Diurnal Pine Bark Structure Dynamics Affect Properties Relevant to Firebrand Generation

Scott Pokswinski,E Louise Loudermilk,J Kevin Hiers,Joseph J O’Brien,Michael R Gallagher,Nicholas S Skowronski

doi:10.3390/fire3040055

Scott Pokswinski, E Louise Loudermilk + Show 4 more

Open Access

https://doi.org/10.3390/fire3040055

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Abstract

Firebrands are an important agent of wildfire spread and structure fire ignitions at the wildland urban interface. Bark flake morphology has been highlighted as an important yet poorly characterized factor in firebrand generation, transport, deposition, and ignition of unburned material. Using pine species where bark flakes are the documented source of embers, we conducted experiments to investigate how bark structure changes in response to diurnal drying. Over a three-day period in a longleaf pine (Pinus palustris Mill.) stand in Florida, we recorded changes in temperature, moisture content, and structure of bark across different facing aspects of mature pine trees to examine the effects of varying solar exposure on bark moisture. We further compared results to bark drying in a pitch pine (Pinus rigida Mill.) plantation in New Jersey. Under all conditions, bark peeled and lifted away from the tree trunk over the study periods. Tree bole aspect and the time of day interacted to significantly affect bark peeling. General temperature increases and moisture content decreases were significantly different between east and west aspects in pitch pine, and with time of day and aspect in longleaf pine. These results illustrate that bark moisture and flakiness is highly dynamic on short time scales, driven largely by solar exposure. These diurnal changes likely influence the probability of firebrand production during fire events via controls on moisture (ignition) and peeling (lofting).

Highlights

Firebrands, combusting airborne objects lofted during fires [1], are an important consideration for wildland fire managers because they directly influence fire spread and represent the primary cause of structure ignitions during wildfires [2,3,4,5]
Firebrand structure plays a vital role in each component, influencing combustion characteristics [9], aerodynamics [9,10,11], and patterning of firebrand deposition [7,8,12], as well as the probability of new materials igniting from firebrands [13]
We found that exposure to solar radiation was a primary driver of patterns of the drying and

Summary

Introduction

Firebrands, combusting airborne objects lofted during fires [1], are an important consideration for wildland fire managers because they directly influence fire spread and represent the primary cause of structure ignitions during wildfires [2,3,4,5]. Firebrand hazard has been described conceptually as having three components, (1) generation, (2) transport, and (3) ignition of new material [3]; results of recent studies suggest considering firebrand deposition (location and density of deposited firebrands per m2 s−2 ) as a fourth component [7,8]. Firebrand structure plays a vital role in each component, influencing combustion characteristics [9], aerodynamics [9,10,11], and patterning of firebrand deposition [7,8,12], as well as the probability of new materials igniting from firebrands [13]. An understanding of conditions and physical processes that drive firebrands remains limited, which restricts the ability to quantitatively evaluate firebrand hazard or its use in fire behavior models [4,12].

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