Vertically challenged Steve Lennie/Alamy Stock Photo

Never mind the Leaning Tower of Pisa – this is the leaning tower of pines.

Cook pines are towering trees that were once restricted to their native home of New Caledonia, an archipelago in the Pacific Ocean. Through cultivation, they have taken root across tropical, subtropical and temperate regions around the world.

The trees often have slightly tilting trunks. Scientists have now noted a bizarre pattern in their tilt: they lean south in their northern range and north in their southern range.


More strange behaviour: Trees seen resting branches while ‘asleep’ for the first time

Matt Ritter at California Polytechnic State University in San Luis Obispo was writing up a description of the Cook pine (Araucaria columnaris) for a book on the urban trees of California when he realised that the pines always leaned south. So he rang up a colleague in Australia to see if that was the case there. It turned out it was – but this time the pines leaned north.

“We got holy-smoked that there’s possibly a tree that’s leaning toward the equator wherever it grows,” says Ritter.

He and his colleagues studied 256 Cook pines scattered across five continents. They collected tree data at 18 locations between latitudes of 7 and 35° north, and 12 and 42° south. The team estimated that the trees tilt by 8.55 degrees on an average – about double the tilt of the Leaning Tower of Pisa.

The trees also slant more the further they are from the equator in both hemispheres. “It’s a shockingly distinct pattern,” says Ritter. One tree in South Australia slants at 40 degrees.

Drawn to the centre

Trees normally correct for such asymmetry in their growth, but for some unknown reason the Cook pine is unable to do so, says Ritter. “We could be just dealing with an artefact of its genetics that we are seeing now when we have spread it all over the world,” he says. Alternatively, it could be an adaptation to catch more sunlight at higher latitudes.

“The tilting phenomenon is not unusual,” says Steven Warren of the US Forest Service in Utah. In 2016, he reported that the inflorescence of the yucca palm found in the US always points south, thus cutting the cost of transporting nutrients to its flowers. Some cacti lean towards the sun too, he says. However, “this is the first time I have heard of a tree doing this”, says Warren.

Winslow Briggs at Stanford University finds the north-south leaning pattern “fascinating.” Some pines, he says, strictly grow upright, whereas others are pretty sloppy about it.

Journal reference: Ecology, DOI: 10.1002/ecy.1850