Locking away seeds shouldn't be relied upon to save our cherished native species from disease, scientists say.
Seed-banking has been used as a way to preserve our flora for the future - and last year's incursion of fungal disease myrtle rust sent hundreds of officials scrambling to collect the seeds of more than 30 native plant species.
But a new international study, co-led by New Zealand scientists, found this supposed ark may not be as effective as first thought.
The study analysed whether seed banking could achieve the international goal of conserving 75 per cent of the world's threatened species ex situ – meaning away from their natural habitat.
Ex situ conservation offered a type of insurance policy, in case localised disruption or disaster threaten a species.
But the scientists concluded seed banking could not achieve this goal, because many seeds don't survive the storage process.
Twenty-one percent of New Zealand's trees and shrubs, including iconic species such as totara, rimu, and tawa, fit into this group of plants with "recalcitrant" seeds.
Swamp maire, which was threatened by myrtle rust, also had recalcitrant seeds.
And while kauri seeds were not recalcitrant, they were viable for less than 10 years after seed banking, so they also could not be stored this way.
"Most of the plants with recalcitrant seeds are from canopy trees from tropical rain forests," said study co-author Dr Sarah Wyse, of New Zealand's Bio-Protection Research Centre.
"New Zealand has quite a high number of plants with recalcitrant seeds because many of our North Island trees are closely related to tropical species."
Seed banking remained a good way to conserve crop and medicinal plants, and also worked for most New Zealand trees and shrubs, including, for example, pohutukawa and southern rata, Wyse said.
But with the other authors she said it was vital to develop alternative conservation methods, such as cryopreservation, for many plants.
For seed banking, seeds were dried and then frozen at minus 20C.
Once thawed and planted in soil, they grew just like fresh seeds.
However, cryopreservation involved removing the embryo from the seed then using liquid nitrogen to freeze it at an extremely cold temperature of nearly minus 200C.
Once thawed, the embryo then had to be grown in a culture in the laboratory.
"It is a much more complex process than seed banking, and requires many more resources," Wyse said.
In the paper, the scientists say "intensifying research efforts into cryopreservation as a conservation tool" was vital to improve the chances of conserving many of the world's tree and shrub species ex situ.
However, they noted, "it may even be naive and dangerous to assume that ex situ conservation is a valid means of safeguarding a high proportion of threatened tropical moist forest trees from extinction".
The same was true of many of New Zealand's endangered endemic species, Wyse said.
"If we are serious about wanting to save our unique trees, we need to do everything we can to save them in their natural environment, because other methods may simply not work."
Other Kiwi scientists have also highlighted uncertainties around seed-banking.
"For some species, such as kauri, freezing provides a medium-term option for storing seed and maintaining genetic diversity, but there is a lot of uncertainty around the long-term viability of stored seed," said Dr Nari Williams, a plant pathologist at Scion.
For threats like kauri dieback – which had now spread across the top of the North Island - selection for resistance presented as the most promising strategy for the long-term survival of the species, with seed storage having a role in such programmes.
"There is a need for expanded research to develop off site species-specific methods and long-term strategies for plant conservation in the forest, across the diverse range of New Zealand plant species," Williams said.
"But, these need to be supported by efforts to conserve plant species through sanctuary plantings and re-vegetation programmes."
Associate Professor Bruce Burns, of the University of Auckland's School of Biological Sciences, said New Zealand's native flora was already under threat from a range of plant pathogens – with more were undoubtedly still to come.
"Although conventional seed banking is a hugely valuable tool to preserve the genetic potential of many of our plant heroes, knowing which species can't be conserved by this technique should provide the impetus for the timely investment in development of alternatives."