Why are leaves so big in the tropics?

It was in the 19th century that travelling European plant geographers became fascinated with the pendulous giant leaves of tropical plants, and over time observed that plant leaves tended to be biggest near the equator and to shrink with increasing latitude. However, the the precise reasons for this phenomenon have remained a conundrum. A new paper published in Science may have an explanation tying the global patterns of leaf size to key climatic drivers. 

This latest effort, led by Ian Wright of Macquarie University in New South Wales, has involved analysing the leaf size of 7,670 plant species from 682 areas around the world. By modeling the balance of leaf energy inputs and outputs, the analysis says the key to geographic gradients in leaf size are daytime and nighttime leaf-to-air temperature differences. Variation in leaf size can be explained by the combination of temperature, irradiance and moisture, with viable leaf sizes bounded by the risks from overheating during the day and chilling at night.

Large-leaf species predominate in hot and humid environments, while small-leaf species are the norm in both hot and cold arid conditions as well at high latitudes and elevations. The tropical conditions that sustain large leaves have until now been explained according to established theories based on daytime “leaf energy budgets”.

Larger leaves, being thicker, have slower heat exchange with the surrounding air. That means, all else being equal, they develop larger leaf-to-air temperature differences. Like all leaves, they cool themselves through transpirational water loss. Facing greater risk of heat damage from strong sunlight and higher air temperatures, they thus need more water. 

Based on these principles, the established theories have emphasised the disadvantage to larger leaves in hotter, drier, sunnier locales. However, in bad news for the energy-budget theories, studies show average leaf sizes clearly increase with higher average annual temperatures. Another clue for the authors that a more complex theory was needed was the paucity of large-leaved species at cold and high-elevation sites, which pointed to the importance of night-time energy balances. 

The new paper generalises its results in a global map showing geographic trends in maximum leaf size. Maximum leaf sizes are particularly small in warm deserts and in cold regions at high altitude, though for different reasons.

In arid areas, the researchers argue, daytime temperatures place an upper boundary on leaf size because the rapid transpiration needed to avert heat damage is impossible with a limited water supply. In areas with intermediate moisture, daytime constraints appear more limiting between the latitudes of ~20° S and 20° N, but outside this zone night-time constraints dominate. 

In wet climates, there is plenty of water for effective daytime transpirational cooling so night-time constraints determine leaf size. In the tropics where it is always warm and wet, the researchers suggest there may be no thermal constraint on leaf size. Instead, other constraints may come into play, such as biomechanical limits on how big a leaf the plant can physically support or pump liquids through.

So what is the advantage of growing leaves as large as possible? “This is not well understood,” the authors say, though they suggest two possible explanations.

First, having fewer, larger leaves could be more efficient in terms of the amount of branch and twig required to support a given amount of leaf mass. Second, the wider leaf-to-air temperature differences possible for larger leaves may make photosynthesis more efficient, as the leaves would heat up to temperatures for photosynthesis more quickly on cool mornings. Those wider leaf-to-air temperature differences may also allow larger leaves to operate at temperatures substantially lower than the surrounding air in hot and sunny conditions.

By providing a quantitative explanation for the latitudinal variation in leaf sizes Wright and his colleagues hope their contribution will aid climate reconstructions from the deep past using leaf fossils, as well as enriching future vegetation models that predict the consequences of future climate change.