November 12, 2014

A smelly situation: Ginkgo biloba trees and the evolution of their dispersal

by Eric Dillalogue

The fall foliage of a Ginkgo biloba tree ©Riosnestor

In autumn, one of the prettiest trees to change colors, from a vivid green to a brilliant yellow, is the Ginkgo biloba.  Along with this beautiful golden color, though, is often a potent smell from the ginkgo seed – it’s smell has been described as “vomit-like,” “animal scat,” “rancid butter,” or just plain stinky.  Those unlucky enough to tread over the fleshy seeds, or “nuts,” often lift up their shoes to check what they stepped-in and frown.  But there is an evolutionary reason for the smelly ginkgo seeds.

Often called a “living fossil,” the Ginkgo biloba tree is the last living species of the Ginkgophyta phylum, an early branch of the seed plants in the Plantae kingdom (see our modules Taxonomy I and Taxonomy II for more information on scientific classification). Fossil records indicate there were once many members of the group, with forests of ginkgo trees blanketing parts of the prehistoric world. The modern ginkgo is extremely similar to its ancestors and it looks structurally similar to fossils from 200 million years ago. (For an example of another living fossil that looks like its ancestors, read about the coelacanth in our Scientists and the Scientific Community module.)

 

A fossilized ginkgo leaf (left, ©Kevmin) and a modern one (right, ©LERK)

It is this ancient heritage that helps to explain why the ginkgo stinks.  Ginkgoes are dioecious, meaning they have both male and female sexes, and only the female trees produce fruits.  Once the female trees drop their fleshy seeds, the seed’s soft outer layer starts to break down after a few days and produces butyric acid, CH3(CH2)2COOH, which gives off the offensive smell.  But the smell isn’t offensive to all animals, as Teris A. van Beek notes in his book Ginkgo Biloba:

… reports of carnivores consuming whole Ginkgo seeds and defecating intact nuts, raises the possibility that the foul smelling [seeds] may be attracting these animals by mimicking the smell of rotting flesh …. Projecting this line of speculation back into evolutionary time, it seems likely that if dinosaurs were involved in the dispersal of Ginkgo seeds, then it was probably done by carrion feeding scavengers, with teeth adapted to tearing flesh ….

What smells disgusting to us, smells tasty to many carnivores and may in fact have smelled delicious to prehistoric animals.  So from an evolutionary viewpoint, the rotten smell of the ginkgo seed was beneficial because it helped to spread the ginkgo species.  As those ancient animals went extinct, though, so did the ginkgo’s means of seed dispersal, resulting in the loss of diversity.  In fact, as of two million years ago, Ginkgo biloba existed in only a small area in China and seemed marked for extinction.

The fleshy ginkgo seeds among dropped foliage. ©Fritz Geller-Grimm

But human intervention helped to save the species.  It is said that Buddhist monks 1000 years ago in northern China cultivated ginkgo trees in their temples for their medicinal value and beauty.  Though now it is more accepted that the trees were first cultivated for their nutritious seeds, it is still due to humans that the ginkgo has spread from small wild patches in China to global distribution.  As Peter Crane, author of Ginkgo: The Tree That Time Forgot, notes, “by cultivating plants like ginkgo that are very rare in the wild, we’ve sort of taken out insurance for their long-term survival.… I think conservation through cultivation is an important part of the toolkit for preserving plant diversity for the future.”

Today, the ginkgo thrives in urban environments, especially in areas of high pollution and restricted space like roadsides.  Their hardiness and disease resistance have made them a favorite of urban planners throughout the world.  And though the planners may only intend to plant male trees to avoid the stinky seeds, female trees invariably are planted and thrive.  So we continue to support this living fossil, even if we pinch our noses around it at times.

 

More info:

The Living Dinosaur by Peter Del Tredici – Harvard Magazine – describes his search for wild ginkgo populations

Ginkgo: the life story of the oldest tree on Earth – interview with Peter Crane at Environment 360

Introduction to the Ginkgoales – University of California Museum of Paleontology

Ginkgo: the tree that time forgot by Peter Crane, Pollyanna Von Knorring, and Peter Raven (2013), Yale University Press

Ginkgo biloba by Teris A. van Beek (2003), CRC Press

Eric Dillalogue

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Eric Dillalogue holds a MS in Library and Information Science and a BA in English. He has worked in a variety of roles from service industry management, academic libraries, and grant administration. He has taught courses on information literacy, web research, and developmental reading. Eric joined the Visionlearning team as a project manager in 2014.