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Out-of-control plants

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Author Topic: Out-of-control plants  (Read 114 times)
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Having fun in the hills!

« on: October 23, 2014, 09:01:40 pm »

from The Press....

Busy as a bee — garden inhabitants keep looking over the fence

Everyone knows New Zealand has a runaway wilding pine problem. But what
about sycamores, cherries, rowans and palms, the next wave of weed trees?

By JOHN McCRONE | 1:46PM - Saturday, 4th October 2014

BUSY AS: Honey bees working the gorse hedge rows near Ellesmere.
BUSY AS: Honey bees working the gorse hedge rows near Ellesmere.

WHEN gorse was first introduced to Canterbury for stock fencing in the 1830s, one fond owner apparently sat up nights with his shotgun to prevent the precious seedlings being filched.

Massey University ecologist Dr Jill Rapson chuckles. Yes, hindsight is a wonderful thing.

Because New Zealand is now the weediest nation on Earth. Half our plants found in the wild are exotic invaders rather than native species, says Rapson. And worse, we are still on the bend of an accelerating curve. The onslaught of the weeds is only just getting going.

Rapson says it takes time for intruders — especially trees, which are a particular story for New Zealand — to get a hold. There is a lag of as much as a century between being planted in some garden to becoming a pest rampaging across the countryside.

Wilding pines are the problem everyone knows about. But now marching up behind is the following wave of sycamores, rowans, cherries and ash. A motley crew of ornamental escapees.

In Canterbury's Porters Pass, white flowering hawthorn has reached critical mass. Around Nelson, dark pink Taiwan cherries have just been fingered as a new worry. Tourists love the colour of Tekapo's rowan stands, but they are getting out of hand too.

Turn your back for a few years and any spare gully or hillside will soon be taken over by something new which is not meant to be there.

Rapson, whose earned her PhD tracing the arrival of browntop grass back to stuffed mattresses dumped off a Scottish boat in Northland in 1854, says people think pest plants are a legacy issue.

Pines, gorse, lupins, broom. A smallish list of familiar names that ran away on us and now we are wise to them. However, ecologists can see the country's future problems just queued up.

“New Zealand has about 2,000 native species and there are over 20,000 exotic plants sitting around in back gardens and parks, waiting to nip over the fence and naturalise in the wild.”

Even if only a percentage have what it takes to thrive, says Rapson, you have to wonder what the New Zealand countryside is going to look like in another 50 or 100 years.

Lincoln University ecologist Dr Jon Sullivan agrees we are still slap in the middle of the changes. He says even with herbaceous weeds, there is more of a lag period than the public realises.

It takes decades to build a seed population, reach critical mass, but then the spread goes exponential. The plant is popping up everywhere.

“The things that are new to Canterbury's pest list, like purple loosestrife, the bad wetland weed, first showed up in the wild in the 1950s and we're only just starting to worry about them,” says Sullivan.

When you look at gorse, thistles or pines, these are the farming associated pests that were introduced by the early settlers. “They were used for shelterbelts and fodder, or came in with the soil used as ballast on boats or whatever.”

But Sullivan says the next wave is going to be characterised by the ornamental trees and plants that more recent generations of Kiwis have used to fill their gardens. Take palms for instance.

“Auckland has a real problem with those. We're already seeing some of the old time favourites like Bangalore palms and windmill fan palms getting going. Phoenix palms are starting to roar in the wild too.”

“But gardens follow fashions and there have been so many new types of palms planted in the last few decades that eventually many other ones could be a worry. It's like a slow motion time bomb going off.”

Sullivan says New Zealand is notorious as a country generally transformed by invasive species. It has the largest proportion of introduced mammals and second largest proportion of introduced birds anywhere in the world.

At least the lid has gone on the exotic fauna. But Sullivan says the plant invasion is different in that the future invaders are here, brought in by our enthusiasm for anything with an attractive form or flower, and are simply taking their time to break out across the New Zealand landscape.

“In the wider Auckland area, there's close to three times as many exotics as native plants growing in the wild now. And there's no reason to think the rest of the country won't be like that soon. We can't stop it, because so many species are out there, already going or gone,” Sullivan says.

Some renegade Taiwan cherry saplings are this week's concern for ex-Nelson Landcare researcher. Dr Peter Williams. Even in his retirement, he is still chasing weed trees. And the spring flowering has suddenly made their extent obvious he says.

It is going to be a delicate business, Williams confesses. The source trees are mature specimens in just three backyards on Nelson's northern edge.

Blackbirds have been eating the fruit and pooping out the seeds across the local hillsides for a good few years. Now these slopes are throwing their own blossom festival.

“Taiwan cherries are a real problem in the North Island, but this is the first decent infestation we've seen in the South Island. With milder winters, they've taken off.”

Can the home owners be persuaded to chop down loved trees on their own land? Will the town-folk think wilding cherries are in fact an attractive addition to the landscape, doing no great harm, and so resist any council eradication programme?

Williams says it is like the rowans that are bringing autumn colour to the Mackenzie Country.

“I have a wonderful picture of the rowans spreading up the hill beside Lake Tekapo. Beautiful blue lake against lovely red trees. The rowans are taking over from the broom and matagouri scrub. Now you're starting to get them showing on the tourist posters and calendars as a feature.”

So gorse and pine are one thing. No-one much cared for these agricultural species in the first place. However, as our most favourite garden plants start to turn on us too, will Kiwis be galvanised to do what is necessary, Williams wonders?

First the analysis. What are the mechanics of this invasion? Why does New Zealand seem so particularly prone to weeds? Do its natives lack resistance? And how would it look if we let this “new New Zealand”, this internationalised species mix, find its own equilibrium?

Massey's Rapson says early settlers, inspired by a Victorian view of Darwinian natural selection, indeed thought New Zealand wildlife to be weak and impoverished. Drab forests of beech and fern creating a dark blanket across the land. They thought it ripe for improvement by the importation of more vigorous and showy species.

But there is no evidence that New Zealand ecosystems have some basic vulnerability, she says. Look around the world and a passion for gardening novelty has produced thuggish invaders everywhere. Often names that will surprise.

“I've just been to Western Australia where they have a massive weed problem with freesias if you can believe it. And hydrangeas have gone completely feral on the Azores islands — I'm telling DoC we need to get rid of them on roadsides of the West Coast.”

Rapson says New Zealand has even managed to export a few notorious weeds of its own. Flax now clogs the cliff faces of Cornwall. Pohutukawa is forming impenetrable tangles in coastal South Africa.

Dr Dave Kelly, biology professor at Canterbury University, says when it comes to colonisation by non-natives, recent statistical research shows that the size of the land mass is a major factor. Smaller islands are more likely to be swamped by imports. Proportionately, places like Hawaii are in an even worse state than New Zealand.

Australia, with its similar settler history, has about the same number of exotics gone wild — about 2,000 species. It is just that Australia is large enough for this to seem well diluted by the 20,000 natives it has of its own.

Kelly says the reason for New Zealand topping lists with half exotics is that foreign plants will flourish where they can get a foothold. And large tracts of New Zealand were first fired by the Maori, then systematically cleared by the Pakeha. “It's easy to take root where the landscape's been disturbed by human action, where it's suddenly open.”

After that it does come down to vigour, says Kelly. Native bush is generally slow growing with small seeds which are adapted to having to spend years developing in dim light before breaking through the thick canopy overhead.

BUT Kelly says the introduced plants and trees giving all the trouble were brought here in the first place because they were quick growers with bountiful seed production. In picking species that would fast fill a space, we were selecting the natural weeds.

Then as settlers brought the right seed-spreading birds with them too, and possibly because rabbit and sheep found the native vegetation less equipped with chemical defences to discourage nibbling, the scene was set.

Kelly says throw in a nice average climate, a lack of home nation insects and viruses to keep the invaders in check, and the reason for New Zealand being the weediest nation, about to get much weedier yet, is no great puzzle.

Landcare's Williams says the widespread land clearance for pasture also explains why it is weed trees and woody shrubs in particular that are the notable feature of the New Zealand story.

Elsewhere, a weed is normally thought of as something small, green and leafy. But New Zealand was once a solid wall of forest up to the mountain tree-line and would naturally return to that state given half a chance.

“When I first joined the DSIR botany division, a wonderful old chap who studied tussock grass said to me: ‘The problem Peter is wood. It all wants to be wood.’ That wasn't so clear to us back then, but he turned out to be right.”

Williams says this is evident with wilding pines. They would takeover the South Island high country from horizon to horizon if let go.

We, of course, imported the very species that grow fast on barren land and do well in our climate. They produce plenteous wind-blown seed that can travel kilometres on a northwester. And Williams says we not only created the extensive logging plantations but for a good while followed a policy of using conifers to stabilise scree slopes.

“Some of the biggest invasions of pines were brought about because of catchment boards planting them in remote places for soil conservation reasons,” says Williams.

It was only later that the scientists realised such erosion was natural, not a result of over- grazing. “Someone said hang on, the weathering on these scree rocks is an eighth of an inch deep. They've been sitting here like this for thousands of years.”

But by then the damage had been done. Pockets of seed trees had been established in even the most inaccessible spots.

Politics did not help. There was a decade of confusion after responsibilities for pines were transferred from the old Forest Service to the Department of Conservation (DoC).

Tenure review added to this with large tracts of land switching from farming to conservation park without enough money set aside for upkeep. Now a romping tide of saplings is having to be held in check all over the back country.

It is Pinus radiata and Pinus contorta but also Douglas fir that threatens. Williams says Douglas fir is emerging as a risk as it is more shade tolerant so can creep in and replace even native bush where there is wind damage or a river bed to ramble up.

“I can take you to river basins in Marlborough where there are just acres of the stuff coming up like hairs on a dog's back.”

So wilding conifers are the extreme problem. The easily identifiable enemy. But Williams says New Zealand now needs to be on the watch for the sycamores, hawthorns, rowans, palms — the whole swathe of trees that would quickly turn remote hills into exotic forest as well.

No single species will rival the pines, however the sheer variety of what could follow is the issue. “If humans and their grazing animals disappeared off the New Zealand landscape right now, within 100 years you would end up with this one bush that was a mosaic of species — exotic and native — just all jumbled together.”

CRUSADER: DoC’s Keith Briden who almost singlehandedly has been keeping South African boneseed from spreading away over the hills past Taylors Mistake.
CRUSADER: DoC’s Keith Briden who almost singlehandedly
has been keeping South African boneseed from spreading
away over the hills past Taylors Mistake.

PEST species can be controlled. It is just a matter of time, cost and knowledge.

Williams says at least biosecurity legislation was brought in during the 1990s to stem the flow of further plant species. “Before that, you could bring in pretty much anything that had leaves.”

Williams himself was part of the technical advisory group that drew up an accord with the Nursery and Garden Industry Association to limit the sale of the worst plant offenders.

He also worked on the DoC strategy on how to spend money wisely. A policy of trade-offs. Williams says if it is early enough in the invasion curve, it makes sense to have a total control strategy for a species — aim at eradication. But once wild populations are widely dispersed, it becomes about targeting the high natural value areas of the countryside.

Williams says there is some good news on pines as recently a “devil's brew” of herbicides has been developed which just needs to be sprayed as a patch on the base of baby trees.

Before workers had to get in on foot and cut a nick for poisons to work. Now helicopters with a long spray boom can take out even the last of the cliff-face stragglers that shed their seeds across whole valleys.

Reports are that the Ministry for Primary Industries will shortly take advantage of the new control methods and announce a national-level wilding pine management programme, putting money into something that has got beyond what regional councils can deal with.

When it comes to plant pests, vines are another emerging concern.

Lincoln's Sullivan says vines represent a special threat to native bush as if something like the rampant clematis, old man's beard, gets a hold, it can smother 30-metre-high trees.

There are many familiar garden species like banana passionfruit vine and Japanese honeysuckle on the loose. But for a while, the Bay of Plenty had a renegade kiwifruit epidemic too, says Sullivan, who was involve in the cleanup.

“In the early days, orchards weren't being good with their hygiene and just dumping reject fruit and cuttings into local gullies. Or it was being given to farmers to spread on the ground as stock feed.”

Luckily, because the source of the contamination was agricultural, the authorities got on top of it quickly, says Sullivan.

“The numbers were looking frightening 10 years ago. It was just getting into that real, go forward, exponential increase phase like wilding pines. But it's been effectively stomped on.”

When PSA came along in 2010, a wild kiwifruit population could have made the vine disease impossible to contain of course. So Sullivan says the experience shows exotic incursions can be dealt with, given the effort.

Sullivan says New Zealand has plenty of local volunteer weedbuster operations too. He is part of a weekend group, the Friends of Ernie Clark Reserve in Cashmere, where they knock back the sycamore seedlings to encourage the natives.

There are individuals like DoC's Keith Briden who almost singlehandedly has been keeping South African boneseed, a pretty yellow daisy bush, from spreading out of Christchurch's beach suburbs and away over the hills past Taylors Mistake.

When people are aware of the dangers, it can make a lot of difference, says Sullivan.

But what of the long-term? Like Williams, Sullivan feels New Zealand is being reduced to managing a bad situation. There are too many weed species lined up and so much disrupted countryside where they can take hold.

Then there are the unforeseeables. Will the public secretly favour a more colourful New Zealand with palms, vines and cherries running riot? There are the famous examples of stubborn guerrilla gardening like the sewing of bright lupins all along the main highway through Central Otago's Lindis Pass.

Against that is the drive to encourage natives. The purists are pushing for strict controls on the new rural subdivisions popping up around the country — covenants to restrict their plantings to indigenous species.

Well, the real purists like Rapson are not even so sure about that. Personally, she says, she would rather gardeners even in country towns stick to exotics unless they are willing to eco-source natives that have a proper genetic match to the locality.

“In Manawatu, people want to plant rimu in their gardens because they think it's helping. But if those rimus are sourced from Northland or Southland, and they shed pollen into my local bush, then they are polluting that bush. So if you live anywhere near a native area, I say plant exotics, because when I find them straying, I know to weed them out.”

This is when you realise how straightforward it has perhaps been with gorse and pines. The future debate over New Zealand's now compromised wilderness looks set to get complicated. All that is certain is we live in a weedy nation that still yearns to get a lot weedier.

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