Fire enjoying fresh oak litter during the perfect December burn window. Credit: Lenya Quinn-Davidson, Fire Adapted Communities Learning Network

Topic: Fuels treatment / Prescribed fire Type: Research Synthesis

Evolving with Fire: Understanding Flammability and Rethinking Burn Windows [Science Thursday]

Authors: Lenya Quinn-Davidson

When was the last time you changed your mind about something important — something that you thought you knew?

This question was posed at a conference I attended recently, and it has lingered with me ever since. I’ve been mulling it over, coming up with my own personal examples, and I’ve been asking friends and colleagues to ponder it, too. I love the question because it’s asking us to consider how open-minded we really are. And in this era of political and social divisiveness, where it’s so easy to become siloed, I think it’s more important than ever to open ourselves to new ways of thinking and doing — even around topics that we know well.

I had a revelation last year that seems, in hindsight, embarrassingly obvious. In fact, it concerns something that I’ve thought and read about for almost a decade, and a topic on which my husband and several of my close friends are published authors: litter flammability. No, not litter on the side of the road (though I have actually considered writing a blog on burn barrels!) — I’m talking about the flammability of leaf litter, and the traits that various plants have evolved to either facilitate or discourage fire.

A few years ago, my husband and our colleague Morgan Varner published a paper on the role of leaf traits in the flammability of California oaks (Engber and Varner 2012; PDF, 505 KB). For that study, they collected samples and conducted lab burning of litter from 18 different species of oaks, including deciduous and evergreen oaks and both tree- and shrub-form species. They found that leaf size accounted for most of the variability in the flammability of oak litter, and that California black oak and Oregon white oak — two deciduous species with large, lobed leaves — were the most flammable of them all. This finding corroborated what those of us who burn in these systems know to be true: the big, lofty litter under black and white oaks burns really well, whereas only the hottest fires will burn through live oak litter, which typically consists of dense mats of small, thick leaves. Other studies have done similar comparative analyses of tree species in various regions of the U.S., offering important insight on the relationships of these adaptive leaf traits to woody encroachment, mesophication and fire management (Kane et al. 2008, Kreye et al. 2013, Mola et al. 2014, Varner et al. 2015). Through this lens, we can see that fuels are not random — plants shape fire regimes, but fire also shapes plants.

More recent literature has delved even deeper into the topic of flammability, urging us to move away from a focus on individual traits and toward a more holistic, multiscalar view. Last year, Juli Pausas, Jon Keeley and Dylan Schwilk published Flammability as an Ecological and Evolutionary Driver, which proposed a novel framework for thinking about flammability (2017, PDF, 1.75 MB). In that paper, they describe three “flammability strategies” for plants that grow in fire-prone ecosystems: the non-flammable, fast-flammable and hot-flammable, which are defined by different degrees of ignitability, heat release and rates of fire spread (see Table 1).

A table showing the relationship between flammability strategies (ranging from non-flammable to fast-flammable to hot-flammable) and fuel/plant characteristics

Credit: Pausas et al. 2017

Non-flammable plants have special traits that allow them to persist in fire-prone environments, such as thick bark, plant architecture that prevents ignition, leaf structure that provides a dampening effect when it piles up at the base of the plant, and high moisture contents. The authors give the example of some succulents that live in fire-prone systems but successfully avoid burning. Fast-flammable plants are those with traits that promote quick, frequent burning, which then gives the plant a competitive edge over its neighbors. These plants produce fuels that ignite easily and burn quickly, fueling fires that spread rapidly but have relatively low heat release (i.e., are of low intensity). Deciduous oaks and long-needle pines are good examples of fast-flammable plants. Hot-flammable plants are those that burn hot and for which individual survival is sacrificed in the name of reproduction. These plants tend to be effective post-fire seeders and/or resprouters, and naturally include serotinous species, whose cones require high-intensity fire to open and release seed.

Intellectually, I find the flammability topic compelling. It’s neat to think about how fire not only shapes the current distribution and composition of plant communities but that it may also be the driving force behind the evolutionary structure of the plants themselves. How cool is that?! But from a management perspective, it can sometimes be hard to understand how these slightly esoteric concepts translate to our work on the ground.

And that’s where my revelation comes in. For years, I’ve been burning in California black oak and Oregon white oak habitats here in northern California. The bulk of my prescribed fire experience is in these systems, where we burn to reduce competition from the native invasive Douglas-fir, a fast-growing conifer that is encroaching on oak woodlands at alarming rates. And until last year, every burn I had ever done in an oak woodland was in or around October, when we can take advantage of dry grass from our Mediterranean summers and hope to kill as many small firs as possible before the rains come in. Everyone I know who burns in oak woodlands burns during this window, including my husband and his crew at Redwood National Park, who probably have the most robust oak woodland burn program in the state. That window is the norm. But guess what? In October, our deciduous oaks still have their leaves! Which means we — the very people who have nerded out on leaf flammability for the last decade — aren’t taking advantage of the leaf litter that those trees have evolved to provide! In fact, it’s not uncommon for these October burns to blaze beautifully through the open grass, only to peter out under the oaks.

In December, I worked with some local ranchers and did my first winter burn in a white oak woodland, and I finally got to see those fast-flammable leaves in their full glory. A hard freeze the night before sucked the moisture from the litter and one-hour fuels, and without their leaves in the canopy, the trees let in plenty of sun to dry the winter dew. The fresh, fluffy litter was crunchy by midmorning. The forest adjacent to the unit was wet from fall rains, and the surrounding grasslands were imperviously green from the first flush of winter grass. The fire moved quickly through the woodland, roasting every last conifer seedling and posing little threat of slop-over, escape or overstory damage. It was the perfect burn window, and one that came again in short spurts over the next two months. And my friends at Redwood National Park were green with envy, because their burn plans don’t allow burning after December 31, and most of their crews are unavailable after fire season ends in late October.

Burned fir seedlings with a backdrop of a smokey oak tree stand

Roasted fir seedlings in an oak woodland burn unit completed last December. Credit: Lenya Quinn-Davidson, Fire Adapted Communities Learning Network

Dead fir seedlings with oak trees in the background

Dead firs under deciduous oaks, in an area that was burned by a Humboldt County rancher this February. Credit: Lenya Quinn-Davidson, Fire Adapted Communities Learning Network

So that’s my example of the last time I changed my mind about something important — something I thought I knew well. The information had been in front of me the whole time, but it took me opening my mind to let the information in, to think outside the social norm. And this has me wondering: what else am I missing? What other enlightenment — social, political, scientific — lies in wait for me to evolve my thinking? And it has me asking everyone I know, now including you: what’s your example?


Engber, E. A., and Varner III, J. M. (2012). Patterns of Flammability of the California Oaks: The Role of Leaf Traits. Canadian Journal of Forest Research42(11), 1965-1975.

Kane, J. M., Varner, J. M. and Hiers, J. K. (2008). The Burning Characteristics of Southeastern Oaks: Discriminating Fire Facilitators from Fire Impeders. Forest Ecology and Management256(12), 2039-2045.

Kreye, J. K., Varner, J. M., Hiers, J. K. and Mola, J. (2013). Toward a Mechanism for Eastern North American Forest Mesophication: Differential Litter Drying Across 17 Species. Ecological Applications23(8), 1976-1986.

Mola, J. M., Varner, J. M., Jules, E. S. and Spector, T. (2014). Altered Community Flammability in Florida’s Apalachicola Ravines and Implications for the Persistence of the Endangered Conifer Torreya Taxifolia. PloS One9(8), 103933.

Pausas, J. G., Keeley, J. E., and Schwilk, D. W. (2017). Flammability as an Ecological and Evolutionary Driver. Journal of Ecology105(2), 289-297.

Varner, J. M., Kane, J. M., Kreye, J. K., and Engber, E. (2015). The Flammability of Forest and Woodland Litter: A Synthesis. Current Forestry Reports1(2), 91-99.


Want to repost this story? Please contact our communications manager.

3 thoughts on “Evolving with Fire: Understanding Flammability and Rethinking Burn Windows [Science Thursday]”

  1. Outstanding thoughtful story, thank you. It read as a real life lesson, applicable to all that we do and think about (and feel confused about but know there is some kind of a resolution).

  2. Eamon Engber says:

    Honing in on all the potential burn windows in our oak woodlands is a good thing. Questions remain related to post-rainfall leaf litter drying times, particularly in the more coastal oak woodlands. Often I’ve observed matted wet leaves following rainfall events, especially on northwest aspects, that don’t seem to dry well. However, south aspects on upper slopes would probably have a longer winter window, in addition to the typical fall window.

  3. Johnnymac says:

    Well, after preparing hundreds of burn units for that “October” burn window in oak woodlands with the objective of killing Douglas fir and only occasionally meeting those objectives, I am very open to the insight Lenya presents. I say let’s try to replicate that same prescription again and if successful, I buy the first round. Thanks for a great (outside the box” boost.

Leave a Reply

Per our policy comments are manually approved by a website administrator and may take some time to appear. Your email address will not be published when you comment.

Required fields are marked with an asterisk.