4.2.4 Fire Regime Biotic Schemata

The three quantitatively and qualitatively distinct fire regime classes of low-severity, mixed-severity, and high-severity exhibit intrinsic behaviours that are shaped by energetic constraints (Archibald et al, 2013) [Fig. 24]. Fundamentally, fire regimes express a dichotomy between ecoregions dominated by woody vegetation, and those dominated by graminoids. The latter taxon, which is populated by an estimated 12,000 species worldwide (Christenhusz and Byng, 2016), is comprised a mix of annual and herbaceous perennial grasses (OST, 2017) of which the physiology enables coexistence with fires as frequent as every 1-3 years (Archibald, 2013). In contrast, ecoregions dominated by woody species, such as members of the genus Pinus, are physiologically adapted to fires of frequencies ranging from decades to centuries.

Analysis of datum documenting historical fire regimes, together with the topography of the sites thereof, indicates a general correlation between fire regimes and landscape gradients. Fuel-driven, the low-severity fire regime is affiliated to lower elevations (Agee, 1997). Weather-driven, the high-severity fire regime is affiliated to higher elevations (Bessie and Johnson, 1995; Agee, 1997). Whereas, the mixed-severity fire regime can be fuel or weather driven, but is most commonly found on mid to high elevations (Agee, n.d). While the physiological traits that enable flora to coexist with fire predate the climatic conditions of the present (Lamont and He, 2017; He, Lamont, and Manning, 2016), the synecological character of the symbiotic relationships that have evolved between flora and fire are evident whereupon historical fire regime records are interrogated. In the United States, and the wider Neoarctic realm, the historical fire regimes of the Holocene [11.7 tya to present], and their affiliated dendrological chronologies, illustrate an affinity between biotic assemblages predominantly populated by Oak [including Quercus garryana and Quercus wislizeni], Ponderosa Pine [Pinus ponderosa], Mixed Confer, and to a lesser extent Douglas-fir [Pseudotsuga menziesii] and the low-severity fire regime (Agee, n.d, 1998; Gray and Riccius, 1999; Foster, 1998). In contrast, species including Subalpine fir [Abies lasiocarpa], Whitebark pine [Pinus albicaulis], Mountain hemlock [Tsuga mertensiana], and Lodgepole pine [Pinus contorta] (Pierce and Taylor, 2011; Agee, n.d, 1998) are associated with the mixed-severity and high-severity fire regimes.

Thus, while fire’s inherent stochasticity limits our capacity to predict precisely when and where a wildland fire will become manifest, whereupon we know a site’s topography, and more particularly, its elevation, together with its biomass composition, and we triangulate this datum with meteorological forecasts, we can reasonably assess the probability of a low, mixed, or high-severity fire at a given site. If expressed through the medium of a pyro-polygon, a fire behavior triangle forms comprised fuel, weather, and topography (Agee, 1997, 1998).

In order of historical frequency, the fire regimes are described below:

Low-severity

The low-severity fire regime is associated with biomes comprised open canopy structures (Jenson and McPherson, 2008) of limited tree density and basal area [67] (Agee, n.d; Baker 2017), therein forests, shrublands [68], and grasslands of which the biomass structure has a lower overall load of hydrocarbons than those affiliated to mixed and high severity regimes. This regime is characterised by very frequent, but relatively low-intensity fires that swiftly burn through understory vegetation, such as leaf-litter, logs, and low-lying branches, as well as snags [69], but leave soils unheated, and the overstory vegetation generally unaffected. Hence, the low-severity fire regime is sometimes referred to as non-lethal, killing no more than <20% of basal area (Agee, 1993, 1994). Spatially, low-severity fires tend be small, (Agee, 1998, 2005); burning out comparatively quickly compared to mixed and high severity fires. Visually, the low-severity fire regime creates relatively homogeneous landscapes, within which the legacy of one fire subtly blurs into that of the next (Agee, 2005). While individual fires tend have little apparent effect on the biome, cumulatively they sustain the landscape mosaics that are fundamental to the survival of the species adapted to this regime type (Agee, 1998). Dendrochronological techniques, including cross dating of tree rings and fire scarring [70], have enabled the creation of hypothetical reconstructions of historical low-severity fire regimes. While the spatial complexity of fire spread within the landscape renders the accuracy thereof uncertain (Jensen and McPherson, 2008), analysis of datum relating to the case study area suggests that once fire has burned through an area it will rarely return within 3 years (Heyerdahl, 1997; Agree. n.d), the exception thereto being some graminoid-dominated ecoregions (Archibald, 2013).

Mixed-severity

The mixed-severity fire regime is associated with biomes comprised landscape mosaics that evidence combinatory fire severities; a hybrid of low, moderate, and high-severity fires; thus the regime constitutes the foremost complex of the fire regimes (Agee, 2005). While technically, the mixed-severity fire regime is associated with an average 25-75% biomass topkill [71] (LANDFIRE, 2014), its legacy is a highly heterogeneous, patchwork quilt of a landscape comprised juxtapositions of burned and unburned vegetation. Common in the mid-elevation forests of the western United States (Jensen and McPherson, 2008; Agee, n.d), the mixed-severity regime supports floral and faunal diversity, as it provides a rich and complex array of biotic habitats, both living and dead. Many species benefit from the abundant coarse woody debris [72] [CWD] born both of mixed-severity fires past (Agee, 2005), together with wide- ranging other biotic disturbances, including windfall, heat stress, and insect outbreaks. In the case study region, this regime averages an MFRI of 25-75 years (Agee, n.d). The timing and behaviour of the mixed-severity fire regime is shaped by a generally wider spectrum of influences than its low and high counterparts. While biomass condition [fuel moisture-levels, structure, and composition, the latter two both shaped by fires past] and weather are generally cited to be the foremost influencers in this regime, topography also plays a fundamental role (Jensen and McPherson, 2008; Agee, 2005; Schoennagel et al, 2004). The mixed-severity fire regime constitutes a microcosm of the wider fire regime family, sometimes manifesting low-severity patches on lower elevation North/East facing slopes, and high-severity patches on higher elevation South/West facing slopes (Agee, 2005; Taylor and Skinner, 1998).

High-severity

The high-severity fire regime is principally associated with biomes that are largely comprised swathes of even-aged stands [73] at the late-successional stage, therein biomass structures that are susceptible to crown fire behaviour (Agee, 1998). The exception thereto is subalpine forests where harsh environmental conditions and limited seed distribution hinder regrowth in the aftermath of a fire, and/or other natural disturbance (Agee, n.d). Occurring at higher elevations, and widespread in boreal (Agee, 1998; Johnson, 1992), subalpine (Agee, 1998; Agee and Smith, 1984), and wet coastal forests (Heinrichs, 1983) of the western United States, this regime is characterised by relatively infrequent, but high-intensity fires, of which the MFRI historically averages 75-100 years (Agee, 1998). Usually weather-driven (Agee, 1998, 1997; Bessie and Johnson 1995), the high-severity fire regime typically burns through many thousands of hectares (Agee, n.d), and especially whereupon winds widely distribute sparks and firebrands, therein igniting spot fires far ahead of the fire front (Scott and Reinhardt, 2001). Extreme weather conditions tend result in high-severity fires burning through biomass at all successional stages (Bessie and Johnson, 1995; Romme and Despain, 1989; Romme, 1982). Whereas, more usual weather tends result in high-severity fires slowing or stopping at the boundaries of early successional-stage stands [i.e. where the fuel-load lessens] (Agee, 1998; Romme and Despain, 1989; Despain and Sellars, 1977). In either scenario, the tree mortality rate is high, and especially given that the physiology of the species affiliated to this regime is generally not adapted to survive the intensity of the fires it commonly manifests (Agee, 1998). Hence, why the high-severity fire regime is associated with stand-replacing fires, and thus sometimes referred to as lethal (Agee, 2005). However, whereupon the regime resides within the usual parameters of the historical MFRI, ‘lethal’ is somewhat of a misnomer, in so far as the biota affiliated to this regime has adapted to reproduce in the aftermath of a fire (Hutto, 2008).

>Continue to Chapter 4.2.5 here.

Footnotes

[68] Shrubland is a biome type comprised evergreen sclerophyll shrubs, which comprised hard leaves, are woody plants that grow to less than 10m tall.

[69] Snag refers to standing dead tree, or part thereof (NOAA, 2016).

[70] Whereupon its tissues are damaged by fire, a tree develops physical and chemical boundaries at the injury site: a process that helps to reduce the probability of infection. As in humans, this process is called ‘scarring’, and in the record thereof helps dendrologists to establish information about the timing and possibly intensity of an historical fire (USDA, 2016).

[71] Topkill refers to mortality of aerial biomass, which may, or may not recover by resprouting in the aftermath of a fire).

[72] Course woody debris is described as “dead woody materials in various stages of decomposition, including sound and rotting logs, snags, and large branches” (Enrong, Xihua, and Jianjun, 2006).

[73] Stand refers to a biotic unit comprised a single species that is homogeneous both in composition and age (Lincoln, Boxshall, and Clark, 1998).

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.