The huge variety of leaves in the plant kingdom has long been a source of wonder and fascination.
Working with a global team of researchers, Jacobs and her colleagues cracked the mystery of leaf size. The research was published Sept. 1, 2017 as a cover story in Science.
The researchers from Australia, the U.K., Canada, Argentina, the United States, Estonia, Spain and China analyzed leaves from more than 7,600 species of plants over the past 20 years, then pooled and analyzed the data with new theory to create a series of equations that can predict the maximum viable leaf size anywhere in the world based on the risk of daytime overheating and night-time freezing.
The researchers will use these findings to create more accurate vegetation models. This will be used by governments to predict how vegetation will change locally and globally under climate change, and to plan for adaptation.
Jacobs contributed an extensive leaf database — research that was funded by a National Science Foundation grant. She analyzed the leaf characteristics of 880 species of modern tropical African plants, which occurred in various combinations among 30 plant communities. Jacobs measured leaves of the plant specimens at the Missouri Botanical Garden Herbarium, one of the largest archives of pressed dried plant specimens from around the world.
Jacobs is one of a handful of the world’s experts on the fossil plants of ancient Africa. As part of a team of paleontologists working there, she hunts plant and animal fossils in Ethiopia’s prolific Mush Valley, as well as elsewhere in Africa.
By Helen Briggs
The leaves of a banana plant, for instance, are about a million times bigger than the leaves of heather.
The conventional wisdom is that leaf size is limited by the balance between how much water is available to a plant and the risk of overheating.
However, a study of more than 7,000 plant species around the world suggests the answer may be more complex.
“A banana leaf is able to be so huge because bananas naturally grow in places that are very hot and very wet,” said Ian Wright of Macquarie University in Sydney, Australia.
“Our work shows that in fact that if there’s enough water in the soil then there’s almost no limit to how large leaves can be.”
He says this is only part of the puzzle of leaf size.
“The other part is about the tendency for larger leaves to freeze at night,” Dr Wright explained.
“And, you put these two ingredients together — the risk of freezing and the risk of overheating — and this helps understand the pattern of leaf sizes you see across the entire world.”
There are hundreds of thousands of plant species on the planet, from tiny alpine plants to massive jungle palms.
Their leaves vary in area from less than 1 square millimetre to greater than 1 square metre.
Large-leaved plants predominate in tropical jungle — something that was noted as early as the 19th Century. Meanwhile, small-leaved plants thrive in arid deserts and at high latitudes.
Some decades ago, scientists realised that variability in leaf size was related to water and temperature. They proposed that the limit to leaf size was set by the risk of overheating.
Thus, when rainfall is high, plants can get away with having larger leaves.
The new research, published in the journal Science, suggests this idea applies only in certain regions of the globe.
“There were some pieces in this puzzle that were clearly missing,” Dr. Wright told BBC News.