Why is a banana leaf a million times bigger than a common heather leaf? Why are leaves generally much larger in tropical jungles than in temperate forests and deserts? The textbooks say it’s a balance between water availability and overheating.

But it’s not that simple.

A global team of researchers, led by Associate Professor Ian Wright from Macquarie University, revealed that in much of the world the key limiting factor for leaf size is night temperature and the risk of frost damage to leaves.

Ian, and 16 colleagues from Australia, the UK, Canada, Argentina, the USA, Estonia, Spain, and China analysed leaves from more than 7,600 species, then teamed the data with new theory to create a series of equations that can predict the maximum viable leaf size anywhere in the world based on the risk of daytime overheating and night-time freezing.

“The conventional explanation was that water availability and overheating were the two major limits to leaf size. But the data didn’t fit,” Ian says.

“For example, the tropics are both wet and hot, and leaves in cooler parts of the world are unlikely to overheat.”

They will use the findings to create more accurate vegetation models. This will be used by governments to predict how vegetation will change locally and globally under climate change, and to plan for adaptation.

Banner image credit: Ian Wright