Scion study reveals lodgepole pine produces three times more seeds in New Zealand than in native North America; research suggests pine's enhanced invasion success is due to increased seed holding capacity, lack of natural 'seed predators' like squirrels

Lodgepole pine – a wilding conifer considered one of New Zealand’s worst weeds – has evolved to grow and spread faster here than it does in its native North America.

A Scion study, the focus of a New Zealand Journal of Ecology article published on February 21, seeks to understand why, to help inform strategies for addressing the problem. This study is part of Scion’s Vive la résistance programme, which started in 2021 and has $12.85 million in funding through the Ministry for Business, Innovation and Employment’s Endeavour fund.

The study investigated cone and seed traits of lodgepole pine (Pinus contorta) from several major invasion sites across NZ compared with its native range. The comparison across six sites showed the average number of filled seeds per cone was 74 – at least three times higher than that recorded in its native range and about 50 percent higher than previous NZ estimates.

Scion’s researchers predict this increase in the number of seeds has likely enhanced the invasion success of lodgepole pine by allowing it to spread quickly and overwhelm ecosystems.

“The average number of seed scales per cone is similar between NZ and the native range,” Scion invasion ecologist and article lead author Tom Carlin says. “Therefore we suggest this trait shift towards a greater seed holding capacity may be due to an increase in the proportion of fertile scales.”

“This demonstrated trait shift is unsurprising given lodgepole pine has fewer natural enemies in New Zealand, thrives in our growing conditions and has potentially produced more invasive hybrids since its introduction.”

Some sites were found to have produced fewer seeds, such as Craigieburn near Christchurch, where the national wilding conifer control programme has had some success in removing pollen-producing individuals. “Keeping the number of pollen-producing adults low slows population spread as seeds don't get fertilised,” Tom says.

In North America, lodgepole pine has specialist seed predators such as squirrels and crossbills which put pressure on cone defences. Those predators are not present in NZ, which could have allowed the trees to dedicate more resources to producing seeds rather than investing in cone defences. Lodgepole pine is not a forestry species in NZ and is not actively planted.

Wilding conifers (non-natives) are estimated to affect over 1.7 million ha of productive and conservation lands in NZ (Ministry for Primary Industries, 2015, p.9). Their rapid growth and spread poses a significant threat to ecosystems worldwide by outcompeting native species, increasing fire risk, supporting invasive mammal populations, reducing water yields and limiting future economic land uses.

Scion’s VLR programme works alongside New Zealand’s National Wilding Conifer Control Programme (NWCCP), which aims to contain and eradicate wilding conifers and prevent their future spread. Existing populations of wildings are being treated but cleared land is being re-invaded. For the NWCCP to be successful it is critical to develop effective strategies to create long-term resistance to re-invasion on treated land.

Scion scientists Matthew Scott and Thomas Paul co-authored the journal article.