Sunday, April 19, 2009

The Fire Gene Described

A lengthy introduction during the initial post on this topic (available Here) contrasted a harmonious view of nature with the perspective of nature as a series of oppositional organisms struggling to gain a competitive edge over rivals. As a model of this outlook, the ecotone boundaries between various sets of differing plant communities were offered as a case study. More specifically, the prairie, savanna and hardwood hammock ecosystems of the Big Cypress Preserve were forwarded along with the proposition that members of these communities actively challenged each other for limited resources. In staging this proposition the question was asked, “Why don’t trees invade - and take over – the prairie communities currently occupied by grasses?” After eliminating the likelihood that densely growing stands of grass crowded-out young saplings by denying them access to sunlight, cyclic wildfire were explained and presented as an alternative explanation. Moving forward with this production, a profile of one of the previously introduced characters is in order – the conifer tree Pinus elliotti.

Slash Pine Growing from a Log

Slash Pine is a species of the genus Pinus (pine tree) which branched from genus Picea (spruce tree) during the Cretaceous Period, somewhere between 87 and 193 million years ago. There are two distinct varieties of slash pine, variety elliotti and variety densa, although there are several important distinctions, for purposes here both varieties can be considered one and the same. Pine and spruce trees are grouped together with cycads, gnetophytes and ginkgo as gymnosperms, which had a start back in the Pennsylvanian Period of the Carboniferous more than 300 million years ago. The long history of the pine trees, and the slash pine in particular, is significant because these trees have one of the largest and most complex genomes of any organism on the planet today – a result of varied evolutionary forces. Of specific interest in regards to evolutionary history is that gymnosperms arose from the Carboniferous swamps during a period of rapid plant adaptation. In addition to the advent of the bark fiber “lignin,” plants during that period underwent a multitude of morphological changes - many of these changes were adaptations to wildfire. Unlike the 21% atmospheric oxygen present today, the carboniferous boasted 35% oxygen content, this in conjunction with an abundance of herbaceous material (remember Carboniferous = “coal age”) resulted in frequent – and intense – wildfires.


So, do wildfires prevent trees, such as slash pine, from invading prairie strongholds held by grasses? Not really, some especially intense (“intensity” being a measure of a fire’s maximum temperature and duration) wildfires may destroy slash pine, but fires capable of doing so are relatively rare. The typical “fire seasons,” as described in the first post, may have sufficient intensity to kill some young saplings, but remember - slash pines also have “initial rapid growth genes” which provide a solid head start in defending themselves. Essentially, any sapling greater than two years old has a good chance of getting through the “average” wildfire. As for the periodic “non-average” wildfire, one that is of an unusually high intensity, slash pines may need to rely on evolutionary adaptations other than “initial rapid growth genes” – they may need to lean on morphological phenotypes resulting from a “fire gene.”

A fire gene is a genetic compliment possessed by an organism that is expressed in such a manner that the presence of fire improves the likelihood of that genotype being passed on to future generations. In other words, if a population of trees exists in which some members have a genotype that provides phenotypical resistance to fire, and that population is then exposed to fire, killing a certain percentage of the population, those trees with fire gene advantage will have higher survivability and greater measures of fitness than will those not possessing a fire gene. Through this process of “selection by fire,” the fire gene would become more prevalent in the population, eventually becoming so common as to be called characteristic.

This is precisely what has occurred with the pine trees of the Big Cypress. Through millennia of “trial by fire,” only those trees expressing the most fire tolerant phenotypes have survived. Morphological features such as thickly armored plates of bark shield the trunk from heat, scale plated meristems guard against flames and the pine’s reproductive strategies take into account spring fires by germinating in the fall and producing periodic mast crops. However, these products of natural selection are merely defenses, what is truly remarkable is that another aspect of the fire gene contributes to offensive maneuvers.

As a thought experiment only, image being a tree with the cognitive function of a human and the knowledge that you have an inherit resistance to fire; a resistance that many of your competitors do not posses. If locked in a battle for survival, and you had a match in hand, (or rather, a match in “branch”) would you start a fire?

Of course, matches are of little use to trees outside of thought experiments, but what if there was an adaptation that would provide not only defense, but also allow trees to harness naturally occurring fires to their advantage? Genes don’t exist in isolation; frequently they form partnerships to gain mutual advantage. Epistasis, the interaction between genes, has occurred in pine trees to accomplish the same goal. Not only do the trees have defensive morphologies, they have also adapted the chemistry of their leaves (i.e. pine needles) such that while on the tree the leaves produce flame resistant chemicals, but when wildfires are absent for extended periods of time leaf chemistry changes. In the absence of wildfires leaves are randomly shed, accumulate in the area around the tree and - as opposed to being flame retardant - they become easily ignited at low temperatures and burn at an intensity that, well… An intensity that only a slash pine would love…

Some fires do adversely affect slash pine, but the presence of a “fire gene” provides both defensive and offensive adaptations that can –and have been – utilized to survive. So, why don’t trees such as slash pines invade prairies? It’s a “one-two punch.” Through heat stressing the trees, fires slow down advancing slash pines; however it is what happens after the fire season that stops them cold in their tracks – flooding. Summer rains pile on additional stress to what has already accumulated due to fire defense investment. Grasses are in the same boat, but due to a better water tolerance they can bounce back more readily. The slash pine can survive fires or flood, but taken together these two modes of environmental disturbance overwhelm the trees and limit their prairie-ward charge. This is, however, a function of seasonality, climate and cyclic wildfires; with climate change and alteration of these natural processes all bets are off. (But that’s a topic for another time…)

Beckage, B., Gross, L., & Platt, W. (2006). Modelling responses of pine savannas to climate change and large-scale disturbance Applied Vegetation Science, 9 (1) DOI: 10.1658/1402-2001(2006)9[75:MROPST]2.0.CO;2

Nordlund, D., & Lewis, W. (1976). Terminology of chemical releasing stimuli in intraspecific and interspecific interactions Journal of Chemical Ecology, 2 (2), 211-220 DOI: 10.1007/BF00987744

Morse, A., Peterson, D., Islam-Faridi, M., Smith, K., Magbanua, Z., Garcia, S., Kubisiak, T., Amerson, H., Carlson, J., Nelson, C., & Davis, J. (2009). Evolution of Genome Size and Complexity in Pinus PLoS ONE, 4 (2) DOI: 10.1371/journal.pone.0004332

Platt, W. J., J. M. Huffman, M. G. Slocum, and B. Beckage. In press. Fire regimes and trees in Florida dry prairie landscapes. In: Noss, R. & Singh, S. (eds.) Land of fire and water: The Florida dry prairie ecosystem. Avon Park Air Force Range and Department of Defense, Avon Park, FL,

US.Kabrick, John M.; Dey, Daniel C.; Gwaze, David, eds. Shortleaf pine restoration and ecology in the Ozarks: proceedings of a symposium; 2006 November 7-9; Springfield, MO. Gen. Tech. Rep. NRS-P-15. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station: 28-32.

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