Is there a more enigmatic island than Rapa Nui (Easter Island), deforested and depopulated and studded with giant stone statues? We know it transformed from palm-clad to treeless within a few centuries, but the nature of its path there – while undoubtedly due to humans and their behaviours and accompaniments – causes academic quarrels.
Regardless, the island’s indigenous trees are all gone. Its last one clung on until about 1960, sheltering in a volcanic crater. It was a kōwhai lookalike called toromiro, complete with pendulous golden flowers, found only on Rapa Nui. Toromiro and kōwhai share the genus Sophora. Fortunately, toromiro seeds were collected during James Cook’s and Thor Heyerdahl’s expeditions, and it is grown in some botanic gardens.
Rapanui people want toromiro back. But in early attempts, the saplings died after a year in the island’s impoverished soil. Eventually, Chilean scientists realised the toromiro were missing soil bacteria that commune helpfully with their roots. Kōwhai, gorse and other legumes need the same help. The bacteria, called rhizobia, create nodules on their hosts’ roots, and these make an enzyme that turns atmospheric nitrogen into a form of nitrogen that fuels plant growth.
Kōwhai specialist Peter Heenan says that ability to self-produce fertiliser allows such plants to colonise new and often bare surfaces. “Kōwhai love open, disturbed and stony sites with young soils,” he says.
Scientists searched Rapa Nui’s soil for the missing bacteria to no avail. But they struck a successful match with rhizobia of the most common kōwhai species, Sophora microphylla, supplied by Lincoln University.
Heenan, who is based at Manaaki Whenua Landcare Research in Lincoln but worked with the university team, says Sophora microphylla will unfussily pair with rhizobia from other kōwhai species.
Each of our eight kōwhai species has a preferred match, but experiments have shown “they fundamentally worked with whichever one it was,” he says.
“The different bacteria species assisted all of the kōwhai plants to grow.”
Like toromiro, kōwhai seedlings without rhizobia fail to thrive.
Macarena Gerding, of Chile’s Concepción University, leads the toromiro reintroduction project. She says the biggest toromiro on Rapa Nui, planted in 2018, are inoculated with kōwhai rhizobia. However, they’re pampered in a nursery area.
“What we are planning to do now is to work with the community. If we hand people one tree per family, they can look after the tree and put it in their backyards. Maybe it’s a way of bringing the tree back to life. Although it’s not going to be in the wild, you have a protected environment within their shrubs.” Some 7750 people live on Rapa Nui, up from 110 at its lowest, and about half are indigenous.
The latest saplings are inoculated with rhizobia from a Sophora species from Chile, Rapa Nui’s closest large land mass. “We didn’t use the New Zealand strain this time, not because it’s not working, but we are trying to use more native ones,” says Gerding. That matters to the Rapanui people, who once prized the tree for carving.
There are other missing ecosystem actors, too. Instead of native palms (now extinct) protecting toromiro from wind and sun, another Polynesian tree will nurse them. The island receives less rain than it used to, says Gerding, and the groundwater that islanders might irrigate their trees with is slightly salty. Then there’s the question of what will pollinate toromiro flowers. “The island’s ecology is so degraded that they have few insects or birds, apart from sea birds.”
That could make it impossible to re-establish a self-sustaining toromiro population. In New Zealand, kōwhai depend solely on tūī for effective pollination, says Heenan. Kōwhai can self-pollinate, but one study reported that over half the resulting seeds were inbred and “genetically doomed”.
When it comes to toromiro, time will tell whether Rapa Nui is still teaching lessons on the perils of interfering too severely with nature’s interconnections.
