Rosie Sargent has been testing thermally modified tōtara
It could only be a matter of time before a locally grown indigenous and specialty wood species is seen on the outside of iconic buildings in Aotearoa if early research results are anything to go by.
Scion scientists are announcing promising results in researching thermally modifying tōtara for exterior building uses.
Tōtara heartwood was once popular on farms for strainer posts. The thermal modification process offers a new lease of life for the timber and enhances the durability of the sapwood.
Thermal modification is basically cooking the wood in a low oxygen environment. Scion’s research into thermally modified tōtara began around 2017 using relatively young trees from Northland – about 80 years old.
Tōtara (Podocarpus totara) was chosen because it has natural durability and tests have been done with both the generally nondurable sapwood – living, outermost portion – and potentially durable heartwood – dead, inner wood.
Scion senior technologist Rosie Sargent said while any thermally modified species would have some improved durability, tōtara and lusitanica had been the most successful to date. To get sufficient durability for New Zealand conditions, the wood must be modified at very high temperatures.
“It’s a question of does it perform for specific applications and can you do it without destroying the wood.”
After thermal modification, the wood is tested for durability using long-term accelerated field tests and fungus cellar stakelet trials. The tests are chosen with the aim of being able to assess if the product can meet building and durability standards.
The fungus cellar creates ideal fungus growing conditions to speed up decay. Small wooden stakelets are left in the controlled high-decay situation and the level of decay is assessed over time and can be compared to the performance of known products.
The stakelets remain in test until all are rotted away. The thermally modified tōtara has been in the fungus cellar for about six and a half years, and the lusitanica for more than three years. In comparison, untreated radiata pine stakelets can severely decay in as little as six months.
In the field, durability testing is being done on accelerated decking and L-joints made from thermally modified tōtara and lusitanica as per international testing protocols.
Both species have been in field testing for about two years. Tests can take anywhere from five to 10 years and non-durable wood such as radiata sapwood will fully decay much quicker.
While testing in both the field and fungus cellar is ongoing, early results are positive, Rosie said.
“We’re finding that it increases the durability of both the sapwood, which isn’t durable, as well as the heartwood which is.”
Thermal modification is a well-established method in which timber is heated to high temperatures without oxygen. The process enhances wood dimensional stability – meaning it shrinks and swells less with changes in moisture content. A high degree of modification can increase wood durability making it more likely to be suitable for exterior uses like cladding and decking.
Radiata pine is not naturally durable but has been successfully thermally modified to increase durability and this product has been commercialised for cladding.
Scion interim portfolio leader (Trees to High Value Wood Products) Elizabeth Dunningham, said it was important to have data about indigenous products and trees to make good planting and product development decisions, as existing data was limited. She said there were also strategic reasons for choosing tōtara.
Scion is directly involved with Taitokerau Māori Forestry Inc. as partners in the Tōtara Industry Pilot. This resulted in new markets for the farm-based tōtara, with the view of developing a regional industry.
Taitokerau Māori Forests chairman Ernest Morton said the group supported Scion’s work: “Tōtara haemata (lofty leader) he rākau rangatira o te ngahere (chief of the forest).”
The Farm Forestry website states that timber from relatively young naturally regenerated totara trees from farmland is an excellent native softwood timber. Relatively easy to mill, dry, work and finish, it is suitable for all interior uses, particularly feature linings, joinery and furniture.
The Forests Amendment Act covers naturally regenerated totara trees on farms. Development of an industry based on the commercial-scale use of this native timber tree is seen as a practical way to encourage the integration and management of the regeneration on private land within existing largely pastoral land use.
