4.2.6 Pyrophyta: Systema Naturæ per ignem regnis [77]

“The typical pattern of development varies from system to system, but it is the sharing of different states of successional development that maintains diversity in virtually any ecosystem. The natural forest becomes a tapestry of patches in different stages of succession, and hence a tapestry of diversity” Levin, 1999.

Biota exhibits wide-ranging evolutionary responses to fire. Collectively known as ‘pyrophytes’, some species selected morphological means of co-existence, while others chose biochemical, physiological, phenological, and/or behavioural. Pyrophytes are classed as either ‘passive’ or ‘active’, depending on whether their fire- defences are principally armoury, as is the case for the Cork oak (Quercus suber) or biochemical in nature, as for the California lilac (Ceanothus L.). In some instances, pyrophytes have synced their reproduction with the frequencies, intensities, and resultant severities of wildfire, and these are classed ‘pyrophiles’, of which Knobcone pine (Pinus attenuata) is one. However, whatsoever functional traits were selected, as is always the case in biology, a trade-off was involved. While many are the means by which plants may be classified, the foremost relevant with respect to this study [78], (sensu Vogl, 1974) assigns flora to one of five “modes of persistence” (Rowe, 1983, p.140): Invaders; Evaders; Avoiders; Resistors; and Endurers.

In order of the fire frequency with which they are principally affiliated, the functional groups are described below:

Endurers

Dubbed “phoenix species” (Ibidem, p.144), endurers regenerate upon the passage of fire. Their resilience resides in their perennating parts [rhizomes, roots and root crowns], which, excluding in the instance of ground fires, tend be protected by both humus [organic] and mineral soils (Ibidem). Survival is coupled to the vertical positioning of the belowground organs, the depth of which correlates with the proportions of the humus stratum (Ibidem). Ecoregions that incur low frequency, but high-severity fires will tend accumulate more biomass, therein deeper duff, of which a consequence is the “zone of maximum biological activity and nutrient release” (Ibidem, p.145) moving upward, with the perennating parts following suit. Given that mineral soils are relatively retardant to fire, but organic soils are significantly less so, a result thereof is that endurers may be all but eliminated from a site that experiences a high-severity fire. Thus, their functional traits primarily befit endurers to the low and mixed-severity fire regimes, and to a high rate of fire frequency. The exception is a subclass that Rowe describes as “superspecies” (Ibidem, p.145), which, though primarily surviving by means of resprouting, are nonetheless prevalent in ecoregions experiencing a range of fire frequencies. Some species, including many fire-adapted shrubs, have both the capacity for resprouting and postfire seedling recruitment. Termed ‘postfire facultative seeders’ (Keeley, 2012), we might posit these to be ‘superspecies’.

Evaders

Also referred to as “bankers” (Rowe, 1983, p. 142), evaders have evolved the ecological equivalent of an insurance policy. While the species within this functional group succumb to fire, they do so having stored seeds that rapidly germinate upon its passing. One variant store their seeds in the canopy, the other in organic and mineral soils. When stored aerially, serotinous [79] cones and fruits shield the seeds from the flames. When in the ground, the soil itself performs this task. In both instances, an abundance of seeds remain dormant until fire-related mechanisms release them from their protective casing (Ibidem). Their survival strategy harnessing the abundance of space and nutrients within post-fire landscapes, evaders are adept at enduring high- severity fires. Like endurers, evaders fall into two subclasses: one comprised semi- tolerant and shade-tolerant perennials [80], the other shade-intolerant ephemerals [81], the former subclass storing their seeds over prolonged periods of time, the latter in “one shot” deposits during “brief post-fire flowerings” (Ibidem, p. 142). Thus, the subclass populated by perennials is commonplace in ecoregions where fire frequency is low, and vice versa. While Rowe proposed evaders as being primarily adapted to short and intermediate fire cycles, subsequent studies evidence that the mosaic nature of mixed and high severity fires enables this group to co-exist with long fire cycles (Wallace and Christiansen, 2004). Furthermore, other studies assert that whereupon fire becomes especially frequent some perennials within this class have not sufficient time to accommodate the length of their reproductive cycle (Nijhuis, 2012).

Resisters

In keeping with Rowe’s analogies, in a nod to novelist Roderick Thorp’s character John McClane, we might dub resisters ‘die hards’. While juvenile plants have low resistance to fire, adults are protected by an array of pyro-armoury. Resisters defences include thick bark to protect against ground fires; self-pruning and peripheral foliage that help prevent against fire scaling from surface to canopy [fire ladders] (Rowe, 1983); and in some species, biochemical mechanisms, such as flammable oils in their exterior parts, which create the biological equivalent of a flashover [82]. Resisters resilience varies from one species to another, with those of the genus Pinus and Eucalyptus amongst the most [die] hardy. Factors that contribute to the heterogeneity in resilience limits amongst species include variations in the distribution of defences. For example, whereas some resisters select for defence against surface fires, others are adapted to withstand crown fires, the former’s pyro-armoury restricted to the base of its trunk (Jackson, Adams & Jackson, 1999), the latter’s protecting the plant to its crown (Pausas, 2015). Rowe suggested that resistors are principally affiliated to fire cycles of intermediate duration. This notwithstanding more recent studies have illustrated some species within this functional group are well adapted to relatively frequent fires (Ibidem).

Invaders

Their propagules dispersed by the wind, invaders are the “fugitives” (Rowe, 1983, p. 141) of the functional quintet. Prolific in their populous, invaders are amongst the first on the post-fire scene. Present in the aftermath of fires of any frequency and intensity, invaders may be found in quantities large or small in wide-ranging ecoregions (Ibidem). Generally shade-intolerant species, invaders tend flower and fruit in abundance in the period immediately following a fire, thereon dwindle as other functional groups re-establish. Thus, their populations pulse with perturbations, including both wildfires and other categories of natural hazards, including volcanic eruptions.

Avoiders

The antithesis of the above four functional classes, avoiders are the laggards of the ecological arena. Featuring no known evolutionary adaptions for co-existence with fire, avoiders are late successional species within landscapes where fire is infrequent. Mainly shade-tolerant mesophytes [83], most often, their propagules will only persist where invader, evader, resistor, and/or endurer species are well established, therein humus and aerial coverage have accumulated (Ibidem). Hence, while all flora persists in a state of symbiosis, avoiders tend do more so than do most others.

However, an interdisciplinary researcher working within and across the disciplines of botany, forestry, and landscape ecology, the creator of the above classification system, the late Stan Rowe, recognised that functional groups persist within an “open geographic system” (Ibidem, p. 147). Therein, in and of itself, autecological [84] information provides not a flawless means of modeling future ecosystemic states. Rowe also acknowledged the possibility that, in some instances, functional traits that are advantageous in fire-prone landscapes may, in the initial instance, have evolved in response to other environmental factors. Citing examples, Rowe referenced a study that posited that high concentrations of volatile substances in a species, which, under his system, fell into the resistor group, might primarily have evolved as a herbivory defense (Chapin and Van Cleve, 1981). He also highlighted how many species exhibit not one, but two or more stratagems for coexisting with fire, therein while assigned a structural-functional type, are akin to hybrids (Rowe, 1983). Nonetheless, Rowe’s ‘modes of persistence’ provides a scientifically plausible, coherent and elegant framework to use when examining the nature of the relationships at play within fire- prone landscapes. While all five of the functional types will be referenced in later chapters, those of primary interest to this study are the three that feature fire-response mechanisms: endurers, evaders, and resisters, the former two of which may more generally be described as fire-tolerant, the latter fire-resistant, and species with neither properties, as fire-intolerant.

>Continue to Chapter 4.2.7 here.

Footnotes

[77] Systema Naturæ per ignem regnis [Nature System by means of the three kingdoms of fire] is a reference to the title of the 10th edition of Caroli Linnæi’s publication of Systema Naturæ, which published in 1758, had the extended title Systema Naturæ per regna tria naturæ, Secundum Classes, Ordines, Genera, Species, Cum Characteribus, differentiis, synonymis, locis.

[78] Ecologist Richard Vogl thought the concept of successional stages of limited in fire ecology. He considered it more useful to conceive of species through the lens of a 5-part functional group comprised increasers, decreasers, neutrals, invaders, or retreaters (Vogl, 1974).

[79] Serotiny refers to a functional trait, which found in some flora species, enables the retention of seeds in resistant structures, including cones and pods, whereupon dispersal follows a major environmental disturbance (Lincoln, Boxshall and Clark, 2003).

[80] Perennial refers to plants of which the lifespan exceeds 24 months. 81 Ephemerals may be described as flora that lives fleetingly.

[82] Occurring at temperatures of 500 °C and above, flashover refers to the near-simultaneous ignition of organic matter within an enclosed area.

[83] Mesophytes are plants adapted to environments where precipitation is not scarce or excessive.

[84] Autecology is the study of how individual organisms or single species interact with both living and nonliving entities within their environment.

The thesis is also available in PDF format, downloadable in several parts on Academia and Researchgate.

Note that figures have been removed from the digital version hosted on this site, but are included in the PDFs available at the links above.

Citation: Sterry, M. L., (2018) Panarchistic Architecture: Building Wildland-Urban Interface Resilience to Wildfire through Design Thinking, Practice and Building Codes Modelled on Ecological Systems Theory. PhD Thesis, Advanced Virtual and Technological Architecture Research [AVATAR] group, University of Greenwich, London.