Long-term impacts of prescribed burns on soil thermal conductivity and soil heating at a Colorado Rocky Mountain site: a data/model fusion study

Abstract

Heating any soil during a sufficiently intense wild fire or prescribed burn can alter that soil irreversibly, resulting in many significant, and well studied, long-term biological, chemical, and hydrological effects. On the other hand, much less is known about how fire affects the thermal properties and the long-term thermal regime of soils. Such knowledge is important for understanding the nature of the soil’s post-fire recovery because plant roots and soil microbes will have to adapt to any changes in the day-to-day thermal regime. This study, which was carried out at Manitou Experimental Forest (a semiarid site in the Rocky Mountains of central Colorado, USA), examines three aspects of how fire can affect the long-term (post-fire) thermal energy flow in soils. First, observational evidence is presented that prescribed burns can alter the thermal conductivity of soils to a depth of at least 0.2 m without altering its bulk density. Second, data are presented on the thermal properties of ash. (Such data are necessary for understanding and modeling the impact any remaining post-fire ash layer might have on the daily and seasonal flow of thermal energy through the soil.) Third, observational data are presented on the long-term effects that prescribed burns can have on soil surface temperatures. In an effort to quantify long-term changes in the soil temperatures and heat fluxes resulting from fire this study concludes by developing and using an analytical model of the daily and annual cycles of soil heating and cooling, which incorporates observed (linear variation of) vertical structure of the soil thermal properties and observed changes in the surface temperatures, to synthesise these fire-induced effects. Modeling results suggest that under the dry soil conditions, typical of the experimental forest, the amplitudes of the daily and seasonal cycles of soil heating/cooling in the fire-affected soils will greatly exceed those in the soils unaffected by fire for several months to years following the fire and that these effects propagate to depths exceeding one metre.