Saturday, April 18, 2009

The Fire Gene: One Organism’s Ability to Exploit Fire

Gazing across the tranquil landscape of the Big Cypress Preserve, nature seems to be in balance, unchanging and at peace - picturesque beyond any poetic description. Here, anthropogenic throngs of sharply angled concrete and glass edifices suspend their battle for roadside dominance and yield themselves to a sea of sparsely treed savanna, rolling prairies of grass, and randomly scattered islands of thickly vegetated hammocks; the perfect environment for a relaxing stroll, a picnic, or even a quick nap. All may appear calm within this enchanting panorama; however, the perceived tranquility is but a chimera. A mere illusion of serenity resulting from shortfalls in the ability of Homo sapiens’ photoreceptors to see beyond the narrow range of the electromagnetic spectrum called visible light, an inability to hear sound outside of 22000 Hertz, and the failure of the human olfactory system to nose its way into the vast chemo-landscape of pheromones and other volatile chemicals in which it is continuously assailed. If the sensory apparatus of Homo sapiens was keener - more finely calibrated – the landscape of the Big Cypress would appear very different.

Very different indeed, imagine the ecological interplay that could be interpreted if humans could see ultraviolet light through the eyes of a bee, smell pheromones from six-miles’ distance like a moth, or interpret chemical stimuli through soil like a plant… Far from serene, if viewed through time, adaptive maneuvers, survival strategies and arms races would be manifest in every action undertaken by the immense diversity of organisms on Earth. If these actions could be viewed more directly, the landscape would appear saturated with war. Even the plant community boundaries which demarcate prairie from savanna from hammock in the above described landscape are maintained by way of fierce battles waged over evolutionary time. These ecosystems, which appear stable and so pleasingly haphazardly scattered, are in fact tightly ordered armies of competing plants struggling for resources and existence. In these recurrent ecotonal conflicts one species has honed a new weapon – it has adapted to exploit the power of fire.

Naturally existing plant communities exist in a continuum of ecosystems which through evolution have adapted to almost every available habitat on the planet; from “box thorns” (Lycium pallidum) in Death Valley to fully aquatic hyacinths (Eichhornia paniculata) floating around the lakes of Brazil, genetic plasticity in plants is the product of natural selection. Although diverse habitats represent a surmountable challenge, a multitude of both biotic and abiotic factors determine the overall abundance (density), composition (diversity) and ultimate success of plant communities at any given location.

For example, looking across the landscape of the Big Cypress, densely concentrated hardwood trees form hammocks which, due to the broad area of their collective canopies, limit the amount of sunlight available to underlying herbaceous groundcover. This is a straight forward relationship - no sun reaching the ground means fewer plants on the ground. Following this rationale, if the tree canopy should be opened (by a storm, hurricane or by the death of older trees) and sunlight is able to temporarily penetrate to the floor, a rapid emergence of both herbaceous plants and new saplings would be predicted. This is precisely what happens; sunlight is the limiting resource, once made available, those plants best able to take advantage of the situation (through rapid growth) will be able to literally overshadow their competitors; plants with genetic compliments favoring a period of “initial rapid growth” are at an advantage and will be positively selected.

Extrapolating this scenario to the prairies of Big Cypress begs the question – why aren’t there any trees in the prairie?

Prairies, typically found on relatively low topographical gradients in Florida, have an abundance of soil nutrients and water; at least enough to support the enormous quantity of grasses and herbaceous plants currently found there. Additionally, from the perspective of a tree, grasses present little competition for sunlight. So, what is it that prevents trees from invading the prairie?

One often suggested possibility is that because prairies are occupied by dense populations of grasses - some of which more than six feet in height - young trees are prevented from taking hold; sort of like a reverse hammock scenario in which the grasses overshadow the young trees thereby starving them of sunlight instead of vice versa… This is plausible, but why wouldn’t trees take hold after wildfires? Wildfires have been historically inevitable in Florida and have the effect of clearing grasses long enough for those trees possessing an “initial rapid growth gene” in their arsenals to stake a claim.

Slash Pine (Pinus elliotti), for example, has just such an initial rapid growth gene. This permits the tree to take advantage of any opportunity to seize real estate, whether it is in a forest or a prairie. Slash Pine is even capable of expressing secondary needles in less than six month’s time – seedlings grow rapidly. Once present, this conifer could easily out-compete grasses for sunlight.

A quick word about wildfires: Florida’s climate cycle is punctuated by alternating dry and wet periods. November to February is the dry season, with relatively little precipitation, and is followed by heavy rains and thunderstorms (particularly near the coasts) during the months of June, July and August. The spring season, February through May, represents a transition from dry to wet, but during this period lightening strikes often cause wildfires due to the parched conditions of plants – parched, having just come out of the dry season. Regularity of climate has resulted in a cyclic “fire season” arriving during the early spring.

Speaking of wildfires… Being a regular occurrence, they are often offered as another explanation for limiting the advance of trees into prairies. Although this suggestion is partially correct, it isn’t the whole story - in some instances trees have even wielded fire as a weapon to destroy its grass competitors.

4-19-09 UPDATE: The Second Half of this Post can be Viewed HERE.

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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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