The Pines That Lean Towards The Equator
- 2 weeks ago
Most trees grow vertically straight, but under challenging conditions where individuals have to compete for light, or when mechanical stress is intense, trees may grow at an angle.
Araucaria columnaris, or Cook pines —named after Captain James Cook, whose second voyage around the globe carried the first botanists to classify the tree— is a tree endemic to New Caledonia in the Melanesia region of the southwestern Pacific Ocean, but have since been planted in temperate, subtropical, and tropical areas throughout the world. When grown outside of its native range, the Cook pines have a pronounced lean that’s so ubiquitous that it is often used as the identifying characteristic for the species. But until recently, nobody paid much attention to which direction it leaned or by how much.
Matt Ritter at the California Polytechnic State University in San Luis Obispo was researching the Cook pine for an upcoming book on the urban trees of California, when he realized that the pines always leaned south. To ascertain whether that’s always the case, he rang up a colleague in Australia and was surprised to learn that the trees down under leaned north.
Intrigued, Matt Ritter and his team expanded their efforts and studied 256 Cook pines scattered across five continents ranging from latitudes of 7° and 35° north, and 12° and 42° south. They found that the trees always leaned towards the equator, and the magnitude of the lean increases the further they went from the equator. On average, the trees tilt by 8.50 degrees, although one specimen in Australia was found to be leaning at nearly 40 degrees.
It’s not clear why the Cook pines exhibit this peculiar behavior, but the researchers feel it’s due to phototropism—the same phenomenon that causes houseplants to lean towards the sun. It’s possible the Cook pines bend themselves to better catch the slanting rays of sunlight at higher altitudes. In most trees, the tendency to lean towards the sun is counterbalanced by their sensitivity to the Earth’s gravitational pull, a phenomenon called gravitropism, that keep trees upright. The researchers speculate that the Cook pines might be lacking this ability.