In a universe of unexplained phenomena, add this: a tree that leans toward the equator no matter where it grows.

The Cook pine tilts northward in the southern hemisphere and southward in the northern hemisphere, according to new research published recently in the journal Ecology.

"That has never been seen ever in any plant, let alone trees," said botanist Dr. Matthew Ritter, a professor at California Polytechnic State University and co-author of the paper.

The Cook pine is native to New Caledonia, a group of islands in the South Pacific, and has been successfully cultivated in other warm, temperate climates like Australia, New Zealand, California, Mexico and Hawaii, where it grows 15-24 metres high, on average, and up to a towering 30 metres (roughly 10 storeys).

Ritter and his colleagues measured 256 Cook pine trees on five continents in 18 different regions (defined as distinct areas that are more than 500 kilometres apart).

"The pattern is shockingly clear: they lean toward the equator, wherever they're found," Ritter, who heads up the Cal Poly Plant Conservatory, told CBC News.

Data collected by the researchers found the average tilt was 8.05 degrees — double that of the Leaning Tower of Pisa

They also consistently found that the farther away the trees are from the equator, the more they lean: one tree in southern Australia tilted 40 degrees.

Cook pine 40 degree lean

This Cook pine at Brighton Beach, South Australia, tilts 40 degrees northward. (Matthew Ritter)

"No [other] plant, as far as we know, is sensing where it is on the planet," Ritter said.

Trees of any species can grow crooked due to factors in their environment, like wind and snow.

But Ritter says this is a "worldwide pattern in which every individual in the species is leaning  ...  and also [has] this very distinct directionality associated with it."

'All kinds of theories'

Why?

No one is sure.

"We have all kinds of theories — some of them are more testable than others," Ritter said, adding he's received email suggestions from all over the world since the paper was published.

Other species of trees, even from the same family, don't lean. If they bend slightly toward the sun as they grow, they'll get straightened out when the wood of the trunk develops.

So one theory of the Cook pine is that Cpt. James Cook, who landed at New Caledonia in 1774, may have brought to Hawaii a bunch of mutant seeds and cones, ones that can't respond to light properly. Or perhaps there is a mutation in the wood so that it doesn't mature properly. Or it might be an adaptation to the calcium-rich white beach sand it grows in.

cook pine lean pattern

The magnitude of lean is given by lines radiating from the centre and measured in degrees from vertical, as indicated by concentric circles. Red points represent samples from the southern hemisphere, and blue points those from northern hemisphere. (Ecology)

Whatever the reason, it hasn't hampered the Cook pine's success in areas where suitable conditions exist. But Ritter said that doesn't necessarily mean the anomaly is an asset. 

"Not everything has a selective advantage, and some things are just artifacts that have been carried through the evolutionary history of the species," Ritter said, like "nipples in human men."

Ritter isn't yet sure what the larger implications of the discovery might be.

"Why it's a big deal globally is it's a scientifically interesting thing," he said, "and we, because of this, now are realizing plants are responding to their environment in ways we don't understand."