Why Are Some Leaves Massive and Others Minuscule?

Researchers have found that the rainfall, sunshine and the threat of frost or overheating set the maximum size for leaves

Big Leaf
Wikimedia Commons

The size and shape of plant leaves​ seems endless. There's the massive fan-like leaves of the palm, the skinny prongs of the fern, the mid-sized hand-shaped leaves of the buckeye, and more. But why leaves take such diverse forms have long remained a mystery to scientists. 

Past researchers suggested that water availability and risk of overheating determines leaf size. But as Helen Briggs at the BBC reports, a new study published in the journal Science suggests it’s a bit more complicated.

The idea of water availability and leaf shape can be traced to the 19th and early 20th century Danish botanist Eugenius Warming who hypothesized that high precipitation in the tropics allows things like palm fronds and banana leaves to grow to enormous sizes, writes Ian Wright, first author of the study, for The Conversation.

That idea was refined in the 1960s and 1970s when scientists figured out that large leaves can overheat in the summer. To prevent this from happening, they rely on transpiration cooling—the leaf-equivalent of working up a sweat. This model helped explain why the leaves of desert plants were tiny (they can't afford to "sweat" out their precious water) and the leaves of plants in cool, shady spots often grow large.

But researchers still couldn't adequately explain everything, such as the abundance of large-leaf species that dominate in the searing heat of the tropics or the tiny leaves of plants who live in cooler parts of the world.

To sort out these contradictions, a global team of scientists analyzed the leaves of 7,670 plants from latitudes and ecosystems all over Earth, according to a press release. By taking into consideration average daytime and nighttime temperatures, rainfall and humidity, the researchers found a series of rules that govern the maximum size of leaves. 

“We came up with a new framework to model the way leaves should vary in their temperature, in the relationship to the site climate,” Wright tells Briggs. “What we’ve been able to show is over perhaps as much as half the world the overall limits to leaf size are much more set by the risk of freezing at night than the risk of overheating during the day.”

As Alice Klein at New Scientist reports, avoiding nighttime freezing or daytime overheating is a balancing act between two factors: the amount of surface area a leaf has that it can use for transpiration (sweating) and the size of its boundary layer, an area of still air immediately surrounding the leaf that acts as insulation.

Because large leaves have thicker insulating boundary layers, it’s more difficult for them to absorb heat at night, making them more susceptible to cold temperatures. The boundary layer also makes them unsuited for desert climates, where the insulation puts them at risk of daytime overheating. But large leaves can survive in the tropics because with abundant water they are able to take advantage of transpiration to cool off. 

“It’s basically a trade-off between how much they heat up and how much water they have available to cool themselves down,” Wright tells Klein. “This new knowledge helps us to understand a fundamental aspect of how plants do business.”

The new understanding of leaf dynamics could help paleontologists reconstruct ancient climates and ecosystems from leaf fossils. But this new knowledge could also help researchers understand how vegetation will shift as climate change progresses and which species may be vulnerable to declines because of changing rain and temperature patterns, plant conservation biologist Andrew Lowe of the University of Adelaide tells Klein.

So this Fall, when the leaves begin to drop from the trees, take a look at the variety of shapes and sizes. That the surprising diversity happened for a reason.

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