On the Tendency of Species to Form Resilience

Panarchistic Architecture :: Chapter #4 [4.2]

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.

4.2.7 On the Tendency of Species to form Resilience [85]

“Adaption is the act of bending a structure to fit a new hole. Evolution on the other hand, is a deeper change that reshapes the architecture of the structure itself – how it can bend – often producing new holes for others”. Kelly, 1994.

In diagrams of the phylogenetic tree of life, fire is conspicuous by its absence, and particularly so given that, in some species, pyroendemics, both seedling germination (Moreira and Pausas, 2012) and successful recruitment, are limited to “immediate postfire environments” (Keeley and Pausas, 2016, p. 1). However, dated molecular phylogenetic [86] and Bayesian [87] ancestral state reconstructions, together with the fossil record, make evident the emergence of fire-related functional traits within the coevolution of fire and flora (He and Lamont, 2017, 2016). Furthermore, enduring, evading, and resisting were the first three of the five modes of persistence to evolve.

In no particular order, the five foremost functional traits of fire-resilient species are described below:

Pyriscence

Originating in the common ancestor of the conifer (Ibidem), pyriscence, a type of serotiny [Fig. x], is a functional trait found in endurer species in ecoregions including Mediterranean forests, woodlands, and scrubs, and temperate coniferous forests. Evident in members of the genus Pinus since the Late Carboniferous [332> mya], pyriscence evolved in angiosperms no later than 74 mya (Ibidem). Seed release in pyriscent species is fire-stimulated. In conifers and some angiosperms the release mechanism is the melting of the resin that holds the protective parts together (Ibidem) [Fig. 25]. In the genus Pinus, pyriscence is a heritable trait, which recent studies suggest is encoded via genes into enzymes, and of which the expression level varies throughout the evolution of species (Ibidem). Thus, the temperature at which the resin in pyriscent cones and fruits melts is variable over time, which, over generations, enables pyriscent species to adapt to changing fire conditions [i.e. match the melting point to the intensity of shifting fire regimes, and in turn, the environmental scenarios as underpin them]. However, some species stoke their own fires, having tightly coupled pyriscence with abscission. In this instance, the litter produced therefrom is highly flammable, an example thereof being the genus Banksia (He, Lamont, and Downes, 2011). Regardless of whether abscission is present or otherwise, in the Northern Hemisphere, together with an abundance of nutrients, light, and space, upon germination, pyriscent species benefit from the removal of allelopathic leaf litter, therein, part removal of pre-existing hierarchical ecological structures that may inhibit their species populous.

Pyrogermination

Several-dozen plant families found in fire-prone Mediterranean-type climate (MTC] ecoregions possess fire-stimulated germination (Baskin and Baskin, 2014; Keeley, 2012: Keeley et al, 2012), which for the purposes of this thesis shall be termed ‘pyrogermination’. Potentially an exaptation derived from earlier evolutionary changes in primary metabolism (He and Lamont, 2017), pyrogermination has been identified in 2,500 species (Bradshaw et al, 2011) spanning a “vast phylogenetic range” (Keeley and Pausas, 2016 p.3) [Fig. 26]. Of the various hypothesis of origin, at the time of authorship, the foremost plausible appears to be that of convergent evolution arising manifold times across diverse taxa in temporally and spatially divergent MTC ecoregions (Ibidem; Pausas and Keeley, 2009). For example, in the Poales species Anarthriaceae-Restionaceae, fire-stimulated germination was already well established by 91 mya (He and Lamont, 2017). Whereas, in the flowering plant family Proteaceae, of which genera include the national flower of Australia, the Banksia, and the genus Embothrium, which is more commonly known as Chilean firebrush, pyrogermination evolved 81 mya (Ibidem). In the study region, California, pyrogermination is prevalent in many annual species, most of which are pyroendemics, as are a significant number of woody species (Keeley and Pausas, 2016). Primarily associated with evader species, the trait is also exhibited in many invaders.

The trait is triggered by a variety of autecological mechanisms, including:

Heat cracking of seedcasings, which enables the process of imbibition [88] to occur (He and Lamont, 2017).

Response to chemical signals emitted in smoke, charate, and ash upon the breakdown of organic compounds (Ibidem). For example, when living, some plants use cyanide as a defence against herbivorous animals. However, when these same plants burn they emit the nitrogenous compound cyanohydrin (glyceronitrile), which has been found to stimulate germination in pyroendemics (Flematti et al, 2011; Downes et al, 2014). Additionally, a class of organic molecules known as butenolides have been identified as highly active stimulants (Flematti et al, 2004; van Staden et al, 2004), and studies evidence that butenolide derivative Karrikins [KAR1, KAR2, KAR3, KAR4, and, to a lesser extent, KAR5 and KAR6], of which the origin is strictly combustion (He and Lamont, 2017, p.11), trigger the germination of dormant pyroendemic seeds (Keeley and Pausas, 2016; Nelson, D. C., et al, 2012; Daws et al, 2007). However, KARs and glyceronitrile are but one of several compounds found to stimulate germination, others include another nitrogenous compound, nitrous oxide, and the hydrocarbon ethylene (He and Lamont, 2017). Having been transported in smoke and residues, the compounds are absorbed by soils, thereon, the seeds that are stored in those soils, ultimately reaching the embryos, their proteins, and the genes therein, the latter of which are encoded to respond to the compounds (Ibidem). The earliest report documenting the process thereof was published just a decade ago (Keeley and Pausas, 2016), thus the field is one of active enquiry, with many questions left as yet unanswered. However, studies suggest that pyrogermination cued by chemical signals likely incurs combinations of compounds to catalyse the process, but that butenolides [KARs] are central thereto (Ibidem). In some instances, pyrogerminatic species, as we might call them, have been found to tightly couple chemical signalling with flammability, thus ensuring that enough ethylene is emitted to trigger pyrogermination (He and Lamont, 2017).

Abscission

A process of “co-ordinated breakdown of the cell wall matrix at discrete sites and at specific stages during the life cycle of a plant” (Roberts et al, 2000, p. 223), abscission is a common trait in floral species, which facilitates the shedding of parts for reasons including reproduction, conservation of resources, and defence. Appearing in the conifer lineage in the Carboniferous, branch abscission and healing (He and Lamont, 2017, Looy, 2013), and the sparse crown that is a consequence thereof, is a trait associated with resisters.

Retardant Rhytidome

Foremost in the pyro-armoury of resisters, rhytidome is the outer of a tree’s two bark layers. Mainly comprised of dead tissue, but penetrated by periderms [cork layers], rhytidome is hypothesised to have evolved by means of protecting plants from the elements [the climate hypothesis] and/or from pests, infections herbivory species [the biotic hypothesis] (Pausas, 2015). In resisters, rhytidome is thicker, as is the overall bark assemblage, this being a trait that protects the inner, meristematic [89] tissues from fire (Pausas and Keeley, 2009). The genus Pinus provides evidence of the evolution of thick bark as a means of protection from fire [the fire hypothesis] [Figs. 27, 28], as lineages that are prominent in fire-prone ecoregions are endowed with thicker bark than are those that are present elsewhere (Pausas, 2015; Keeley and Zedler, 1998).

Resprouting

As described above, resprouting is an endurer hallmark. Abundant in its taxonomical distribution, the trait is present in several ancient woody lineages, including that of ‘living fossil’ Ginkgo biloba (Pausas and Keeley, 2009), a gymnosperm, which has remained largely unchanged in over 200 million years (Cohn, 2013). Whereas pyriscent and pyrogerminating species may endure the passing of one fire, but not another whereupon the interim between the two is short, resprouters will usually persist (Agee, 1998). Hence, why Ginkgo, and its evolutionary descendants are well placed to endure for many millennia to come.

>Continue to Chapter 4.2.8 here.

Footnotes

[85] On the Tendency of Species to form Resilience is a reference to a joint presentation made by Alfred Wallace and Charles Darwin at the Linnaen Society of London on July 1st 1858 to announce their theory of evolution by natural selection.

[86] A method of molecular systematics, which resides within field of phylogeny, Molecular phylogenetics involves the analysis of hereditary molecular differences by means of establishing the evolutionary relationships between species (Brown, 2002).

[87] The Bayesian method enables calculation of the probability that an unknown organism belongs to a specified taxonomic lineage (Lincoln, Boxshall and Clark, 2003).

[88] Imbibition is a diffusion process, which in seeds involves the absorption of water.

[89] Meristematic tissue is comprised “unprogrammed” living cells that are yet to be “assigned a role” within a plant, otherwise described as “undifferentiated cells” (Steadham, 2017, online).