Why Leaf Sizes Vary with Latitude
Hiking through the northern Canadian forest, you won’t find any elephant ear plants among the conifers. Plants with big leaves hardly typify the chillier latitudes, nor do trees with needle-like leaves dominate in tropical areas.
The reason isn’t simply that hot, sunny climates favor large leaves that can take advantage of the abundant sun. That doesn’t happen in hot, sunny, but arid regions, where small leaves are the rule. How latitude interacts with other factors to influence leaf size was the subject of a recent study by an international team of scientists who wanted to improve predictions of how climate change will alter vegetation through its effects on leaves.
“We sought to explain the latitudinal gradient in leaf sizes first noted by 19th-century plant geographers—a longstanding ecological conundrum,” says team member Peter Reich, a Regents Professor in the University of Minnesota Department of Forest Resources. “It has prevented realistic embedding of this key trait in global vegetation and Earth system models.
“We wanted to improve predictions of how climate change will alter vegetation through its effects on leaves, and also improve how we characterize leaf size—and thus its effects on carbon uptake and water loss—in models that project future carbon cycling and climate.”
The team gathered data on energy budgets—i.e., energy inputs and outputs—of leaves from 7,670 species that varied by 100,000-fold in surface area. They then constructed a model to predict leaf sizes in which the “under-appreciated influence” of nighttime energy balance exerted a major influence. The study, “Global climatic drivers of leaf size,” was a cover story in Science.
Night and Day
Central to the global patterns of leaf size was the fact that large leaves are insulated by a thicker “boundary layer” of calm air that slows their ability to lose or absorb heat.
The new model shows that in arid regions, leaf size is limited by daytime temperatures. Leaves must lose heat to avoid overheating, and in hotter zones the thick boundary layers around large leaves may keep their heat from radiating away fast enough. Leaves also lose heat by “transpiring” water—not unlike sweating—but in arid areas, this isn’t an option.
At wet sites, the new model predicts that daytime heat exerts little downward pressure on leaf size because with water plentiful, even large leaves can lose enough heat through transpiration. In those cases, it’s the nighttime situation that controls size at all latitudes.
At higher, colder latitudes, large leaves are at a disadvantage at night, when leaves radiate away heat. Then, large leaves’ thick boundary layers hamper their ability to recapture heat radiated by soil, air, and other plants, leaving them prone to frost damage.
The study was led by Ian J. Wright of Macquarie University in Australia.