New research says invasive radiata pine trees are spread more widely across New Zealand than previously appreciated.
The study suggested a levy on economic uses of invasive species would help pay for the costs of managing invasions and also suggested stricter regulations to protect vulnerable ecosystems.
The researchers, led by Dr Peter Bellingham of Manaaki Whenua - Landcare Research, set out to reassess the prevailing view, stemming from the late 1980s, that radiata pine is only a minor invasive species in New Zealand.
They undertook the first comprehensive review of where invasive radiata pine occurred in New Zealand and evaluated whether climatic conditions were suitable for it to invade nationally.
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The modelling showed that up to 76 per cent of the country's land area is climatically capable of supporting pine trees.
Plot and site data from the National Vegetation Survey database also showed that radiata pine occurred far more widely across New Zealand than previously thought.
Radiata pine has mainly invaded grasslands and shrublands, as well as some forests. It has also invaded ecosystems mostly below 1000m altitude, including revegetating landslides, down to sea level, where it was planted to stabilise sand dunes.
Notably, it has often invaded areas of lower-statured vegetation, and at least three classes of naturally uncommon ecosystems - geothermal, gumlands, and inland cliffs.
Researchers said that because of its rapid growth rates and its flammability, radiata pine was likely to alter naturally uncommon ecosystems, and could drastically alter forest regeneration.
Although management of invasive radiata pine was already widespread in New Zealand and other southern hemisphere countries, researchers said more proactive planning was needed, given ongoing large-scale afforestation efforts and potential future increases in an invasion.
Professor Euan Mason, from the New Zealand School of Forestry at the University of Canterbury, said the researchers did a good job identifying where radiata pine would grow and the instances where it was a wilding problem.
"Ability to grow and ability to reproduce are not the same thing, however," Mason said.
Natural regeneration of radiata pine was much more severely restricted by frost than that of other, more problematic wilding species such as lodgepole pine, Corsican pine, Douglas fir and Scots pine, he said.
"It is also intolerant of shade and will not prosper under an existing forest canopy."
Mason said this meant that radiata pine was much less likely to be a wilding problem in the high country, even on open sites, where low grazing pressure often favoured wildings of other species.
"Moreover, on unforested, lower elevation, warm, wet sites where it might reproduce easily, high grazing pressure on neighbouring properties usually keeps it in check," he said.
While Mason agreed that people who plant trees that reproduce as wildings on neighbouring properties should be held responsible for controlling them, he thought a levy on the whole non-native conifer industry would be "unfair".
Radiata pine (Pinus radiata, or Monterey pine) is native to North America and had naturalised in New Zealand by 1904.
At present, it is by far the largest contributor to New Zealand's forestry industry, comprising 90 per cent of the total plantation area.
It supplies most domestic wood products, and is the third largest export earner, contributing around 3 per cent of GDP.
