Advancing Investigation and Physical Modeling of First-Order Fire Effects on Soils

William J Massman,Sacha J Mooney,John M Frank

doi:10.4996/fireecology.0601036

Abstract

Heating soil during intense wildland fires or slash-pile burns can alter the soil irreversibly, resulting in many significant long-term biological, chemical, physical, and hydrological effects. To better understand these long-term effects, it is necessary to improve modeling capability and prediction of the more immediate, or first-order, effects that fire can have on soils. This study uses novel and unique observational data from an experimental slash-pile burn to examine the physical processes that govern the transport of energy and mass associated with fire-related soil heating. Included in this study are the descriptions of 1) a hypothesized fire-induced air circulation within the soil, and 2) a new and significant dynamic feedback between the fire and the soil structure. The first of these two hypotheses is proposed to account for the almost instantaneous order-of-magnitude increase in soil CO2 observed during the initiation of the burn. The second results from observed changes to the thermal conductivity of the soil, thought to occur during the fire, which allow the heat pulse to penetrate deeper into the soil than would occur without this change. The first ever X-ray computed tomography images of burn area soils are consistent with a change in soil structure and a concomitant change in soil thermal conductivity. Other ways that current technology can be used to aid in improving physically-based process-level models are also suggested.

Highlights

Intense wildland fires can alter the successional trajectories of plant communities and soil biota and transform the physical, chemical, and structural properties of soils
Because all first-order fire-related effects on soils are the result of soil heating, this study focuses on the critical processes that govern the transport of energy and mass during more extreme fires, with the intent of providing guidance for developing the generation of models of soil heating during such fires
Several improvements in current processbased models of soil heating during fires are possible with current technology but, as we have argued here, should be based on increasing the observational data base

Summary

INTRODUCTION

Intense wildland fires can alter the successional trajectories of plant communities and soil biota and transform the physical, chemical, and structural properties of soils. The boundary conditions of the current version of the First Order Fire Effects Model (FOFEM 5.0) include specific parameterizations for the soil thermal properties and soil heating rates and can predict reliable soil temperatures during fires (Reinhardt 2003). Improving predictive models of fire-related heat penetration into soils requires the ability to quantify soil heat conduction and radiation absorption, which are the physical processes responsible for the thermal energy flow through the soil surface during a fire. The ideal upper boundary condition for modeling soil heat flow during a fire is a linear combination of Gcon(0, t) and Grad(0, t) that can be determined from the type of fire and the physical properties of the surface This ideal may never be completely achieved. Improving models of the soil heat flux during fires will lead to improved modeling capability and improved understanding of the thermal impacts of fire on soil, and to improved understanding and modeling of the mass (and soil moisture) transport in soils during fires

Mass Transport within Soils during Fires

Effects of Heating on Soil Nutrient Status and Soil Chemistry

MODELS OF SOIL HEATING USING CURRENT TECHNOLOGY

LITERATURE CITED