4.2.2 Forests: The Biogeographic Realms of Fire Regimes.

Low-severity, mixed-severity, and high-severity fire regimes are indigenous to the Western United States, which comprised several mid-latitudinal biome [56] types, falls within the Neoarctic [57] biogeographic realm [58] (Udvardy, 1975). More specifically, the fire regimes are endemic to the ecoregions [59] the WWF classifies as Temperate Coniferous Forest, and Mediterranean Forests, Woodlands, and Scrubs (Olson et al, 2001; Olson and Dinerstein 2002, WWF, 2012a, 2012b, 2012c), thus both are subclasses of the dominant terrestrial ecosystem: forests [60] (Pan et al, 2013). Covering an estimated 5.9 billion hectares (ha) in the pre-industrial age (Adams, 2012), and roughly 3.8 billion ha worldwide [30.6% of Earth’s terrestrial surface] today, (FAO, 2015; Lindquist et al, 2012), forests account for 75% of terrestrial gross primary production (Beer et al, 2010), and 92% of biomass worldwide (Pan et al, 2013). A greater volume of CO2 is stored in forest biomass and soils than in Earth’s atmosphere (Pan et al, 2011). Approximately 42% of the sum thereof is stored in living forest biomass, the remainder stored in necromass [61] (Pan et al, 2013).

In consequence of the albedo effect [62], forests sizeably alter the reflectance properties of Earth’s surface (Feddema et al, 2005; Avissar and Werth, 2005), therein further impacting upon atmospheric and climatic conditions, and to the extent that deforestation can indefinitely alter local and global weather patterns (Miller and Cotter, 2013). Peaking at a gross rate of roughly 16 million ha p/a [8.3 million ha p/a net whereupon expansion through planting and/or natural processes is accommodated for], deforestation was at its highest in the 1990s, whereas today the gross has dropped to an estimated 13 million ha p/a [net 5.2 million ha p/a] (Adams, 2012). In totality, the latest global assessment estimates that 129 million ha of forest has been lost since 1990 (FAO, 2015). However, the biological integrity of the world’s forests is varied: whereas some remain in pristine condition, others are in various states of degradation, the former resilient to disturbance events, the latter less so.

The distribution of the world’s forests is coupled with the climate (Davis et al, 2001; Davis and Shaw, 2001; Jackson et al, 2000; Prentice, Bartlein, and Webb 1991). Multiple studies evidence that, as average global temperatures rise, treelines are shifting in response (IPCC, 2007; Parmesan, 2006); in some instances, headed upwards (Liang et al, 2016), in others polewards (Iverson and McKenzie, 2014; Zhang et al, 2013), and in others still, longitudinally (Fei et al, 2017). Several-fold are the biological reproductive processes and ecological phenomena that enable flora to migrate by means of mitigating shifting local and global environmental conditions, and details thereof will be discussed in the following chapter.

A recent study of ground-based surveys and satellite imagery indicates there to be approx. 3 trillion trees worldwide, of which 1.30 trillion are in the tropics and sub- tropics, 0.74 trillion are in the boreal forests, and 0.66 trillion are in the temperate regions (Crowther et al, 2015). 93% of the world’s forested areas are comprised primary or secondary forest, the former that which has been subject to comparatively few human disturbances, the latter that which has naturally regenerated (FAO, 2015). The remaining 7% of forests are planted (Ibidem); therein generally contain sizeably less biodiversity than natural forests. An estimated 80% of living forest biomass worldwide is aboveground, with the remaining 20% belowground (Cairns et al, 1997; Jackson et al. 1996, 1997). However, the ratios thereof vary considerably from one ecoregion to the next. Whereas the percentage of below ground living biomass is relatively high in tropical deciduous [25%] and boreal forests [24%], it drops to 16% in tropical evergreen forests, and to just 15% in temperate coniferous forests (Jackson et al, 1996). Disturbances, both natural and anthropogenic, are so very wide-spread within forests that at any given time, a sizeable portion thereof will be recovering from one or several such events. A relatively recent study indicated that 60%> of the some 3.8 billion ha of forests worldwide are in the process of recovering from a disturbance (FAO, 2006).

In totality, 40% of the Earth’s terrestrial surface is covered by fire-prone ecosystems (Bond, Woodward, and Midgley, 2004), of which forests are the dominant biome- type. Ecosystems affiliated with low-severity fire regimes are more commonly found on the Western side of continents within 30-40° latitude and with a Mediterranean climate, including California and Northern Baja, Central Chile, Mediterranean Basin, Western Cape South Africa; and Southwest and South Australia. Whereas, ecosystems affiliated with mixed and high severity fire regimes are associated with both these, and with other regions worldwide, including in the northern latitudes. For example, analysis of data including charcoal distribution in lake sediments evidences both mixed and high severity fire regimes indigenous to the Artic tundra, such that vast swathes of forest burn with a periodicity spanning decades to millennia (Sheng Hu, 2015; Kolden and Rogan, 2013; Mack et al, 2011; Jones et al, 2009). Therein, forests constitute the foremost spatiotemporally dynamic of the several primary terrestrial biome-types; so intimately integrated to Earth’s abiotic systems as to be inseparable thereof, and no less so than in relation to fire [Fig. 22].

>Continue to Chapter 4.2.3 here.

Footnotes

[56] Broadly corresponding with a climatic region, “biome” describes the largest biogeographical region (Allaby, 2012).

[57] Covering 22.9 million square kilometres, the Neoarctic is a biogeographic region extending from North America to Central Mexico (Escalante et al, 2010) that was first delimited in the 19th Century by Sclater (1858), and Wallace (1876). The latter delineated the region into four subdivisions, of which he stated, though “pretty clearly indicated by physical features and peculiarities of climate and vegetation”, zoologically, “while the species of several sub-regions are in most cases different”, at the taxonomic level of genus, “even the vast range of the Rocky Mountains has not been an effectual barrier against this wide dispersal of the same forms of life”. Following in Wallace’s footsteps, the WWF also divides the Neoarctic into four bioregions having applied a similar methodology (Ricketts et al, 1999).

[58] ‘Biogeographic realm’ is the broadest delineation of taxonomic composition. First introduced by Miklos Udvardy in a paper for UNESCO’s Man and the Biosphere Programme (Udvardy, 1975), the classification system was adopted by WWF in its Global 200 scheme in the 1990s, employing the same general methodological approach (Olson and Dinerstein, 1998). Udvardy’s system defined 8 categories of biogeographic province, with a further 193 subcategories characterized by one of 14 biome types. Whereas, WWF’s system is more detailed, specifying 8 forest types and 867 ecoregions (Olson et al, 2001).

[59] An ‘ecoregion’ designates a “large unit of land or water containing a geographically distinct assemblage of species, natural communities, and environmental conditions” (WWF, n.d).

[60] Some several hundred definitions for ‘forest’ exist worldwide (United Nations, 2010). However, within the research domain of dendrology a ‘forest’ is defined as an area greater than 0.5ha of land featuring trees of 5m or more in height, of which the density is at least suffice to meet a minimum of 10% canopy cover (FOA, 2005).

[61] The term necromass describes “any quantitative estimate” of the sum of the mass of dead organisms p/unit area or volume within a specified time (Lincoln, Boxhall, and Clark, 1998).

[62] Quantified on a scale of 0-100, albedo describes the percentage of the Sun’s energy (radiation) reflected back into space from the Earth’s surface. On average, Earth’s surfaces have an albedo of .31. However, forests have a relatively low albedo, averaging between .08 and .15 (ESSEA, 2017).

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

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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.here