Why do tropical plants have such big leaves?

Leaf size and shape is incredibly important to plants, as it helps them cope with the environments in which they live. We’ve long known that plants in the tropics have larger leaves than those in cooler climes (see image above of one of the world’s biggest leaves from tropical Borneo – the Giant elephant ear plant), but the reason for this has been hotly debated. We now think we know why…

Researchers at Macquarie University have recently published a plausible explanation for why tropical plants have large leaves. As with many things in life, it turns out it’s a compromise, and in this case between heat capture and loss.

Large leaves, so the theory goes, are more susceptible to extremes in temperature.  The extra insulation around large leaves makes it harder for the leaf to extract heat from their surroundings at night, which means they risk suffering frost damage in cold climates. Large leaves are also poorly adapted to hot desserts, because they tend to overheat during the day. Under both these circumstances, small leaves are favoured.

However, in the tropics, large leaves can counteract the daytime heat-trapping effect using transpiration – the plant version of sweating – something desert plants cannot afford to do as they would lose too much water. Large leaves also allow tropical plants to capture more sunlight energy and together with a ready supply of water are able to convert this energy readily into rapid growth.

We’ve conducted quite a bit of work over the years on leaf shape, including examining changes in leaf shape along climatic gradients (leaves get larger with increasing rainfall and decreasing temperature), over time (leaves have been getting narrower to cope with increasing temperatures due to climate change) and on the advantage of broad leaves over needles for conifers.

Some of this work also featured in a New Scientist article covering the theory described above, and on which this blog post is based.

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