Fire vs. Plants: Some Plants Win!

“The fundamental chemistry of combustion lies at the core of the living world. When it happens within a cell it's called respiration. When it happens outside organisms, it's called fire.” (Stephen Pyne, NOVA)

Plants cannot move when threatened by fire. So how do they survive the intense heat and maintain the ability to reproduce? Their defense strategies fall into three main categories: 1) Resistance: the above ground parts of a plant protect it (as in the thick bark of eucalyptus and cork oak trees).  2) Recovery: plants evade permanent destruction by sprouting after the fire recedes. 3) Recruitment: plants evolve so that their seeds actually need heat, smoke, and seed scarification in order to germinate.

In one example of resistance, ponderosa pines shed their lower branches as they mature. This strategy prevents lower branches from becoming ladder fuel, thereby protecting the branches and needles higher up on the trees.  Most mature trees have fire-resistant bark from two to four inches thick which insulates the inner tissues from heat (similar to the way Nomex or Kevlar suits protect firefighters). It might seem counterintuitive, but South African aloes and Australian grass trees retain dense dead leaves around their stems; the dead matter acts as insulation against the heat.

Plants recover from fire in an astounding variety of ways, as you already know if you have ever tried to remove a tree and later been faced by a yard full of returning suckers. Buds that lie dormant deep under the bark will develop when exposed to light and air. Even if all the stems of a eucalyptus burn, these deep-seated (“epicormic”) buds are still able to germinate, so the trees are able to regrow their canopies quickly. Scrub oaks and chamise plants resprout from their roots.  Many plants and trees that have evolved to thrive in fire-prone areas have woody growth (lignotubers) at or below ground level.  These contain dormant buds and starchy nutrients to fuel the growth of new basal shoots. Similarly, in a phenomenon known as clonal spread, the loss of above ground stems triggers regeneration. California redwoods and aspens can send up suckers far from the burned mother tree. In Utah an aspen colony 40,000 strong has developed from one quaking aspen.

In recruitment strategies, seed dispersal is triggered by fire instead of by natural maturation. This process is known as serotiny. An example of serotiny is the Jack pine: In the heat of a fire, immature pine cones open and release small, brown, hard-shelled seeds. You can observe this phenomenon by tossing a young pine cone into a campfire and watching as it opens and releases its seeds. Conifer survival is enhanced with a higher germination rate than other plants due to thermal scarification (the breaking or softening of the seed shell, which facilitates germination). 

Many other plants that have evolved in fire-prone landscapes actually need fire in order to propagate. Heath can wait up to two years after a fire to germinate. The germination of yellow rock rose is triggered by smoke. In experiments the germination rate of sugar bush increased from one in ten seeds to nine in ten after being submerged in boiling water while kept dry in waterproof bags. Lodgepole pine depends on fire to melt the resin that surrounds its seeds so they can be released.  The acacia tree “recruits” ants to carry its resinous seeds underground where they can then germinate after a fire. 

Another form of recruitment is pyrogenic flowering, found in plants that have adapted to periodic fires by increasing their rate of flowering after a fire event. In this situation, fire actually facilitates and accelerates flowering and seed development.  Plants and new sprouts get an extra benefit because fire creates an environment that has increased light due to decreased tree canopy; soil amendments and nutrients from ash; and greater opportunity for birds, bees, and winds to carry seeds to bare earth.  Fireweed and the grass tree have particularly impressive floral displays after a fire.

In our area, and in chapparal and forest ecosystems in general, landscapes have received periodic fire for thousands of years, and the plants that thrive here have evolved to withstand or even rely on it. But fire is disastrous if your home is located in the path of one, as many of us have experienced.  Regular prescribed or controlled burning at the proper time of year can control the size and intensity of future unplanned fires as well as result in greater germination rates.

We humans can help resist disastrous fire results on our own properties by choosing more fire-resistant species and maintaining proper defensible space around buildings. Well-watered plants are less flammable. Stress increases risk, so plants that are stressed from drought or poor soil can become more vulnerable to fire due to stunted growth and the buildup of dead material. Enriching the soil with compost and watering regularly can increase fire resistance, which depends on leaf moisture and the chemical content of sap, among other factors. As an example, fig trees hold lots of moisture in their large juicy leaves. Planting trees like figs can help deter fire. Deciduous trees like maple, poplar and cherry are generally less flammable than conifers.

In many ways, plants have developed strategies to live with and even win over fires! Overcoming the disadvantage of not being able to run away, plants have evolved ways to use fire to survive, multiply, and thrive. Hopefully humans will do so as well.

For more information on this topic, see the following references, used in writing this article:

NOVA Online | Fire Wars | How Plants Use Fire | PBS

Andrew C. Scott, et al, Fire on Earth: An Introduction (2014).

Improving California's Forest Health | Our Strategies | CAL FIRE

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