Abstract
Prescribed burning is a primary tool for habitat restoration and management in fire-adapted grasslands. Concerns about detrimental effects of burning on butterfly populations, however, can inhibit implementation of treatments. Burning in cool and humid conditions is likely to result in lowered soil temperatures and to produce patches of low burn severity, both of which would enhance survival of butterfly larvae at or near the soil surface. In this study, we burned 20 experimental plots in South Puget Sound, Washington, USA, prairies across a range of weather and fuel conditions to address the potential for producing these outcomes. Risk to diapausing butterfly larvae, assessed by measuring subsurface soil temperatures and heat dosages, was lower when air temperature was less than 26°C and dead fuel moisture was greater than 9 %. The likelihood that unburned or low-severity patches would be left was affected by dead fuel moisture, but also required pre-existing fuel discontinuities. Burns conducted in the morning hours during the summer drought season (the main prescribed-fire season in this system) were cooler and had lower severities. This research increases our understanding of how fine fuel moisture and fuel continuity during grassland burning can affect fire intensity, severity, and spread. It also provides support for burning earlier in the day as a way to increase burn heterogeneity and has allowed us to create recommendations for burning in sensitive butterfly habitat.
Highlights
Prescribed burning is one of the primary tools for fuels reduction, agricultural residue consumption, and habitat restoration across ecosystems; in the United States, almost 1.2 million hectares of land were intentionally burned in 2015 (National Interagency Fire Center 2016)
Average air temperatures ranged from 14 °C to 32 °C, relative humidity (RH) ranged from 32 % to 88 %, and wind speed ranged from 0.5 m s-1 to 3.7 m s-1
Average peak soil temperature was strongly positively related to air temperature and negatively related to dead fuel moisture (R2adj = 0.829, P ≤ 0.001)
Summary
Prescribed burning is one of the primary tools for fuels reduction, agricultural residue consumption, and habitat restoration across ecosystems; in the United States, almost 1.2 million hectares of land were intentionally burned in 2015 (National Interagency Fire Center 2016). Concerns about potentially detrimental effects of burning on diapausing butterfly larvae near the soil surface, as well as other ground-dwelling arthropods and bryophyte and lichen communities, can sometimes inhibit the implementation of restoration treatments (Fimbel 2004, Hamman et al 2011, Schultz et al 2011, Calabria et al 2016). These organisms are more likely to survive a fire in areas that experience lower soil temperatures during the burn or in unburned and low-severity patches (Bradstock et al 2005, New 2014). This analysis focuses on the relationship between weather and fuel conditions and several response variables that serve as metrics of the risk to fire-sensitive organisms near the soil surface during prescribed fires
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