Wattle silviculture and utilisation of tannin extracts

• Chinese Academy of Forestry, China
• Chinese Academy of Forestry, China

China suffers from substantial, and increasing, shortages of tannin extract, fuelwood and building timber. Some 40 tannin-extract factories in China currently produce more than 30,000 tonnes of extract a year, but the material they rely on is low in quality, laborious to collect and expensive to transport. Moreover, apart from being inadequate in quantity, the product is mainly hydrolysable tannins, which are unsuitable for some uses, and so China imports large amounts of condensed-type tannins.

The Australian black wattle (Acacia mearnsii), which is adaptable and fast-growing, yields suitable tannin and has already been introduced into China. It grows in more than 10 provinces there, but only in small areas that currently total 3300 ha, and efficient commercial operation requires much larger plantation areas. However, the genetic history of the early introductions is unknown as the seed came from a number of poorly documented sources, including exotic plantations, and this project seeks to establish a pool of well-adapted genotypes for seed production. The first objective will be to identify the most productive provenances of black wattle which will adapt to a range of environments in China.

Black wattle has a very wide-ranging natural distribution, and evidence from other widely occurring species suggests that there should be substantial variation within the species in growth rate, in adaptation to drought and low temperature, and in wood and bark characteristics. Provenance trials will assess the potential value of this variation for increased productivity, by field-testing seed from a number of sources over a range of environmental conditions in China. Field collections of seed in Australia may continue for the duration of the project, but initial supplies of seed should be sufficient to establish some replicated trials in the first year.

A second objective is to determine the capacity of related Acacia species to produce tannin and other wood products in environments unsuitable for A. mearnsii. Parallel species and provenance trials will investigate several species, including A. dealbata, A. decurrens, A. irrorata and A. silvestris.

The third objective is to produce purified extracts of uniform quality for specific purposes and ultimately to develop wood adhesives from them. Australia has developed a high-quality wood adhesive based on wattle tannin extract which has been used for more than 20 years, but currently imports the uniform-quality extracts required for this purpose. Recently, CSIRO developed an ultrafiltration method, using special membranes that can purify condensed tannin

Extracts and partly concentrate them, yielding a product of uniform high quality. Project workers will attempt to establish appropriate membranes and conditions for Acacia tannin extracts. They will also determine the best methods for further concentration - for example, by spray drying - since ultrafiltration produces material of about 20% solids content, about half the strength required for use in adhesives.

The economic success and efficiency of this project will depend on the maximum production of highest-quality tannin extract, fuelwood, charcoal and construction timber. Accordingly, the choice of seed, land selection and forestry practices (such as cultural methods, environmental protection and management) should be closely integrated for optimum utilisation. Australia could offer advice on the development of mathematical models to aid decision-making in these fields.

Identifying the best-adapted species and provenances should lead to substantial improvement in wood and bark yields and reduce the incidence of gummosis disease. Introduction of a wide range of well-documented genetic material will form a reliable basis for future tree improvement and breeding. In addition to increasing the quality and quantity of tannins, the research should assist global production of adhesives from renewable resources.

Black wattle is also an important source of tannin in other countries. Results of this research should have application in places where black wattle is already established, such as India, and where there is potential for establishment, such as Indonesia.