Soil scientists find new virtues in willow trees

Press, 9 March 1984, Page 18

Soil scientists find new virtues in willow trees

From the Ministry of Works and Development

Willows have been grown in Neto Zealand for the last 140 years, helping to train the course of rivers and providing protection for Stop banks against flood damage. Yet it is only relatively recently that their potential for other uses has been exploited. Most of this development in breeding and selecting willows and poplars (another genus of the same Salicaceae family) for New Zealand conditions has been undertaken at the Ministry of Works and Development soil conservation centre at Aokautere, near Palmerston North.

Mr Chris van Kraayenoord, who heads the scientific group that researches plant materials, acknowledges that the primary purpose of willows is soil conservation. { ‘But we are always looking to see if the trees can have another use,” he says. His group’s quest is now producing, tangible results. Willows have now been shown to have value for providing shelter, streambank protection, arresting hillside erosion, emergency fodder for stock in times of drought, yearround bee fodder, material for basketry, for cricket bats and as a source of biomass energy. Another particular merit of the willow is that it is reddily-propa-gated from cuttings. Trees grown from cuttings from a single parent tree belong to the same clone and retain identical

characteristics over generations of propagation. So trees with the most desirable qualities can be easily produced. The inherent danger is that if disease strikes it can do widespread damage — always the penalty of a narrow selection — and consequently it has become necessary to widen the range of willows and poplars that are genetically different f

Development is usually a laborious process and no less so with poplars or willows. “All new willow and poplar clones are selections from our own breeding programme. Each year we produce several thousand seedlings, then we have our first screenings for growth and disease resistance. After three or four years, we select only the more promising for field trials throughout New Zealand in different climatic and soil conditions,” says Mr van Kraayenoord. “We may send out 70 different clones for field trials and eventually end up with half a dozen or even one. We really should then take the development of the whole tree through to maturity but we are generally in a hurry to control erosion so we often release material after only seven or eight years of growth and field testing.”

According to Mr van Kraayenoord’s records the first willows introduced to New Zealand were two weeping willows brought to Akaroa in 1840.

They came from the island of St Helena near Napoleon’s grave and were the forerunners of more which came from the same parent trees in later years. They were the source of all the weeping willows planted along the Avon River in Christchurch. Other stock was also planted near the early settlements in Northland, Hawke’s Bay and along the Wanganui River. About the time of World War I no more than 15 species of willow were being propagated in New Zealand.

Many more were subsequently imported apd it was not until Mr van Kraayenoord carried out a national survey of willows and poplars in the 1950 s and early 1960 s that it became known just how

many species were in the country. “Then we began to turn our attention to developing better varieties for river control and hill country planting, and we now have something like 300 different clones available for trials,” he said. There are three types of willows — tree, shrub, and osier or basket willow (in descending order or size). Tree willows, with their single stem, have a wide spreading root system, . making them ideal for erosion control in unstable hill country and for shelter. Shrub and osier willows have multiple stems and pliable shoots and are especially useful for river bank protection and river training. Several are able to withstand a harsher climate and browsing depredations and are also used in revegetating North and South Island high country. Willows originally used for riverbank protection; although no doubt the best in their day, were often far,from ideal, tending to break during flooding because of their brittleness. They were also too palatable to rabbits and possums. However, through a careful programme of breeding and selection and field trials the strains have

undergone significant improvement.

This has been helped by the process oftissue culture of poplars and willows, developed in collaboration with the Plant Physiology Division of the D.S.I.R. and subsequently applied on a practical scale in the centre’s laboratory at Aokautere.

Speed of propagation by this method is quite remarkable. In one year alone the centre has generated 28,000 plantlets from a few cuttings of a poplar introduced from Korea, thus saving five years in propagation. “We were the first laboratory in New Zealand to use tissue culture for the rapid multiplication of hardwood species and one of the first in the world to have produced large numbers of poplars on a commercial scale,” said Mr van Kraayenoord, with some pride. Within the last two years the soil conservation centre has been releasing cuttings of willows suitable for basketry and for bee fodder. The centre has selected and developed willows that flower in rotation right throughout the summer and are thus a continuing source of nectar and pollen.

Several willows have been selected as suitable for emergency stock fodder during droughts.

Other willow selections when grown in one or two year rotation produced the greatest biomass volume, in terms of dry matter per ha per year, of any tree outside the tropics and are a potential source of liquid fuel. Several of these New Zealand selections are now tested in other countries for energy farming. But the principal beneficiaries of the centre’s research are the catchment boards and other Government departments. While the centre supplies nucleus stock, catchment authorities throughout the country grow their own stock in some 250 hectares of nurseries. They annually plant l¥z million rooted and unrooted plants.

What direction will research take in the future?

“We will continue doing breeding trials until we are satisfied we have a sufficiently large range of disease resistant poplar and willow clones for New Zealand conditions, that thrive best in different regions, and are most suitable for the different purposes,” says Mr van Kraayenoord. “Although we have several bitter multi-stemmed shrub and osier willows we do not yet have a bitter tree willow which resists opossum browsing. The problem is that the bitter shrub willow and the tree willow have different chromosomes so that there is an hereditary incompatibility to be overcome.” “We have tried to produce a bitter hybrid and although success was achieved in breaking the incompatibility barrier, the resulting hybrids were not bitter, but we shall keep on trying,” Mr van Kraayenoord says.