Not too hot, not too cold: Uncovering the mystery of variation in leaf size

Have you ever wondered why leaves are different sizes? We know that leaves are much larger and broader on average in tropical ecosystems than in boreal forests and deserts. Why is this? Forest Resources Professor Peter Reich is part of an international team of researchers that uncovered the mystery of why leaves vary in shape. Their research was published as the cover story in the journal Science this fall. There has been a widespread belief that leaf size is primarily a function of water availability and heat dispersion. The belief held that a large leaf could not lose heat fast enough on a hot day and would overheat. Reich and his team discovered that the answer is more complicated.

Field technician fixing LI-COR machine for measuring respiration rates in plants. July 2011.

Professor Ian Wright from Macquarie University in Australia, a former postdoctoral student and long-term collaborator of Reich, led the international team of scientists from Australia, the U.S., Canada, Spain, Argentina and China. Their research findings show that the “old” theory was true in certain places, but in a large part of the world, nighttime temperature and risk of frost damage to leaves are the limiting factors of leaf size. They coupled new theories with analysis of tens of thousands of leaves from over 7,600 species to generate equations for predicting the maximum viable leaf size anywhere on the planet based on the risk of overheating during the day and freezing at night. 

Scientists and technicians setting up data loggers at the Cloquet Forestry Center. August 2008.

The team discovered trends that hold true for plants across climate zones in different continents and across plant types. Their findings confirmed that the risk of overheating during the day in hot and dry regions controls maximum leaf size there. However, in tropical ecosystems, no temperature-related limits to leaf size exist, given that there is enough available water for cooling by transpiration. The key findings that were not apparent from previous research showed that in much of the world, maximum leaf size is controlled by risk of frost at night rather than risk of overheating. “It was long thought that overheating was the primary limit to leaf size. But the data didn’t fit,” said Reich. “For example, although much of the tropics is wet, leaves can still get very hot in tropical heat, yet many tropical leaves are big. On the other hand, leaves in cooler parts of the world are unlikely to overheat, but many of them are small. Our team took a different approach. We asked whether cold as well as heat could be problems for large leaves,” noted Reich. Professor Wright, the lead investigator, added, “Larger leaves have thicker, insulating boundary layers of still air that slow their ability to dissipate heat and cool themselves in hot, dry weather and that also slow their ability to draw heat from surrounding leaves and branches on cold nights, heat that is needed to compensate for longwave energy lost to the cold, nighttime sky.” “It should be no surprise then that these mechanisms influence plants closer to home too,” Reich said. “Minnesota’s deciduous plant species, like maples and oaks, must cope with the risk of frost in spring and fall, and those that stay green all winter, like our pines and spruces, must deal with much deeper cold. As a result, maple and oak leaves are smaller than banana leaves, but much bigger than needles of pines and spruces. We can throw a parka and hat on to stay comfortable at 30 below zero, but since leaves can’t, they have to adapt any way they can.”

The team’s results are important for creating more accurate models of global vegetation, which will allow scientists across the world to predict vegetation changes locally and globally due to climate change. Predicting vegetation changes will facilitate climate change adaptation strategies and policies.