Thursday, November 12, 2009

Fire Ecology Marathon; Nature Red in Tooth and Flame Part-4

The savannas of the southeastern United States are inimitable natural communities that have undergone ecological assembly in the presence of seasonal fire cycles and, as discussed during the first three installments on this topic (available here; Part-1, Part-2, Part-3), are rich in organisms capable of manipulating the regularity, movement and intensity of these wildfires. During the preceding post (Part-3) the phenotypes of two such fire-born species, the longleaf and slash pines, were detailed as exemplars of organisms with traits that not only aid in defending against heat and flame, but also as species that exhibit specific physical structures, chemicals and behaviors that could intrinsically promote fire. In closing that previous discussion, consideration was given to the possible motives behind the longleaf and slash pine’s ability to deliberately provoke fire.

Though it may initially seem to be counterproductive or even a hindrance to survival, through promoting fires the savanna pines obtain benefits that directly enhance their inclusive fitness. Because of the processes that drove the organismal evolution of the longleaf and southern slash pines in geological time, and the processes that propelled community assembly in savannas, the presence of wildfires effectively created a duality in the character of potential pine competitors and that of would-be savanna inhabitants - either they can tolerate fire, or they can’t tolerate fire.

In the absence of wildfires over extended periods of time (i.e. fire suppression) several ecological changes can occur in savannas. Most profoundly, without regular wildfires not only would the already present fire-tolerant plant species survive, but in addition, fire-intolerant species would experience greater fecundity. Without the deterrence provided by fire, resource-rich savannas can quickly become the envy of plants from surrounding hammocks and mixed hardwood forests, thus encouraging invasion and recruitment from these neighboring communities. Such movement of new species into the savannas would contribute to substantial ecological alteration of the natural processes that maintain the system’s predictable boundaries, ecotones and makeup.

Recall from Part-1 of this post that the plants found in hammocks have undergone selection for initial rapid growth and direct competition for sunlight. Just as the natural history of the pines has been shaped by fire, the history of dense-canopy species have evolved to fight for radiance. If unobstructed access to the abundant savanna sun is tantalizingly flaunted, these species would quickly invade, rapidly recruit and hurriedly regenerate to overtake all biologically available space. What was initially a patchwork of invasive species would spread to encompass and overcrowd the savanna, in the process reducing the diversity of appropriate groundcover plants, and adversely impacting the reproductive success of the native inhabitants – slash and longleaf fitness would decline.

In addition to increasing interspecific competition in the savannas, invasive species also create positive feedbacks in the wildfire cycle - magnifying fire suppression. The presence of abundant shrubs and woody species in a normally open savanna formulate densely vegetated landscapes that reduce fine fuel loads on the ground and decrease the likelihood of fire propagation. The lack of fire - in turn - facilitates further invasions, which increases vegetative densities even more, which reduces fire even more, which allows for yet greater invasive proliferation, etcetera…

With continued fire suppression, what was once a savanna, characterized by thinly distributed trees, would transition towards a densely canopied hammock with an impenetrable thicket understory. Growing populations of invasive species would amplify competition for resources, thus pushing the fitness experienced by the longleaf and southern slash pines to dangerously low values. This is precisely why the ‘fire gene’ is so critically important to the pine’s genotype. As crowding increases in this scenario, and essential resources dwindle, hormonal stress responses within the pines intensify. The hormones drive physiological changes in the trees causing leaves to drop and internal hydrocarbon chemistry to move toward increased combustibility. The probability for fire is increased. And, when fire does return, the stems, branches, leaves and roots from newly arrived invasives will serve as kindling for augmented wildfire intensity - to such extremes that only the hardiest of the fire-tolerant will be able to survive.

For clarification, conceptual genes (like the ‘fire gene’) aren’t confirmed as actual chromosomal localities for which variable alleles compete. Rather, conceptual genes are offered as thought-tools for understanding the premise that natural selection operates on phenotypical traits that are the products of genotypical coding. In regards to the ‘fire gene’ specifically, it is a hypothetical genetic compliment that is expressed in such a manner that the physical presence of fire improves the likelihood of that genotype being passed on to future generations. In other words, if a population of trees exist in which some members have a genotype that provides increased fitness in the presence of fire, AND that population is then exposed to fire - ultimately killing a certain percentage of the population - those trees with fire gene advantage will experience higher survivability and greater measures of fecundity compared to those not possessing a fire gene.

Returning to the savanna pines expressly, irregardless or not if there is literally a single gene that provides for all of the phenotypical adaptations to fire described throughout this post, or if these traits are the result of a cooperative epistasis, or if the characters are disparate and independent, it remains likely that their occurrence and continued propagation through evolutionary time has provided a significant advantage.

Through 300 million years of natural selection, wildfires have propelled the savanna defending pines to levels of adaptation in which they are capable of wielding fire. ‘Nature, red in tooth and flame’ has fashioned a true ecosystem engineer, one that is capable of establishing and defending the ecotonal boundaries between natural communities.

Beckage, B., Platt, W., & Gross, L. (2009). Vegetation, Fire, and Feedbacks: A Disturbance‐Mediated Model of Savannas The American Naturalist, 174 (6), 805-818 DOI: 10.1086/648458

Stevens, J., & Beckage, B. (2009). Fire feedbacks facilitate invasion of pine savannas by Brazilian pepper New Phytologist, 184 (2), 365-375 DOI: 10.1111/j.1469-8137.2009.02965.x


  1. You explained nicely a fire environment and some of the fire-affecting behavior of the pines. I didn't notice a description of the fire-suppresing "gene" of invaders which you mentioned in an earlier part. You mentioned a little about prolific invaders overwhelming some fire by their numbers, but little specific.

    You also mentioned that a stressed pine can switch from fire suppression to hydrocarbon-fed promotion. Is that a reference to the turpentine production which Europeans industrialized?

  2. SEWilco,

    Thanks for the comment.

    And thanks for pointing out my shortfalls… (lol)

    You are correct in calling me out for the poor description of the fire suppressing ‘genes.’ It was my original intent to discuss the “Fire feedbacks facilitate invasion of pine savannas by Brazilian pepper” paper in greater detail; however the series as a whole was getting pretty lengthy, and I figured that if it got much longer no one would bother to read it. Honestly, I’m quite surprised that you read the whole thing – thanks! I applaud your tolerance for the monotonous abuse of grammar.

    The Brazilian pepper paper (cited in the post above) discussed experiments in which the plant was shown to lower fire intensities via a reduction in the maximum achieved temperature. Essentially, once the invasive plant reached a certain abundance threshold (that threshold being 2 – 6 plants per 5 meter radius), its presence affectively lowered the temperature of the wildfire by 200-degrees centigrade. The result of this drop in temperature caused the normal fire-induced mortality of the plant to plummet by as much as 80%. With this in mind, my original idea was to describe how the physiology of Brazilian pepper reduced fire in an analogous way to the pine’s increased fire. But as I said, the work was getting a bit lengthy…

    You’re right on the turpentine reference. Turpentine is manufactured from the distilling and processing of chemicals called ‘terpenes’ that are produced naturally by the pine trees in the post – the commercial collection of these chemicals has a long history here in Florida. The natural production of the hydrocarbon based oil varies with season, however when stress hormones accumulate in the trees (as a result of fire suppression) the chemicals can become more abundant.

    Thanks for taking the time to muscle through the write-up; your comments are sincerely appreciated.

  3. It's better to have too much to write about than too little, and you did back link them so it was easy to start at the beginning.

    Brazilian pepper? Hmm. Shrub/tree, across southern Brazil. That's interesting. I'm only aware of terra preta and pre-Columbian earthworks to the north of the Brazilian pepper's domain. I wonder whether the native fire users ever tried to move into the pepper's area. There are also two similar plants which extend north to Mexico, but the scholar was only addressing the single plant.

    On the other hand, making terra preta requires slow charring. The pepper's temperature-lowering behavior would not hinder human use of it in terra preta.

  4. I wonder whether the pepper tree suppresses fire due to its structure, roots, moisture, or chemicals in the wood. If the action works in the wood, then putting the plant in a fire might be interesting.

    I wonder if terra preta scholars who tried to duplicate the charring process tested if Brazilian pepper wood (green or dry?) chars better than other woods. They might have tried random woods without considering whether the farmers might have chosen specific woods for the process. Charring is considered to be labor-intensive due to needing to manage the fire so it burns slowly -- but maybe some woods tend to char rather than burn.

  5. SEWilco,

    Brazilian pepper is an invasive here in Florida, and it’s really modifying natural fire regimes. You raise good questions about the actual physical/physiological mechanisms of fire reduction – I’ll need to check into that…

    I have little doubt that terra preta must have been labor intensive, though admittedly I know very little about the topic. If you’re familiar with its history/process, I’d love to have a short description of it to post here?