Feature

Death In The Forests

Our forests are dying, and this decade may be the last in which effective action can be taken.

by Vicki Hyde

"Our forests are emptier today than (they were) 100 years ago and, if we don't act now, they'll be a lot emptier in 10 year's time," warns forest ecologist Steve King.

The reason? The large number of introduced animal and plant species that are playing havoc with New Zealand's native forests. Deer might make great hunting and possums may look cute, but they are doing far more damage to fragile native forest ecologies than most human colonists.

Frantic campaigns by botanists, biologists and environmentalists have achieved a stay of execution, rather than actually saving native forests outright.

"We shouldn't kid ourselves that we've saved forests like Pureora," Mr King says. He has seen the diversity of bird species and plant species in forests like Pureora and Waipoua "go down the tubes" in recent years. While these forests may have been saved from the "ravages" of loggers, they remain prey to deer, goats, possums, wasps and old man's beard.

Possums kill the canopy layer of trees such as rata and kamahi by eating the leaves and shoots. They seem particularly drawn to the native trees, eating those in preference to many others. In some rata-kamahi forests, up to 40% of the rata trees have been killed in this manner.

"There's an awful lot of trees dead or dying," notes Mr King.

Holes in the canopy layer give other plants a chance to move in, particularly exotic plants like old man's beard or the fast-growing ginger plant.

Forests Failing

"Complete forest collapse can and does occur," says Dr Ian Payton of the Forest Research Institute. He has seen some areas where the canopy trees have died and everything underneath has followed suit. Rata, kamahi and pohutukawa are particularly vulnerable, with native hardwoods such as fuschia, mahoe and wineberry also experiencing problems. Fortunately for the kauri, it is not palatable to possums or other forest browsers.

"In the north, a complete canopy collapse is unlikely," Dr Payton believes. Possums have only just made it to the far north, spreading out from the 138 central liberation sites where they were released. In the last few years, the fight to keep possums out of the Coromandel forests has failed and the whole country is now covered with more possums than sheep. They aren't the only threat to native forests, however.

Deer, goats, thar and chamois eat away at the ground level, reducing the diversity of species and contributing to erosion in steep areas. They also make it difficult for regeneration of the forest, particularly as New Zealand natives evolved in the absence of ground-browsing animals and so have no built-in protection.

The reduction of ground level plants can have its effects on the larger trees above. Less water is retained, the soil dries out more quickly and seedlings have a harder time getting established.

Deer, like possums, have preferential tastes when it comes to plants. An area with the most desireable foliage may be quickly eaten out, but the resulting damage to the forest as a whole may take decades to become obvious.

Dr Payton sees goats and chamois posing a real threat to the environment. These herbivores are gregarious and rather sedentary, he explains, so they tend to stay in one place and graze it to extinction. As their preferred habitats are on steep broken terrain, the grazing pattern leads to increased erosion and soil friability.

Tussock Land At Risk

New Zealanders don't often think of wallabies as imported pests, but the six species roaming the New Zealand landscape have contributed to difficulties in getting forests to regenerate.

On the in-shore islands of Kawau and Mototapu wallabies have severely depleted the understorey of the forest and only remnants now remain. The best thing to be said for these marsupial imports is that they don't affect tawa or rewarewa forests.

Wallabies and their smaller counterparts have had a major impact on tussock grasslands in Canterbury and Otago.

"Rabbits can reduce tussock grasslands virtually to deserts", Dr Payton observes. The burgeoning rabbit population in the Mackenzie Country is causing real problems there, particularly as the rabbits appear to be avoiding recent drops of poisoned bait.

Hare populations are growing too. The reduction of deer numbers in alpine areas by helicopter hunting has helped this. As the deer numbers fall, the hares move in.

Wasps vs Birds

In the top half of the South Island the threat is coming from an even smaller import -- the German wasp. These wasps are competing heavily with native birds for honeydew, an important food resource. Dr Henrik Moller, a wasp expert with the DSIR Ecology Division has studied the competition for honeydew in the Canterbury, Marlborough and West Coast beech forests.

There are ten times more wasps per hectare than found anywhere else in the world, he says. The wasps are drawn by the high-energy food resource that the honeydew represents. Five million honeydew tubes per hectare provide what Mr Moller calls the "turbocharger" of the forests.

Studies of the biomass per hectare -- the total body weight of various species -- are revealing. Bird biomass per hectare adds up to just over 900 grams; stoats contribute 29 grams of biomass. With an average of 12 wasp nests per hectare in the beech forests, the mass of these insects hits 2,900 grams, or three times that of the total bird biomass.

In some areas, where almost 50 nests per hectare are found, the wasp biomass shoots up to an overwhelming 10,000 grams. Birds are not able to compete with this and, consequently, are forced out of the wasp-infested regions into less suitable areas.

There has been some success in the DSIR eradication programme using natural wasp pests and this may hold some hope for the future.

Dealing with plant pests such as ginger plant or old man's beard is more difficult. Ironically one of the most effective ways is clear felling, as it reduces the seed source of the tree-smothering vegetation. This is not usually an option in native forest preserves.

A Necessary Evil

Those attempting to deal with the threat to our forests can be stymied on other fronts. For some years 1080 poison has been advocated as the most cost-effective, efficient means of controlling the burgeoning possum population. There is strong opposition to this from both professional hunters and the public.

The scale of the problem is one thing that is not often recognised. At their peak, hunters were trapping 3-4 million possums and taking around 70,000 game animals per year. With over 70 million possums in the bush, trapping no longer offers an effective form of control.

"Chemophobic reactions are one of the greatest obstacles to protecting our forests," says Mr King. Dr Payton sees a lack of public or political will and inadequate resources as presenting further obstacles in preventing catastrophic damage to native forests. These two factors, and the development and use of appropriate technology, are vital if the forests are to be saved. Can it be done?

"Without all three of these, the answer is most definitely not," concludes Dr Payton.

Vicki Hyde is the editor of New Zealand Science Monthly.