescribed fire that were patchy and resulted in low adult mortality (Penman et al, 2007a). Soil temperatures experienced in such fires are not sufficient to result in mortality for the majority of individuals in the fire area (Penman et al, 2006b;Penman and Towerton, 2008). In contrast, high-intensity fires result in greater heating of the soil (Humphreys and Craig, 1981;Bradstock et al, 1992), causing higher mortality in the adult population.…”