A review of kowhai (Sophora spp.) and its potential for commercial forestry

Abstract

Background: Demand for imported sawn timbers in New Zealand has increased over the last decade, reflecting the lack of New Zealand-grown, naturally durable timber in the domestic market. Therefore, a market opportunity exists for sustainably grown, naturally durable timbers in New Zealand for specialty applications. Kowhai (Sophora spp.) are New Zealand native tree species, known for their bright, yellow flowers and reported to produce coloured, naturally durable heartwood.
Methods: Information on kowhai was collated from literature, focusing on their potential for commercial forestry. The taxonomic relationships, species descriptions, establishment, and growth rates of kowhai were examined, along with timber properties and historical uses, as well as medicinal applications. The review identified potential market opportunities for kowhai and key areas for further research.
Results: Kowhai refers to eight different Sophora species that are endemic to New Zealand. Kowhai is easily established and the different species hybridise readily. While growth and form of kowhai varies with species, site, and management, examples of straight single-stemmed trees and annual diameter increments exceeding 20 mm have been found. Kowhai timber properties might be comparable to those of teak (Tectona grandis L.f.). Kowhai contains alkaloids, a class of compounds used in pharmaceutical applications. The species have been used for timber and traditional medicine by Maori in the past, while European settlers used kowhai for their durable and flexible timber.
Conclusions: Kowhai could be established as a sustainable, domestic source of high-quality timber and substitute imported specialty timbers in New Zealand on account of their natural durability, strength, stiffness, colour, and density properties. The residues could support a secondary industry, as a source of alkaloids for pharmaceutical applications or natural dyes. Key areas that require further study include growth rates and silviculture, mechanical timber properties, machining/processing characteristics, natural durability and cytisine levels in kowhai, as well as the cultural, economic, and ecological framework required for a commercial kowhai forestry industry. Lack of literature on, and expertise in the use of native timbers in general are barriers to promoting native species for commercial forestry in New Zealand.