Tuesday, November 10, 2009

Fire Ecology and Cutthroat Ecosystem Engineering, Part 2

The phrase ‘ecosystem engineer’ refers broadly to the ability of an organism to change or modify the physical characteristics of its surroundings. When these environmental modifications resultantly impact the fitness of the engineering organism itself, the feedbacks created can be thought of as functioning like an extended phenotype. In other words, the feedbacks generated between the engineer and the ecosystem contribute to the reproductive success of the organism, and often (directly or indirectly) affect the life history of nearby competitors. In the closing line of ‘Nature Red in Tooth and Flame - Part 1’ the organisms adapted to use fire are personified as ‘cutthroat’ because they possess a genetic compliment that facilitates the shaping of their environment through a two-fold process that could easily be categorized as self-interested. Firstly, through harnessing fire these engineers are able to create a pattern of ecological disturbance that promulgates increased fitness; and secondly, the application of fire eliminates resource pilfering opposition via direct incineration. But, prior to detailing the precise methods in which ecosystem engineering plants employ fire, it is important to set the stage with a description of the battlefield – the savanna community.

In the southeastern United States savannas are typically found on relatively low topographical gradients with poorly drained soils and ample soil nutrients. Similar in biological composition to hydric flatwoods communities, savannas characteristically differ in regards to tree abundance and exhibit a relatively open canopy with a thin understory and a lavish herbaceous groundcover. Both savannas and hydric flatwoods rely on seasonal rain and fire cycles in order to maintain their soil chemistry, floral diversity and faunal components. Yes, these communities depend on fire cycles…

Prior to modern anthropogenic intervention, and the suppression of natural, seasonally occurring fire cycles, the forests, prairies and savannas of the southeastern United States experienced regular ecological disturbance by means of fire. Using Florida as an example, the annual climate cycle here is punctuated by alternating periods of relatively dry and wet weather. More specifically, the months of November through February represent the dry season and accordingly receive comparatively little precipitation. This dry season is followed by dramatically increased amounts of precipitation during summer with heavy rains and thunderstorms (particularly near the coasts) for the period including June, July and August. The spring season, February-through-May represents a transitional period from dry to wet; however the forthcoming summer brings with it thunderstorms; during this period lightening-strikes often ignite wildfires. The wildfires feed on the parched condition of desiccated plants – the wildfires thrive on the fuels remaining behind from the departing dry season. The regularity of this annual climate has resulted in a cyclic ‘fire season’ that has been recurrent for several millennia. The persistence of the fire cycle has thus contributed greatly to the structuring of local natural communities; however to understand the organismal biology of some of the fire adapted plant species a deeper gaze into evolutionary time is required. So, now that a cursory look at the battlefield has been made, a review of the actual players is in order.

Two exemplars of the fire wielding and ecosystem engineering life style are the longleaf pine (Pinus palustris) and the southern slash pine (Pinus elliottii var. densa). These trees both maintain genetic compliments – fire genes – that enable them to prosper in the flame frequented savannas of the southeastern United States. In order to appreciate the natural history of these organisms, a look at their evolutionary past is obligatory.

The longleaf pine (Pinus palustris) and the slash pine (Pinus elliottii) are two 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.

NOTE: There are two distinct varieties of slash pine, variety elliotti and variety densa, both of which can be found in southeastern U.S. and although there are several important distinctions, for purposes here both varieties can be considered one and the same, though the southern slash pine (var. densa) displays slightly greater levels of adaptation to fire.

Pine and spruce trees are grouped together with cycads, gnetophytes and ginkgo as gymnosperms, which had an initial 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. This was the case because unlike the 21% atmospheric oxygen present today, the carboniferous boasted 35% oxygen content, this in conjunction with an abundance of herbaceous material resulted in frequent – and intense – wildfires. Here, ‘intensity’ can be interpreted as being the ratio of a wildfire’s maximum temperature and duration; both of which can vary. The wildfires positively selected for those plant traits that phenotypically exhibited fire-tolerance, and the wildfires also actively worked to eliminate those plants that displayed fire-intolerant characteristics. Through this dualistic mechanism of natural selection, a long passed Paleozoic ecosystem worked to shape and mold the longleaf and the slash pines into masters of pyrogenic manipulation.

[This blog post continues here with installment Number 3.]

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

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