The spiky, iconic Canadian maple leaf. A large, lush Florida palm leaf. Thin aspen leaves rustling in the crisp Colorado air. You can probably recall many different leaf shapes from many different places. But did you know that the places helped create the shapes?
“Every leaf has a story, and there’s so many different types and genres of story within each single leaf,” said Dan Chitwood, a biologist and associate professor at Michigan State University who specializes in leaf development.
A leaf’s visual characteristics – its size, perhaps, or the shape of its edges – can help tell its story, including the conditions it grows in and even traces of its evolutionary history.
Leaves, to put it simply, vary from place to place.
“We all sort of intuitively know this, right?” said Dan Peppe, a paleobiologist and associate professor at Baylor University who has studied how climate affects leaf features. “If we go to different places, we know that the plants are going to be different. And it’s often that the plants are different because the climate is different.
“Plants are this really cool, interesting and sensitive indicator of the ecosystem.”
What is a ‘leaf shape?’
Everything about a leaf – from the smoothness of its surface to its vein structure and how segmented it is – is part of its function. Even trees from the same species can have different-looking leaves in different climates.
Besides the overall shape and vein structure, the thickness of the leaf – think leathery magnolia vs. paper-thin aspen – and the size, cell structure, and “hairiness” of its surface are all important. Each of these characteristics is an adaptation or optimization that helps leaves absorb sunlight, distribute water, regulate heat loss, avoid pests or address one of many other issues, all specific to the leaves’ environment.
According to Peppe, the two major factors that influence leaf shape are evolutionary history and environment.
Take the maple leaf. Species of maples are native to many regions of the world because of a shared ancestor millions of years ago.
Even just within North America, the same species of red maple can have vastly different leaves under different temperature conditions.
In one study, researchers planted red maple seeds from Ontario and red maple seeds from Florida in both chilly Rhode Island and toasty Florida.
They found that the Ontario seeds grown in Florida produced leaves with characteristics typical of warmer weather. The spiky-lobed cold weather leaves softened into gentler curves.
Conversely, the Florida seeds grown in Rhode Island produced leaves with characteristics typical of colder weather. The softer shape of warm weather leaves became sharper, with more teeth and jagged tips.
“What we’re seeing is that some plants are what we call plastic. They’re able to adapt to different climate conditions,” Peppe said.
Other research has found correlations such as latitude and shape (closer to the equator = simpler leaves), temperature and teeth (colder climate = more teeth), and moisture and size (wetter environment = larger leaves).
Why so many shapes?
Scientists don’t know why shapes are so varied, but they have some ideas.
A leaf’s main purpose is photosynthesis, or using light to convert carbon dioxide into sugars and oxygen, which in turn support nearly all living things.
“These diverse leaves are doing a really important job. They’re the foundation of life on earth,” said Erika Edwards, a professor of ecology and evolutionary biology at Yale University who has studied how plants optimize for photosynthesis.
Edwards outlined several possible theories about leaf shape, including that the configuration of lobes could affect airflow and carbon dioxide absorption, or that shorter growing seasons in cold climates could concentrate leaf growth around the vein system, encouraging lobe development. Her team presented a new hypothesis called “bud-packing,” in which certain lobe shapes let the tiny leaves inside buds fold more compactly, almost like origami.
Size is one feature scientists understand well, according to Ian Wright, who has studied leaf size in relation to climate as chief scientist at the Hawkesbury Institute for the Environment at Australia’s Western Sydney University.
Picture a typical tree in Maine compared with trees in a tropical rainforest.
The large, broad leaves in a rainforest, Wright said, capture more sunlight but also lose more water trying to cool that big surface area. That trade-off works just fine because water is plentiful.
But in a drier, colder northern forest, heat regulation is a bigger problem. Pine trees there have clusters of super-thin needlelike leaves that insulate one another, similar to the way goose bumps help your skin retain heat when you’re cold.
Ultimately, there’s no best leaf shape, and even in the same environment, a variety of leaves might work well.
For example, sassafras, native to the eastern United States, has three distinct leaf shapes on the same tree.
“There’s no evolutionary advantage to one shape over another,” said Susan Pell, executive director of the U.S. Botanic Garden. In that case, she said, different shapes could simply be genetic remnants of the process of natural selection.
What happens when the climate changes?
Some trees may live for centuries, but if their environment becomes less hospitable, they’re stuck in place and can do relatively little but suffer. In that way, Chitwood said, they serve as “sentinels of climate change.”
Edwards said recent studies suggest that plants will suffer dramatically in conditions driven by global climate change, in particular hotter temperatures and drought.
Because of the way seeds are dispersed, plants have been “moving all over the world for millions of years in response to climate change,” Edwards said. “But it’s not been anywhere near the rate of (climate) change that we’re seeing now. I think people are expecting to see a lot of extinction.”
Trees can make some adjustments to their leaves to better survive, but they have limits.
“When I think about climate change, I think about plants first, because human survival depends on plants in so many ways,” Chitwood said. “Through that change in environment, it’s ultimately the responses of the plants that will either do us in or keep us surviving.
“In that whole grand cycle, leaves are like a message.”