Canary Pine bark: research project inspired by a trip

Inspiration for this project was acquired during a trip to the Canary Islands (Gran Canaria) in November 2018 by Claudia and Arne. Gran Canaria may be well known for its tourist hot spots and the beaches but it is also a great place for hiking and experiencing nature. One hike that can be highly recommended is from Mirador del Sargento west of Artenara to the top of Altavista mountain (1377 m). You are walking along a mountain ridge and are rewarded with breathtaking views of the surrounding mountains. Canary Islands pines (Pinus canariensis) can be encountered frequently, and we were amazed by the thickness of the pine bark. Sometimes, the bark is as thick as two volumes combined from the Harry Potter book series …. ca. 15 cm. The thick bark helps to protect the tree from fire damage, similar to what is known from other trees such as Sequioadendron giganteum in North America or Eucalyptus species. Unfortunately, recently, man-made fires have repeatedly affected pine forests, and thousands of hectares of forest have been burned. 

The Canary Islands pine is endemic to the Canary archipelago where it is the only existing native pine (Climent et al., 2004; Plant Ecology 171: 185-196). It can be found mainly throughout the western islands, occupying much of La Palma, Tenerife, El Hierro and Gran Canaria. P. canariensis can reach considerable dimensions of more than 60 m in height and has been known to live for more than 600 years.

The question that comes up when observing the thick pine bark is what makes it so resilient to fire? Is it the sheer thickness of the bark layer, the chemical composition, a combination of these and other factors? Some answers are given in a paper by Tributsch and Fiechter (the material strategy of fire-resistant tree barks, WIT Transactions on The Built Environment, Vol. 97, 2008). The authors analyzed a technical reference polymer, ABS (acrylonitrile butadiene styrene), and fire-resistant tree barks using thermogravimetric analysis. While ABS burnt by leaving only 2.6% solid, the most fire-resistant tree barks only carbonized leaving up to 60% solid. The solid is mostly graphite and carbon which are highly heat insulating and fire protecting as is also known from expanding graphite layers. An important chemical component that confers fire resistance to tree barks is tannin, a polyphenol. A fibrous micro- and nano-structure as well as optimized infrared optical properties (suppression of radiation processes) also contribute to the fire-resistant tree bark properties.

In addition to the resistance of the bark against fire, other factors help the Canarian pine tree to survive against fire: long needles, tall growth habit, self-pruning, deep rooting (ensuring root survival even after intense fires), and the presence of resinous heartwood which prevents heart rotting (Climent et al. 2004). In the FireCellCoat project, we are hoping to use some of the mechanisms and components found in tree bark in nature and to explore them to develop novel, environmentally benign fire-retardant wood coatings based on modified nanocellulose.

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