Fall foliage color is determined by climate change and tree health

What bright autumn leaves can tell us about tree health and climate

Megan Bartels

Before pumpkin spice lattes and Halloween decorations, the surest sign of fall was in the canopies of trees as greens turned yellow and red and leaves fell.

This process is influenced by the weather in the fall and throughout the year, resulting in strikingly different fall foliage from year to year. It also means fall will start to look different as climate change manifests itself in more bizarre situations. Studying what’s happening and why could help scientists understand how trees and forests cope with this important seasonal change.

As the sun sets earlier and temperatures drop, “trees, like us, know that fall is coming and winter is on the way,” says Colby College ecologist Amanda Galinatto. . For some trees, survival strategies do not change when cold weather arrives. Most firs, pines and spruces behave normally and look more or less as green as ever in winter. However, most deciduous trees, such as oaks and maples, follow a different strategy and survive the winter by shedding their leaves and leaving their branches bare.

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These trees become bare because their leaves can no longer store enough energy to replace the water the tree loses during the harshness of winter. And by letting go of their leaves, deciduous trees typically let go of their leaves on their own schedule, allowing them to reabsorb some of the expensive nutrients from their leaves and reuse them in the next growing season. This process manifests itself in the sight of autumn leaves.

During the summer, the leaves appear green. This is because the main pigment in photosynthesis called chlorophyll reflects green light. However, chlorophyll is rich in nitrogen and other valuable components, so when winter approaches, trees remove as much pigment from their leaves as possible.

In many trees, such as ginkgo biloba and Japanese cypress, when chlorophyll is removed, yellow pigment compounds called carotenoids appear. “The species that turn yellow have that pigment all year round, but we’re noticing the absence of green,” Galinat says.

Other species, such as red maple and sweetgum, produce new pigments, red and purple anthocyanins, especially for fall. “These colored pigments are like a sunscreen for the leaf tissue while the plant absorbs nutrients from it,” says Yingying Xie, a plant ecologist at Northern Kentucky University. “They don’t want damage from sunlight because they don’t have chlorophyll to absorb light. So some species produce other types of colored pigments to absorb or reflect radiation. .”

Therefore, leaves that turn bright yellow, orange, red, or purple as the days get shorter indicate a healthy tree that has finished its growing season. However, if the timing or color progression is off, it could be a sign that the tree is struggling. “Stress also appears to be involved, and the early onset of leaf fall is often a very good indicator of tree health,” said Christy Rawlinson, a forest ecologist at the Morton Arboretum in Illinois. and talk.

The two main factors that determine the transition to fall colors are the length of the day and the drop in temperatures, especially at night. The exact balance tends to vary between species, with some species being more sensitive to changes in conditions than others.

But no matter how trees balance the environmental information they receive, they sometimes miscalculate. If something goes wrong, the results won’t be as pretty. Severe summer droughts can cause trees to lose their green leaves because they are unable to maintain their leaves throughout the growing season. Autumn rains can encourage the growth of fungi and pests, which can rob trees of their resources and foliage vitality. If a tree freezes prematurely without realizing it, the same thing can happen, or the leaves can turn straight from green to brown before they fall. That’s because frozen water tears the leaves from the inside. “Freezing leaves is like putting a bottle of water in the freezer,” Rawlinson said. “When ice forms and expands, it can destroy your water bottle.”

So, on the surface, climate warming may appear to be a buffer for trees against loss of leaves due to early frosts. But things aren’t that simple, Xie says, and scientists still have much to learn about how extreme temperatures and precipitation add up over the course of a year and affect fall foliage. . Overall, the outlook for great fall foliage is bleak as climate change continues. “As temperatures rise, we expect the fall vibrancy to fade,” Galinatto said.

One reason for the decline in vibrancy is the prolongation of the autumn foliage season. As autumn temperatures become milder, different trees with different coolness thresholds for color change begin preparing for winter at a more dispersed time. This way, you can see just a few trees change color and then drop their leaves before another group does the same.

“Trees are just like humans, they’re all weirdos.”

Furthermore, changes in the composition of the forest ecosystem itself will affect its normal autumn appearance. Native species are exquisitely tuned to the environments in which they evolved, so they can be caught off guard by unusual seasonal changes. In contrast, non-native species are more likely to be able to adapt to these changes, leading to longer growing seasons in both the spring and fall, increasing stocks compared to native species, Galinatto said. This phenomenon can cause the beautiful autumn leaves to fade further into the fall.

Even taking into account a variety of factors, it is difficult to predict the nuances of fall foliage because individual trees are governed by their surrounding environment rather than in coordination with their neighbors. “Color seasons often vary widely, so it’s a great time to go on a treasure hunt where you live,” says Rawlinson. “Trees are just like humans, they’re all weird.”