Tree-spread survey shows little cause for panic

Press, 21 October 1989, Page 26

Tree-spread survey shows little cause for panic

Country Diary

Derrick Rooney

MAN (and his animals) have exercised a huge influence over the appearance and ecology of the New Zealand landscape. This is a truism, often repeated, that is also, as it happens, true. But what would happen if people, domesticated animals, and all the feral animals were removed for 50 . years? The biggest change to be seen, according to two Christchurch scientists, would be a huge increase in the number of woody plants — especially in the high country, many parts of which would be “invaded” by foreign conifers. Conifers, introduced for shelter or timber production, thrive in the New Zealand high country and in some cases will colonise at altitudes above the natural timberline. Their spread in some areas has brought about changes in land use; in other places concern has been expressed about their impact on the natural environment; and in some parts of the country — such as the central plateau of the North Island — large sums of money have been spent in an effort to eradicate wilding conifers. Concern about all this recently led the Forest Research Institute to make a survey of tree spread, with the aim of pin-pointing critical areas and identifying the species most likely to pose a threat.

The findings suggest that the problem may nqt be as bad as many people believed. Certainly, according to one of the scientists, Nick Ledgard, there is no 1L . .

need to assume panic stations. Trees, he says, have been in the country for more than 100 years — and the amount of land "invaded” is “really very small.” Furthermore, he says, the pattern of tree spread is completelly predictable — and easily controlled.

Mr Ledgard and a research team made a thorough survey several years ago of tree growth — and spread — throughout the South Island high country. Since then he has also looked at spread in the North Island, particularly in the Tarawera area, where conifers spreading across the otherwise effectively barren volcanic terraces pose a threat to tourism values. The Department of Conservation has voted funds for a conifer-eradication study in the Tarawera area. Other areas where conifers are causing concern in the North Island are the Karioi/Waiouru region of the central plateau, and the Kaweka Range. Critical conifer areas in the * ■ ■

South Island are parts of the Marlborough Sounds, the Red Hills, the Branch/Leathern catchment; Molesworth; the Amuri Range (where trees seeded from the Hanmer State Forest have forced a change in land use from pastoralism to forestry); Queenstown; the Mackenzie Basin; and the Blue Mountains and Mid Dome in Southland.

The spread of introduced trees in these areas is causing concern for a variety of reasons, says Mr Ledgard: they threaten biological values, and can force social, cultural, or economic changes. The problem species all originated in the Northern Hemisphere and are colonising New Zealand vigorously— much more vigorously than they grow on their native shores. Lodgepole pine, which can establish itself well above the natural treeline in New Zealand, is considered to be the major problem at higher altitudes. Mr Ledgard identified the other “threatening” species as (in descending order) Scots pine, Douglas fir, larch, and Corsican pine. Radiata pine has the ability to colonise at lower altitudes.

His colleague, Lisa Crozier, has been studying the growth of conifer seedlings, both in the wild and in the research nursery at Rangiora, to determine whether they can be controlled by grazing or herbicides. She has also been studying the germination of conifer seeds, in the hope of establishing the probable “life” of conifer seeds — and has already made at least one significant observation: that seeds

scattered naturally by the wind live several times longer than those sown in nursery plots, and that “seed life” is longer in the drier regions. In the nursery trial, no seeds germinated after 18 months; but at Ahuriri, in the Mackenzie Country, scattered conifer seeds were still germinating after three years. This is significant information because it indicates how long “seedling patrols” must continue after unwanted conifers

have been removed. The two scientists presented their findings at a seminar last week at the Forestry Research Centre, Ham. According to Mr Ledgard, the high-country research has shown that the timing, direction, and to a lesser extent degree of conifer spread can be predicted. Naturalised conifers, he says, produce no viable seed until at least the age of five years for lodgepole pine, and 10 years for

others; the spread therefore occurs in a “staircase” fashion.

With perhaps one exception (Douglas fir, which can establish in beech forest because its young seedlings are shade tolerant) there is ■ little or no spread of conifers where there is a wellestablished vegetative cover — such as forest or improved pasture. Periodic mob stocking has proved to be an effective way of containing conifer spread in tussock pastures.

The sequence of spread of any conifer species can be predicted if its seeding age is known, says Mr Ledgard. To avoid conifer spread, he suggests choosing the correct species forthe site, placing it correctly (avoiding “takeoff” sites), and regularly monitoring the surrounding land, particularly for a radius of 500 metres downwind.

“Take-off sites” is an expression Mr Ledgard uses quite a lot; it refers to situations, such as on a windward slope, where “windlift” can carry seeds for considerable distance. Significant spread is rarely known, he says, from trees sited well down the lee side of a gully. Despite the known ability of some conifers to spread, Mr Ledgard is optimistic about prospects for high-country forestry. There is, he says, enormous potential for trees in this country; and while it is true that trees present potential problems as well as benefits, the problems usually arise through lack of management.

He is not enthusiastic about suggestions that some areas of land, of very low pastoral productivity, should be allowed to become natural conifer forests.

This, says Nick Ledgard, is “forestry by default.” He adds: “You end up with the wrong species, the wrong density, the wrong age structure.”

On the subject of trees, a recent report from the D.S.I.R.’s Plant Physiology Division on the damage to trees in Northland by Cyclone Bola (in March, 1988) makes interesting reading. ■ Sfe. .-. ■

The report details the observations made at 68 sites surveyed within the area of greatest damage, north of Whangarei. What is interesting about it is that while the damage to introduced trees in the surveyed area ranges from 1 per cent for redwoods to 100 per cent for walnuts and oaks, the indigenous tree species all escaped with relatively light damage. The pohutukawa, the familiar coastal tree of the far north, suffered less than 1 per cent recordable damage. Ngaio trees suffered about 10 per cent damage.

Established poplars and silver wattles escaped with light damage — less than 1 per cent in most cases — but eucalypts and radiata pine suffered about 30 per cent damage. Most cypresses were damaged. Weeping and crack willows were badly damaged, but the matsudana willow hybrids generally escaped lightly. Of the tree crops, the most badly damaged was the tamarillo; more than 90 per cent of the young plants were toppled or badly damaged. The lessons for tree planters in this, says the report, are to obtain, good stock and use good planting techniques to promote root systems that develop evenly around the tree; manage trees to preserve good form and a good balance between roots and crown; and to plan for the replacement of defective and overmature trees.

"Organised felling and replacement may be more tidy and rewarding than clearing up after a storm,” says the report. Good