Smuth's milkwood

Forest and Bird, Volume 19, Issue 3, 1 August 1988, Page 37

Smuth's milkwood

by

Victor May

An arranged marriage

here are some uniquely interesting New Zealand endemic plants growing on the Three Kings Islands located approximately 53km northwest from Cape Maria van Diemen in the Far North. One of these is Streblus (Paratrophis) smithii, or Smith's milkwood, a member of the Moraceae family. This is a large family of trees and shrubs with milky sap, mainly tropical, and including many useful plants such as, for example, the figs, the mulberry, breadfruit and jackfruit. In the wild Smith's milkwood is confined to the Three Kings Islands. It is plentiful there but, because there are no fruits in summer, when the islands are normally visited, nobody has recorded seeing them except the plant's scientific discoverer Cheeseman in 1887. Now we have experimented to bring male and female together in cultivation and have realised that the heavy crop of bright red drupes transforms a sombre tree into one of our most spectacular native plants. In 1945 Professor Geoff Baylis brought back to the New Zealand mainland some live material. A cutting from this produced the tree that has grown at Auckland’s Mount Albert Research Centre (MARC) ever since. This tree, a female, now has a twin trunk, joined for the first 10Omm, with a combined overall circumference of 110cm. The tree sprawls over an area measuring 8.25m x 3.66m and, apart from a network of bare branches immediately above the trunk, carries a dense canopy of evergreen leaves supported by a subjacent structure of near-horizontal branches. At the perimeter the young branchlets touch the ground, otherwise the canopy averages, say, 1.25m off the ground with one high point 2.45m. The site the tree occupies must be regarded as optimum as to soil and situation and its growth should not therefore be seen as typical. For instance there was also a seedling brought back from the Three Kings Islands which has grown at Campbells Bay under more natural conditions in competition with pohutukawa and wharangi. It remains like most wild specimens — a sparse slender undershrub. It is male. The Smith’s milkwood we have in our garden at Huia, near Auckland, is of a cutting from the tree at MARC — our tree, therefore, is also a female. In October 1985 it was artificially fertilised with pollen from Streblus (Paratrophis) banksii, also from our garden. This pollen was used as an ex-

periment because, at the time, we were unaware that a male S. smithii existed in Auckland. The female flower spike has 2 rows of closely spaced ovulate segments attached to an axis. Each segment has a tiny flower. After pollination the flowers withered and the spikes remained dormant, intact and unaltered in appearance, throughout the summer months. Then, come March/April, there was a flush of growth — the axis swelled markedly and embryo drupes appeared at the tips of the segments. When fully formed the spikes are up to 145mm in length carrying as many as 50 drupes. However spike size varied considerably. To our delight a large set of fruits was produced. They were green at first, changing through yellow to bright red in June. Subsequently I was to learn of the male Smith's milkwood tree in Geoff Baylis’s garden at Campbells Bay. In general appearance the difference between male and female flowers is that the male is a long, slightly tapering, flexible catkin; whereas the female is a short, parallel, rigid, single spike. Pollen from the male is shed readily and in fairly large quantities. The catkins vary in length up to 280mm and are sometimes branched. In October 1986 pollen from the Campbells Bay tree was used to fertilise our tree at Huia. There was the same long dormancy period before the first signs of success appeared and the same rapid development to bright red fruit. These purebred Smith's milkwood fruits have been distributed (August/October 1987) in the hope that this tree will be more widely cultivated. In a germination test, seed sprouted on 25/6/87 indoors; 26/7/87 outdoors. On 8/8/87 the first of the new seasons flower spikes appeared. The Huia tree is being trained as a weeper, with a single trunk attached to a stake until such time as it becomes self supporting. This way the fruits show to best advantage. The ripe fruit is taken by birds and needs to be protected in perforated plastic bags if required as a seed source. Perhaps seed from gardens might be the means by which Smith's milkwood re-establishes itself in the mainland bush, for it is generally believed that the Three Kings endemics were plants once more widespread that became restricted to the islands during the Ice Ages yt