Humboldt State University

Forest Floor Fire Behavior



Although considerable effort has been invested in studying the effects of moisture on smoldering combustion of duff (Campbell et al. 1994, 1995; Frandsen 1991, 1997; Miyanishi and Johnson 2002, Rein et al. 2008), there is little understanding of its role in fire behavior in the forest floor. The inability to model duff smoldering is a major impediment to the restoration and management of fire-prone ecosystems. Our understanding of duff fire spread and consumption patterns lags, in spite of its wellrecognized importance in tree mortality, emissions, soil heating, and post-burn spatial heterogeneity (McMahon 1983, Swezy and Agee 1991, Miyanishi 2001, Knapp and Keeley 2006). Current empirical models generate binary burn no-burn results based on moisture fraction, measured bulk density, mineral content, and depth (Frandsen 1997). These site specific and environmental condition specific regression models are insufficient for use in other ecosystems and especially to predict duff smoldering behavior across the forest floor. Since forest floor strata are diverse, their presence further complicates modeling; strata have different moisture contents, contain cones and woody fragments that span one or more horizons, and have thus far, been ignored as important fuel strata. Our primary objectives are to better understand the role of fuel moisture relationships on forest floor ignition, spread, intensity, and extinction. The proposed study is focused on the development, parameterization, and validation of a moisturedependent forest floor fire behavior model that will provide a framework for our scientific investigation and understanding.

Project Justification and Expected Benefits

Forest floor fires have been implicated in extensive overstory tree mortality across disparate fire-prone forested ecosystems (e.g., Haase and Sackett 1998, Kolb et al. 2007, Varner et al. 2007). Smoldering duff heats surficial roots and bark, potentially overwhelming the bark and soil properties that protect vascular tissues (Dickinson and Johnson 2004). The key variable in field experiments has been variation in moisture content across the forest floor horizons (Haase and Sackett 1998, Varner et al. 2007, 2009). Smoldering temperature and residence time will play an essential role in any biophysically based model to predict such tree mortality (c.f. Michaletz and Johnson 2007). How spatial distributions of moisture content and physical properties of the forest floor interact to generate temperatures and residence times at various locations is an open question. More precisely, no existing model addresses: how moisture and heat move laterally through the forest floor, heating and pre-ignition drying of the forest floor ahead of a smoldering front, and the duration of smoldering (i.e., extinction).

Post-burn heterogeneity has been identified as a key factor in both limiting soil erosion and promoting species diversity (Benevides-Solrio and MacDonald 2001, Slocum et al. 2003, Knapp et al. 2007,). Again, fuel moisture plays a key role in post-burn heterogeneity (Knapp et al. 2005, Knapp and Keeley 2006). A potential explanation for heterogeneity is that the forest floor is ignited at various sites by vectors (i.e., woody debris or cones; see de Souza Costa and Sandberg 2004, Fonda and Varner 2005) and then moisture limits the spread of these smoldering ground fires to patches. This linkage between vector fuels and fuel moisture may explain results that have found forest floor consumption beyond moisture thresholds, a common result in research (e.g., Little et al. 1982) and is reported by managers. This study will improve our understanding of the role moisture plays in the behavior of fires in the forest floor. More specifically, it will improve our understanding of pre-ignition drying of forest floor fuels ahead of a smoldering front and elucidate moistures role in: 1) ignition of the forest floor by natural vector, 2) the temperature and duration of smoldering combustion, and 3) the formation of spatial patterns of forest floor consumption. The mathematical model we develop will lay the foundation for more userfriendly software for forest floor fire behavior modeling. Ultimately, the scientific knowledge above is integral to understanding important fire effects: runoff and tree stress / mortality.

Project Objectives and Hypotheses

We plan to develop a forest floor fire behavior model that builds on recent advances (Holt 2008) and incorporates moisture and heat dynamics in a spatially explicit manner. We will parameterize and test the model using a series of laboratory experiments and on-going experiments from long-unburned longleaf pine forests in northern Florida (Hiers et al. JFSP 01-1-3-11). The model will be parameterized using measurements from peat and two forest duff types (mixed conifer and pine). Once the model has been parameterized, we will test the ability of the model to make predictions; specifically pre-ignition drying of duff ahead of a smoldering front in the lab, and patterns of smoldering intensity and post-burn duff consumption in the field. Due to the stochastic nature of combustion we do not expect the model to predict exact patterns, but instead statistical properties of the spatial patterns (see Methods). Laboratory experiments will focus on the effects of varying moisture on: the probability of ignition of peat and duff by various vectors (i.e., wooden dowels and cones), and the temperature and residence time of smoldering combustion (i.e., time until transition to extinction).


Our work seeks to improve our understanding of moisture’s role in the spread of fire through the forest floor by addressing the following fundamental questions:

  1. How does forest floor moisture content influence thresholds of ignition by natural vectors?
  2. How does forest floor moisture content influence temperature and duration of smoldering?
  3. combustion?
  4. How does this heat dry the forest floor ahead of smoldering front and allow it to propagate?
  5. How do the above processes interact to generate spatial patterns of forest floor consumption?


Our research will test the following hypothesis:

  1. Probability of forest floor ignition is a function of moisture content by duff type.
  2. Vector type effects probability of floor ignition.
  3. Dry forest floors smolder at higher temperatures, but for shorter durations than moist forest floors.
  4. Smoldering combustion generates observable pre-ignition drying ahead of the smoldering front.
  5. Spatial heterogeneity in pre-burn moisture content generates spatial heterogeneity in consumption patterns in the field.
  6. Multiple ignition points caused by vectors generate spatial heterogeneity in consumption patterns in the field.

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