Foresters, and trees, say thank you

Press, 14 December 1985, Page 19

Foresters, and trees, say thank you

Country Diary Derrick Rooney

One of the pleasant side effects of the recent prolonged rainy weather in inland Canterbury has been an abnormal flush of growth in trees in many parts of the plains and foothills. Despite rural grumblings about prices and prospects for livestock and some crops — including timber - there were happy faces at a recent forestry field, day. Rarely, according to some experienced farm foresters who were there, has there been a growth spurt in Canterbury trees to rival that of the last month. ’

“There are," confirmed a Forest

Service spokesman, “a lot of happy foresters about.”

The rains have not only been generous — up to 200 mm in some foothills areas in the last four weeks — but have come at wellspaced intervals since mid-spring. As there have been few drying winds in the last few weeks, the full benefit of the extra moisture has been realised.

In a normal year peak forest growth is expected during a twomonth period from late September to late November, after which rising temperatures and lessening soil moisture cause growth to slow. This year growth has shown no sign of slackening. According to one experienced farm forester, the central plains and foothills have had the best spring for tree growth for about 12 years. The Forest Service says it has been a “good spring” and that all its plantings are doing as “well as can be expected — or better.” sources say, with V

less reserve, that growth in some shelter and specimen trees has been phenomenal. They are, however, not yet counting their blessings. A change to the “normal” desiccating nor’west winds now could place severe stress on trees which are covered with soft growth; and if the damp weather continues there may be equally damaging outbreaks of fungal diseases in some areas.

Damage from diplodia — a disease which enters pines through “drought cracks” or wounds — has already occurred in numerous plantations where trees had been under stress as a result of last summer’s drought. This disease causes dieback of the growth tips, and can be fatal. It is widespread in Canterbury, though it is rarely very troublesome. It often shows up in firewood as a blue stain.

Forest pathologists also expect cyclaneusma, a needle-cast fungus, to show up soon as a result of the

damp weather. This disease affects the older needles of conifers and gives the tree a yellowish look. It can be confused with “magnesium transfer,” a physiological disorder which also causes yellowing but is rarely seriously harmful. A number of other pathogens may affect plantation trees but the big fear is of an outbreak of dothistroma, which has so far been found only in isolated places in Canterbury. Forest pathologists rule out any likelihood of a widespread outbreak of dothistroma on the plains but-say there is a possibility it may become established in the foothills if the summer continues mild and damp. Dothistroma can severely reduce the growth rate of young pines, and is a problem in the North Island and on the West Coast. It is usually associated with a humid climate, and occurs in inland rather than coastal areas. Until last year it had not been identified in Canterbury, but small outbreaks have been

identified at Hanmer Springs, Ashley, View Hill, Geraldine, and Raincliff, near Pioneer Park but not in the park. The behaviour of the disease in Canterbury has puzzled pathologists because radiata and Corsican pines, two of the most vulnerable species elsewhere, have not been affected here. At Hanmer Springs, Ashley and South Canterbury only ponderosa pine has been affected, and all the diseased trees have been felled and their slash and litter burnt. At View Hill, dothistroma infection was found in a stand of Pinus attenuata. This species, which has little value as timber, is regarded as the most susceptible of all pines to dothistroma. It had been planted at View Hill as an “indicator plot.”

Dothistroma and other diseases which affect shelter and plantation trees are not always readily identified by the layman. The Forest Service says that if there is a [iroblem in a shelter belt or woodot the owner should seek advice

from a forestry extension officer in the nearest conservancy office. Useless but interesting information department: browsing animals such as deer and goats inflict severe damage on some woody plants but on a broad scale the hungriest herbivores of all are insects, which eat about one-twen-tieth of all the annual growth of trees and shrubs. Sometimes the 5 per cent that insects eat has a severely detrimental effect on the ability of plants to reproduce themselves.

It also appears that individual trees vary in their palatability to insects. Two university researchers in Arizona recently made a study of two groups of 145-year-old piriyon pines. Half were normal, treelike specimens; the others were stunted and shrubby. Both groups were infested with a moth whose larvae chew their way inside the stems and cones. The researchers found that on

average the shrubby trees suffered 360 per cent more damage than the normal-looking ones. The shrubby trees reacted to the damage by growing more slowly, but here’s the interesting , bit: the insects ruined the sex life or the trees.

This species of pine has its male “flowere” on side shoots and its female cones at the tips of the branches. Because the grubs destroyed the tip shoots, heavily infested trees had no female cones. No female cones, no seeds; no seeds, pretty soon no more trees. This could be regarded more kindly as an example of a natural defence mechanismat work: the trees take out insurance for the future by reproducing themselves only from individuals which seem to be less palatable to predators. Trees have various ways of achieving- this, ranging from genetic mutation of individual branches to chemical tactics.

One of the avoidance mechanisms used by trees is to grow tough leaves-;

Dr Michael Raupp, of Maryland University, recently published a study of a small beetle which feeds on willow leaves. For a month he fed 40 beetles on tender young leaves, and 40 on tougher, mature leaves. At the end of the period the insects fed on tender leaves had, on average, jaws 13 per cent longer than the others.

The beetles on the tender young leaves munched their way through an. average of 0.25 square millimetres a minute but those on the tough leaves could manage only 0.17 square millimetres in the same time.

Although the beetles on tough leaves compensated by feeding more frequently, or for longer periods, they suffered in consequence through having a likely shorter life span as a result-of the greater exposure to predators, such as birds. Furthermore, as the number of eggs laid, by female beetles is largely governed by the amount they eat, the tougher the leaves, the fewer'the beetles. .//