If you have ever visited exotic countries — or at least enjoyed watching shows about them — you may have noticed the following: Leaf sizes increase as you move from harsh northern climates to tropical regions where humidity and heat prevail.
Leaf sizes can vary greatly: from 1 mm² (approximately 0.00155 square inches) to 1 m² (approximately 10.76 square feet), or even larger. Large-leaved plants — like bananas — thrive in the tropics, while small-leaved plants, such as heather and clover, are found closer to colder latitudes.
The connection between leaf size and location is clear. Just look at a coconut palm and compare it to a spruce tree, and any doubts will vanish.
Botanists first noticed this trend in the 19th century, but for a long time, they couldn’t find a convincing explanation for it.
One theory suggested that large leaves help plants avoid overheating. Due to their large surface area, they can absorb and distribute heat more effectively, preventing it from concentrating in one spot. However, this would also mean the leaf captures more sunlight. So, it would seem that large-leaved plants should evolve in colder regions, not tropical ones.
To solve this mystery, scientists from Macquarie University in Sydney, led by Ian Wright, studied 7,670 species of plants from various latitudes. They looked for connections between leaf size and different climatic aspects such as daytime and nighttime temperatures, precipitation, and humidity.
Wright’s team discovered that it’s just as important for plants to avoid nighttime frost as it is to avoid daytime heat stress.
The scientists determined that there is a balance between leaf size and a plant’s ability to regulate temperature. This balance depends on two main factors. First, the plant needs enough water in its leaves to cool down through a process called transpiration, which works similarly to sweating in humans.
Second, the thickness of the boundary layer of air around the leaves is crucial. This thin zone surrounds each leaf and influences the rate of gas and energy exchange between the leaf and the surrounding environment.
Large leaves have thicker boundary layers of air around them, making them more susceptible to extreme temperatures. The thicker this layer, the slower the transpiration process. Air acts as a form of insulation, making it harder for the plant to lose heat to the environment during the night.
This is why large-leaved plants are uncommon in both northern latitudes and desert climates. The thick boundary layers make it difficult for the plant to stay warm at night, increasing the risk of frost damage. Similarly, in hot desert climates, leaves overheat during the day, and there isn’t enough water for cooling through transpiration.
However, in hot and humid tropical regions, large leaves are ideal. During the day, they can cool down effectively through transpiration, unlike desert plants, which cannot afford to lose as much water. At night, tropical plants are not at risk of frost damage, which occurs in northern regions and deserts.
Large leaves can capture more carbon dioxide from the environment, which is necessary for photosynthesis. They also allow plants to capture more light in dense tropical forests, where sunlight only filters through the thick canopy. In the tropics, where there is plenty of moisture, plants don’t need to worry about water shortages, so they can have large leaves for more intense transpiration.
Lastly, in jungles, plants compete for light, water, and nutrients. Large leaves help them occupy more space and overshadow their neighbors, allowing them to collect more rainwater and sunlight.
In summary, large leaves are advantageous. Of course, even the humble birch might wish it had leaves as large as those of the raffia palm, which can grow up to 16 meters (approximately 52.5 feet) long and 3 meters (approximately 9.8 feet) wide. But such massive leaves would freeze overnight, so plants in temperate regions have to make do with what they have.
