Fire-Resistant Plants: A Review of Plant Morphology, Tissues, Habits, Ecological Adaptations, and Other Factors Contributing to Bioderived Environmental Solutions and Technologies

 

Henry A. Colorado1,* and Joshua M. Henkin2,3,*

 

1 C C Composites Laboratory, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medellín, Colombia.

2 Department of Botany (Department of Science and Education), Field Museum of Natural History, Chicago, IL, 60605, USA.

3 Department of Plant Biology, Rutgers University, New Brunswick, NJ, 08901, USA.

*Email: henry.colorado@udea.edu.co (H. Colorado), jhenkin@fieldmuseum.org; joshua.henkin@rutgers.edu (J. Henkin)

 

Abstract

This review represents an investigation of fire-resistant plants through the areas of materials and characterization, discussing and exploring the implications of their unique properties in several applications, materials solutions, pollution, and perspectives on these plants. The main goals are identifying promising fire-resistant plants from published research with potential applications in engineering and exploring their potential properties in engineering applications including biomimetics. First, Carrot2, results search clustering engine was used to create inclusion criteria and categories for examining the past five years of fire-resistant plant research, materials science-related and otherwise. From these papers analyzed nearly half of research was agricultural or environmental science in nature, with topics and applications in chemistry, engineering, materials science, representing less than 10% each. The unique properties and mechanisms of fire-resistant plants can be classified in areas such as plant architecture, microstructure, and constituent materials, which were discussed. Specifically, the relevance of silica, trichomes (plant hairs), tissue organization, biopolymers, and indigenous knowledge of plants are examined, with an eye towards biomimetics and as inspired structures for engineering as well as in solving pollution/waste disposal and housing/sustainable architecture. Practical considerations, hypotheses to be tested, and conclusions are synthesized, always keeping in mind the intersection of fire-resistant plants and materials science.

 

Keywords: Fire-resistant plants; Silica; Hairs; Biopolymers; Tissue structure; Materials.

 

Table of Contents

 

Innovative Description: Plant fire resistance research in materials science has great potential to improve human life quality, but interdisciplinary work is necessary due to limited research.

 


1. Introduction

There is an increasing concern over the preservation of some species in two very different and iconic regions of South America, the Andes and Amazonia. Due to multiple reasons that include global warming,[1] invasive plants in the context of land management,[2] weather,[3] and deforestation,[4] wildfires are one of the major forces currently devastating native forest ecosystems worldwide, even threatening to change the Amazon rainforest from a net carbon sink to a net carbon source.[5]

Millions of years of evolution have produced some amazing plant species able to survive wildfires, plants that, even when largely destroyed by the action of fire, somehow can resist high temperatures and/or resprout soon after fire exposure, showing that the plant has survived the effects of high temperatures.[6] The resistance of plants, and particularly of trees, to fires, depends on several factors, such as fire type, trunk characteristics, and bark thickness.[6] There are also fire-resistant plants that are now used to protect landscapes and reduce the risk of wildfire.[7]

Fire-resistant plants and plants exposed to fire have been studied, but the topic is far from completely understood due to the complexity of the factors involving diverse plant species. It has been reported from even Cretaceous fossils that some leaves and flowers may be protected and provide protection from direct heat during the fire, due to their hairy texture,[8] a hypothesis that should be investigated in the multifunctional hairy leaves from frailejón (Espeletia spp.).[9] Also, many species have been identified as resistant to fire in various parts of the world, the result of complex natural selection and adaptation, including convergent evolution, which certainly can be very useful to understand their preservation and importance today.[10] The complex microstructure of these plants not only may explain fire resistance but also have direct applications in the fabrication of ceramics, such as silica in bamboo stems.[11] moriche palm fibers,[12] or rice husk,[13] which could explain some of their fire-resistant and mechanical properties. Although the case of rice husk complicates this, as it is used as a fuel, showing that the effect of plant materials on fire resistance is context-dependent and involves a complex interaction between structure, inorganic and organic composition, as well as the fire- and fuel-related variables involved at a macro scale. In fact, the very characteristics of plant materials that make them fire-resistant (e.g., combustibility, rapid rate of fuel consumption, high ignition temperature, elevated pyrolysis temperature range) in habitat may also cause them to be attractive as fuels in certain situations.[14]

This article used the Scopus database to analyze the current trends, limitations, and science related to fire-resistant plants, particularly from the last 5 years, a time interval selected because of the high number of articles to analyze and in the interest of prioritizing more recent research. It was also limited to research and review articles, excluding conference papers and other document types, mainly because this selection usually includes more elaborated research. It presents a detailed literature review, metrics, and statistics to provide basic information about prior research on fire resistance in plants and which areas of basic research could use more attention/work. Also, it addresses indigenous and traditional knowledge as well as ecological and morphological findings in terms of suggesting new lines of research as a driver of fire-resistant plants and its potential importance for mitigating wildfires and inspire new engineering solutions for fire-resistant materials. Finally, it also integrates a broad-ranging view of this issue, incorporating local, practical, and social concerns about the topic. 

 

2. Literature review methods

For this review, bibliographic searches were carried out in the Scopus database, for the area of fire-resistant plants (see Table 1). From these keywords, initially, 761 documents were found. Then, applying filter 1, a total of 590 documents were determined. Thereafter, by applying filter 2, a total of 185 documents were found. Moreover, by applying the two filters in the same search, 160 documents were established. Finally, excluding keywords such as plant disease, genetics, and agricultural and biological sciences, only 71 documents were selected.

Carrot2 software open-source search clustering engine was used to understand the areas found in the search. This software automatically clusters collections of documents, in our case research and review papers, into thematic categories, from the Scopus database search. This was particularly useful because it shows much more specific areas, allowing a different classification and analysis.

 

Table 1. Search algorithms.

Search

All results

Only filter 1 (document type: article or review)

Filter 2 (last 5 years)

Two filters

TITLE-ABS-KEY (fire AND resistant AND plants) 

761

590

185

-

(("TITLE-ABS-KEY (fireAND resistant AND plants) AND (LIMIT-TO (PUBYEAR, 2022) OR LIMIT- TO (PUBYEAR, 2021) OR LIMIT-TO (PUBYEAR, 2020) OR LIMIT-TO (PUBYEAR, 2019) OR LIMIT-TO (PUBYEAR, 2018) ) AND (LIMIT-TO (DOCTYPE, "ar") OR LIMIT-TO (DOCTYPE, "re")) 

 

 

 

160

(EXCLUDE (EXACTKEYWORD, "Erwinia Amylovora") OR EXCLUDE (EXACTKEYWORD, "Genetics") OR EXCLUDE (EXACTKEYWORD, "Plant Disease")

 

 

 

126

AND (EXCLUDE (SUBJAREA, "AGRI"))

Agricultural and Biological Sciences

 

 

 

71

Where TITLE-ABS-KEY is Title-Abstract-Keywords, and PUBYEAR is publication year, all data taken from the analyzed article. DOCTYPE is document type, EXACTKEYWORD is the exact keyword, and SUBJAREA is subject area.

 

2.1 Literature review: results and analysis

Figure 1 shows some of the search statistics obtained from Scopus, where the number of documents in the last years increased significantly, Fig. 1a; only 5% of the documents targeted are review papers, Fig. 1b; the main research areas are agricultural and biological sciences, and environmental science, Fig. 1c. The funding sponsors are led by agencies in China and the USA, respectively.

 

Fig. 1 Scopus statistics regarding the search algorithms from Table 1.

 

Figure 2 shows a tree map generated by the tool Carrot2, with 51 results classified in clusters of fire-resistant plants (5), materials (5), and other topics (9), among others. This graph shows by size the number of studies by area available with Scopus and with the Carrot2 software, which is important to understand the different subjects related to the fire-resistant plants. The subject fire-resistant plants also appear, but it is mostly related to biology or environmental aspects. Other areas are more related to applications and materials.

 

2.2 Materials characterization technologies

From the above classification, several papers that are analyzed below, used diverse advanced materials characterization technologies, opening the understanding of fundamental mechanisms and their associated micro- and nanostructures of plants that contribute to their more significant properties. Among these technologies are Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric and differential thermal analysis (TGA, DTA), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). One study focused on the production and characterization of fire-resistant epoxy composites from plant-derived-ferulic acid used FTIR[15] spectra for chemical bond identification; DSC, DMA (Dynamic Mechanical Analysis), TGA, and TDA for storage modulus and thermomechanical properties; SEM for microstructure identification; and XPS for elemental composition. Another research application has investigated leaves capturing fire smoke particles,[16] involving IR and SEM techniques to understand the smoke composition and compound concentration, as well as microstructure, respectively. Phosphorylated cellulose[17] has been used as an admixture for fire-resistant lyocell fibers, characterized via TGA, SEM, and FTIR techniques. Clearly, much basic research regarding fire resistance in plants and its practical applications requires many of the same advanced characterization techniques. Table 2 summarizes some of the most important characteristics for the fire-resistant plants found in this literature review. First, considering the number of plants interesting for studying this behavior, the number of species with serious studies isvery low, and thereare even fewer studiesusing materials sciences techniques to understand important aspects of plant materials and microstructure. Only a few papers include thermogravimetric analysis (TGA) for precise thermal degradation and quantification, scanning electron microscopy (SEM), or Fourier Transform Infrared Spectroscopy (FTIR). In terms of plant parts, mostly the leaves and bark are the subject of these few studies, and even more limited are the number of countries which have researchers that have undertaken these studies. Besides these studies and considering the number of species worldwide, this review shows amazing opportunities for research and development in this area.

 

Fig. 2 Clustering with more specific topics (search.carrot2.org).

 

Table 2. Fire-resistant plants parameters

Plant species

Fire or temperature exposure

Part studied

Application

Materials characterization techniques

Country´s plant

Ref.

Ferula asafoetida, rattletop, spina date

40-700 °C from TGA

Leaves, fibers, and seeds, respectively with epoxy resin

fire-resistant epoxy thermosets

TGA-DTA, SEM

China

[18]

Seagrass (Zostera marina

leaves, Posidonia oceanica leaves, and wood)

Flame test ISO 5660-1 (over 300 °C), and Cone calorimeter

Leaves and fibers, with polymeric binder

Insulating materials

OM, thermal conductivity

Germany

[19]

Cunninghamia lanceolata and

Schima superba

Portable burner set up 300 °C

Leaves

Wildfires prevention and/or conservation

SEM, IR

China

[20]

Lyocell cellulose (semisynthetic fibers)

Room temperature to 700 °C from TGA, Cone calorimeter, burning tests

Fibers

Fire resistant fibers

FTIR, TGA, SEM-EDS

China

[21]

Banana peel powder (Musa acuminata) (BPP), coconut shell (Cocos nucifera) extract (CSE) and pomegranate rind (Punica granatum)

Flamability tests and TGA up to 700 °C

Extracts and fabric

Fire resistant fabrics

TGA, FTIR, SEM-EDS, mechanical tests

India

[22]

Savanna trees (Crossopteryx febrifuga and 

Piliostigma thonningii)

 

Fire exposure up to 620 °C

All plant

Wildfires prevention and/or conservation

None

CoĂte

d'Ivoire (West Africa

[23]

Eucalyptus spp. and Callitris intratropica

Modified wick-fire technique

Bark

Wildfires prevention and/or conservation

None

Australia

[24]

 

Several, including: pitch pine, balsam fir, eastern hemlock, beech, sugar maple and chestnut oak

Thermal conductivity

Bark

Wildfires prevention and/or conservation

Thermal conductivity

USA

[25]

Cabbage palms (Sabal palmetto)

Direct and controlled fire on site, over 300 °C

All plant

Wildfires prevention and/or conservation

None

USA

[26]

Many, including: annuals,and turf.

Not reported

All plant

Wildfires prevention and/or conservation

None

USA

[27]

Many, including: Ajuga reptans and Achillea spp.

Not reported

All plant

Wildfires prevention and/or conservation

None

USA

[28]

Many, including: Myrica rubra ,  and 

Camellia oleifera 

Not reported

All plant

Wildfires prevention and/or conservation

None

China

[29]

 

3. Plant mechanisms for fire-resistance

The unique properties and mechanisms of fire-resistant plants can be grouped into factors such as plant architecture, material composition, and microstructure. Moreover, there are several parts of the plants that must be preserved so that they can be easily regenerated after a fire: the vascular cambium, the axial or apical shoots, as well asthe phloem in adult woody vascular plants. However, in many cases it may not be known the exact mechanisms by which tissues are protective for their plants – in which case the fire-resistant properties need to be explained by way of further research, the driving purpose behind this review. In this review it was found that some plants contain silica or hairs, for instance,  as components of protective tissues that lead to improved fire-resistance that enables them to withstandwildfires. Even when some parts of the plants are destroyed, many species have an amazing regeneration capacity involving resprouting among other mechanisms.

 

3.1  Silica

One major mechanism for fire resistance in plants that has been broached but that deserves further attention and analysis is related to their silica content. Rather than a main chemical mechanism for fire retardation and resistance, it is proposed that the structural characteristics and physical distribution of silica particles within a plant or in a material are the actual basis for the mechanism.[30] The effectiveness of this flammability reduction is based on certain microstructural characteristics deriving from the physical and physicochemical properties of silica particles, including pore size, particle size, surface silanol concentration, surface area, density, and viscosity.[30] In particular, if the silica particles accumulate near the sample surface, based on the additive’s surface area, density, and viscosity, it seemingly is more likely to perform as a thermally insulating layer for the polymer or composite of interest.[30] This could be an underinvestigated factor in materials where, for instance, hemp fiber is serving as an additive in hempcrete and other composite materials with inorganic binders, since the non-glandular cells of hemp are only silicified on the outer wall surface, and in the case of one study of hemp fiber-reinforced polyester, ignition temperature was increased by the volume fraction of fiber in a density-independent manner.[31,32] Many sources suggest that hemp’s contribution of perceived fire-resistant properties to composites may be more related to the amelioration of their cracking behavior,[32–39] such that the binder itself (e.g., lime) is more responsible for direct fire resistance, and further suggesting that hemp in these materials may primarily be acting much as a fiber temper does in ceramics. This perhaps deserves its own separate analysis, however, and it is not the end of the story where silica and silicified plant tissues, especially where trichomes and epidermal cells are concerned. Anecdotal evidence suggests that in the case of Curatella americana, a fire-resistant shrub that yields various leaf epidermal and hair cell phytoliths (silicified plant remains often corresponding to the shapes of cells and intracellular spaces) present in the archaeological and sedimentary records of Panama, as well as its close relative, Davilla aspera, with similar cellular morphology, these silicified structures are responsible for both the ethnographic use for scrubbing pots and polishing wooden items and also seemingly for their thermal and fire-resistant properties, as is the case with the hyperaccumulating, silicon-dependent scouring rushes/horsetails (Equisetum spp.).[40,41] Phytoliths derived from the ash of tree bark, amorphous silica corresponding to the cellular and intracellular spaces of these tissues, have been considered as a separate category of tempering material in their own right from the outset in the 20th-century development of ceramic science.[42] Certainly, it may be worth considering that there are chemically based fire resistance effects due to silica as well, albeit not necessarily based so much on the natural physiology of plants, given the studies of wood impregnation with silica and titanium dioxide,[43] among other substances. If there are elemental and inorganic biomarkers for fire resistance in plants related to mere concentration rather than microstructure though, including silica, they have yet to be identified with certainty. Thus, there are several complex factors that make a plant resistant to fire, and the microstructure may play an important role in the responses. Fig. 3 shows in two images the microstructure of moriche palm obtained fromscanning electron microscopy, still poorly understood but known by its resistance to fire by ancestral communities from the Amazon region.

 

Fig. 3 Microstructure of a fire-resistant plant Mauritia flexuosa (moriche palm) from Colombia.

 

 

Fig. 4 Non-glandular silicified hairs (left) and calcium carbonate-containing cystoliths (right) in Cannabis sativa L., source of hemp and cannabis; only the outer wall surface of these two types of non-glandular trichomes is silicified.

 

3.2  Hairs (trichomes)

A couple of papers concerning the genus Phylica (Rhamnaceae) and its ancient relatives were published in the past couple of years, which made interesting, if largely anecdotal, claims about the lineage’s ancient adaptation to fire, given the conserved structure and thick covering of trichomes supposedly protecting budding flowers.[44] Possibly more conclusive are the cases of trichome-coated leaf axils and epicormic structures in a couple of myrtaceous genera – Syncarpia and Tristaniopsis – with fire resprouting properties, as well as the presence of taxa with buds protected by a covering of trichomes as a distinct and effective strategy in fire-disturbed habitat, given the more systematic ecological and morphological investigations in these cases.[45,46] The principles here may be relevant to Espeletia spp. (Asteraceae), Dendrosenecio spp. (Asteraceae), and Lobelia sect. Rhynchopetalum (Campanulaceae), among others, all alpine pachycaul plants, although much more, by comparison, has been written about how their convergent evolution in hairy leaves and Bauplan are related to protection from the cold and from UV rays with higher solar irradiance in montane habitats as well as acquisition and conservation of water.[9,47] Evidence from the study of Lychnophora diamantinana (Asteraceae), with similarly hairy leaves and habit to alpine pachycauls, in the Brazilian side, show that, at least in this specific case, the cell walls of non-glandular trichomes may physically degrade and deposit a hyaline layer of mostly pectin that is hygroscopic and protects the plant from desiccation.[48] While there is no guarantee that hairs are functionally providing fire-resistance in the same way they are supporting its survival in dry, sunny habitats, it is interesting to note that the plants are additionally endemic and adapted to fire-disturbed areas. On the one hand, it is suggested that Lychnophora spp. may be mostly resprouters and post-fire germinators and that the tops, including leaves, are destroyed during wildfires or that the fires are not particularly powerful when the aerial parts do survive.[49,50] Not only the leaves are covered with hairs, however, but the stems as well are covered with a thick tomentum composed of trichomes, suggesting that this adaptation attributed primarily and traditionally just to dry environments with high insolation could confer fire resistance as well,[51] a possibility worth at least further and more in-depth exploration either way.

 

3.3 Biopolymers and other materials

To understand the composition of plant materials, fuels, and biomass (and possibly composites and bioinspired materials developed from them), at least one of two main, related techniques should be employed, among others depending on the study: TGA-GC-MS[52] and py-GC-MS.[53] The first, thermogravimetry coupled with gas chromatography-mass spectrometry analysis, allows the scientist to observe the mass percentage to a precise level and volatile chemical composition that evolves with heating at a known temperature program and interpret the results in terms of the sample’s phase transitions and thermal decompositions in concert. This is a powerful tool in terms of understanding the combustion and pyrolysis as well as the initial composition of the material of interest. Ash, cellulose, hemicellulose, lignin, and volatile components and contributions to mass percentage can be monitored, along with those of individual sugar residue types from the decomposition of biopolymers.[52] The second, pyrolysis-gas chromatography-mass spectrometry, is used to quickly decompose and volatilize a sample with a pyrolyzer at elevated temperature and then analyze it[53]. Equally, one can perform a thermal desorption step beforehand to analyze volatiles and semi-volatiles only and then produce a separate pyrogram that corresponds just to the material’s decomposition.[53] Either way, chemical information that would not be recoverable from microscopic and other analytical techniques can be produced and contribute to an understanding of fire-resistant properties.

A relevant study was undertaken in Puerto Rico to determine the carbon, nitrogen, and other essential element resource use and allocation for biopolymers in the metabolism of two species of Neotropical palms was triggered by the excitement about traditional knowledge on the timing of palm frond harvest for thatching, using Sabal mauritiiformis (Arecaceae),[54] during the lunar cycle in Latin America. Examination of the palm fronds showed that their composition during the full moon accordingly demonstrated peak levels of lignification and production of hemicellulose, with the opposite being true for the new moon.[54,55] This is seemingly also verified in the context of wood quality as well since rural Zapotec farmers know to harvest branches for the hafts of axes and the handles of other such implements like hoes as well as structural beams at the full moon in order to ensure their durability and prevent them from being infested by termites.[56] As stated by the authors of the palm frond harvest paper, the best-supported hypothesis, for now, is that these plants’ increased lignification is a response to the higher degree of insect activity and feeding behavior under levels of maximum lunar illumination.[54,55] Yearly, seasonal, lunar, and diel cycles in plants, while they may be known anecdotally or from further observation and scientific study, are not necessarily at all yet attributable to ecological relationships nor to other environmental stimuli.[54] For instance, in the opium poppy (Papaver somniferum), Western empiricism has only recently been able to explain the millennia-old tradition of morning harvest of latex, when the alkaloidal content is highest, physiologically in terms of the plant's daily modulation of water allocation throughout its laticiferous tissue,[54,57,58] and so in certain cases biologically relevant cycles may be more a matter of metabolism instead. It will be well worth appreciating the potential existence and impact of these cycles to identify, reproduce, and understand their effects on the composition and durability of harvested plant tissues, as fuels and as materials in general.

 

3.4  Tissue structure and habit

While chemical and microstructural compositions of tissues may account for some of the fire-resistant properties of plants, higher-level tissue organization and ultimately the habits of plants in their ecological contexts may also account for such adaptations. For Mauritia flexuosa, the moriche palm, for instance, at least some populations seem to be in continuous, long-term interaction with fire, and so long as the individuals are not subjected to wildfire events that kill the apical meristem and/or their belowground biomass, they will likely survive and regenerate.[59] Water retention by leaf sheaths/petiole bases and around the apical meristem keeps moriche palms wet, with fuel moisture being the primary regulator of fire in these environments, in which case they will live through wildfires and grow additional, new green tissue within days.[59] It is entirely possible with Espeletia spp. (frailejones) that the characteristics that make them remarkable as plants in terms of their tissue organization, including stomatal crypts on the undersides of leaves and thickened primary meristems at the shoot apices along with their highly villous leaves, mostly attributable to their high, dry, and cold environments,[9] also may convey fire resistance in some manner. This has yet to be explored, however. Interestingly, it is known that wildfire will sometimes remove the marcescent leaves of Espeletia spp., dead leaves that are retained on plants for decades and even up to and including more than a century, while leaving the rest of the tissues intact.[60] This might suggest that they can serve a protective and preventive role in these cases, for preserving the living giant rosette leaves, stems, and meristematic tissue. Other examples that might be worth studying in greater depth include the lignified outer pericarp of Adansonia spp. (baobabs and boabs; Malvaceae) that serves a role in protecting the fruits from harm during savanna wildfires in sub-Saharan Africa and Australia and seems to help increase the germination rate of seeds after fire exposure;[61] the tissue structures of desiccation-tolerant arborescent monocots such as certain taxa of Boryaceae, Cyperaceae, and Velloziaceae to determine fire resistance;