Why do autumn leaves change color?

A Surprising Way Climate Change Is Affecting Autumn

It’s autumn – time for roasted chestnuts, pumpkin spice, and Oktoberfest. But none of these things would feel quite right without a chill in the air and the leaves turning yellow and orange. If you’ve ever wondered why trees put on this beautiful show, read on to learn what’s happening when leaves change their color – and how studying this natural cycle helps us better predict the effects of climate change.

Before we start, note that we’re talking about deciduous trees (the kind that lose their leaves) in temperate climates (areas of the world with distinct seasons).

By Kristian Peters — Fabelfroh – Self-photographed, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1350193

During the growing season, trees create a pigment called chlorophyll, which allows trees to convert sunlight into the energy they need to grow. (This process is called photosynthesis.) Chlorophyll is what gives leaves their green color. In autumn, cooler temperatures and shorter days tell trees that the growing season is over, and it’s time to stop producing chlorophyll.

As the chlorophyll degrades and the green color fades, leaves turn yellow and orange. Actually, those colors were there all along, but they were hidden by the green chlorophyll! At the same time, trees absorb valuable nitrogen from the leaves, which they can store until the new growing season starts next spring.

For this explanation, our focus is on yellow and orange leaves, but we’ll pause for a few fun facts about other colours. Red leaves are a special case: their red colour results from the creation of a new pigment called anthocyanin. (Interestingly, although we understand how leaves turn red, scientists still debate what purpose it serves.) There are also trees that don’t participate in a colourful autumn: alder and legume trees deliberately drop green leaves without resorbing the nitrogen contained in them. That’s because they have a special symbiosis with bacteria, called rhizobia, which absorb nitrogen from the air, ensuring trees always have enough of it. And in a phenomenon called marcescence, beech and oak trees often wait until the spring to drop their dead brown leaves, maybe as a way to look less appetizing to deer.

We mentioned that cooler temperatures and fewer daylight hours are signals for leaves to change color. But our research into autumn senescence (the fancy name for leaves turning colors and falling from the tree) has revealed a third signal: total energy used. As you’ve probably observed, climate change means spring is starting earlier in the year. It means leaves come out sooner, so trees get to work turning sunlight into energy sooner.

On the other end, warmer temperatures mean autumn is getting later. One might expect that an earlier spring and a later autumn add up to a longer growing season. But there’s a surprising twist: that’s not how trees experience it. We’ve found that there’s a limit to how much energy trees can produce. Once they have produced a certain amount of energy, it’s time for the leaves to change and fall. It’s like if you get up earlier in the morning, you’ll get tired earlier in the evening. (Hopefully your hair doesn’t change color and fall out in the process.) 

In scientific terms, productivity determines senescence. In other words, the earlier spring starts, the earlier leaves turn colors. As we learn more about this phenomenon and the variables that affect it, we are better able to understand the global carbon cycle and how vegetation is affected by climate change. By understanding why spring leaf-out and autumn leaf-off are shifting, we can also predict what this means for species interactions.

For instance, if a tree blooms before a pollinator hatches, how does being out of sync affect these two species and their overall ecosystem? Or if trees lose their leaves earlier, might an invasive plant species be able to take advantage of more sunlight in the understory to spread?

Human-caused climate change is actively destabilizing the complex and sensitive natural systems that developed – in some cases – over millions of years. In addition to taking action to limit climate change, we need to understand the changes that are already happening, so that we can give ourselves and nature a better chance to adapt.