The economics of leaf longevity explained using optimality principles

What is it that decides how long the leaves should live? It turns out, it is all to do with economics.

What determines the lifespan of a tree leaf?

It’s a simple question, but until now, did not have a simple answer. Whilst some trees grow for hundreds, if not thousands of years, the leaves on different species of tree live anywhere from a few months to 20 years or more.

The leaves on a blueberry bush live only a few months, the maple tree leaves live for a season before falling. But Picea trees growing in the Gongga Mountains in China can thrive for thousands of years, growing slowly in severe environments with leaves that last twenty years. 

What is it that decides how long the leaves should live? It turns out, it is all to do with economics.

The leaf economics spectrum

A new paper entitled: “Leaf economics fundamentals explained by optimality principles” was just published in Science Advances. This work represents a theoretical breakthrough by Imperial’s Prof Iain Colin Prentice and Dr Wang Han (Tsinghua University, Beijing) and other researchers from Japan, Norway, USA, and Australia.

The leaf economics spectrum (LES) was first described almost 20 years ago by co-authors Prof Iain Wright and Prof Mark Westoby (Macquarie University) in the paper “The world-wide leaf economics spectrum” in Nature. This paper has been cited more than 7,500 times and has been key in understanding leaf ecology but the theory behind it was still unclear.

The LES described the observed relationships between different leaf traits and demonstrated that plants with thicker and/or denser leaves have longer life spans. We know that evergreen and deciduous trees behave differently along latitudinal gradients. Evergreen trees have longer-living leaves the closer they are to the poles and that deciduous trees do the opposite. Deciduous trees tend to have their longest living leaves the closer to the tropics they are.

Economic decisions

Prof Prentice said: “At last, we have a quantitative theory that explains the leaf economics spectrum, data analyses that support the theory's key predictions; and for good measure, an explanation of the diverging latitudinal trends of leaf properties between evergreen and deciduous species of trees that almost perfectly fits the data."

First author, Dr Han Wang from Tsinghua University in Beijing said: “We knew that long-lived leaves tend to be tougher and thicker, and more expensive to build. Now, we have identified the major environmental factors at play, and summarised them in two equations. These leaf economic traits are fundamental to the carbon cycle and nutrient economy.” 

Co-author Professor Ian Wright from Macquarie University and Western Sydney University, said: “Evergreen conifers growing in poor soil in areas with a long cold winter can only thrive if they make long-term investments in their leaves. Whereas deciduous trees, like the maple, race to create new leaves and capture carbon in the summer sun before leaf-drop in autumn. The economically rational decision for a maple tree is to invest in fast-growing, cheap but flimsy leaves.”

Prof Prentice added: “It’s taken some time to get here, but one key realization was that we can predict the optimum value of both leaf mass-per-area and leaf lifetime (as opposed to the relationship between them) by starting with independent 'prior' distributions of both properties, which are given to us by nature – and assume that natural selection does the rest.”

Towards a comprehensive theory

We live in a changing environment and all plants are subjected to selective pressures that can modify the economic efficiency of their plant traits. This research will not only explain what grows where today, but it will also move ecology into a predictive science that will:

• enable better, more accurate global and regional climate models
• allow land managers to better model forests and other vegetation, and predict how climate change will affect ecosystems
• allow better estimation of crop yield and the impact of climate change on agriculture.

“We're working towards a comprehensive theory for plant functional traits and how they are selected for by the environment. This paper provides a key element,” said Prof Prentice. “The theory will underpin a new generation of land ecosystem models that will rest on firmer theoretical and empirical foundations than those in use today.”

This research was part-funded by Prof Prentice’s European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (Grant No: 787203 REALM). This work is also a contribution to the LEMONTREE (Land Ecosystem Models based On New Theory, obseRvations and ExperimEnts) project, funded through the generosity of Eric and Wendy Schmidt by recommendation of the Schmidt Futures program.

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Wang, H., Prentice, I.C., Wright, I.J., Warton, D.I., Qiao, S., Xu, X., Zhou, J, Kikuzawa, K., Stenseth, N.C, 2023. Leaf economics fundamentals explained by optimality principles. Science Advances, 9(3), DOI: 10.1126/sciadv.add5667