How does climate change affects autumn leaf colour?

Global warming is more present than ever. The hottest summers, the winters that are slow in coming, the extreme cold, the droughts or the leaves of the trees that do not have their natural colour. The intensity and duration of the brown, yellow,… colours give the intensity.

Autumn is one of the times of the year when nature brings out its warmest range of colours. Brown, yellow and reddish tones are the most predominant in this autumn-winter season.

However, each year the seasons arrive earlier. In spring some of the flowers have sprouted. But in autumn, the colour of the leaves of the trees changes colour due to the increase in temperature.

The process of colour change is as follows: temperatures increase, the leaves of the trees come out earlier and their yellow colour comes out much later. This process makes the trees keep their green colour longer. In other words, climate change is generating the alteration of natural cycles.

Key points

• Climate change is altering the seasons and impacting plants that need pollinators, migratory animals, and people.

• One of the recorded changes is that as the global climate warms, events such as flowering may occur earlier in the year.

What is happening with climate change and seasons?

Do you think climate change is just a problem for the future? Think about it one more time. Climate change is already happening: it is changing the seasons and this means a lot for plants that need pollinators, for migratory animals and finally for people.

The study of regular changes in seasons and in plants (such as the start of the rainy season, the first leaf to fall, etc.) over time is known as phenology.

Citizen scientists are observing and recording these changes through projects like Old Weather, Project BudBurst, eBird, and others.

They are tracking changes that can lead to imbalances for plants and animals, like migratory birds that often arrive at the right place in the guise of berries, or bears that come from far away to eat migratory salmon.

Changes in leaf colour

It has long been known that changes in leaf colour and leaf fall depend on changes in temperature and day length. Previous research had shown that global warming leads to a prolonged spring as the number of colder days is reduced. The hope was that the extended growing season would enhance carbon sequestration, the process of capturing and storing atmospheric carbon dioxide, by trees. The new study shows that this may not be the case in the future.

The reason climate change may be detrimental to fall foliage has something to do with plant biology. When fall arrives, and the length of the day and the temperature drop, the chlorophyll in a leaf breaks down, causing it to lose its green colour. Green gives way to yellows, reds, and oranges that create spectacular fall displays.

Worse yet, dry summers can stress trees and cause their leaves to lose their fall colour change entirely, Schaberg said. A 2003 study in the journal Tree Physiology that Schaberg co-authored stated that "environmental stress can accelerate" leaf decline.

These new findings show that rising temperatures don't mean plants can absorb more carbon dioxide. According to projections from the study's predictive model, by 2100, when the growing season is expected to be 22 to 34 days longer, leaf fall would be 3 to 6 days earlier than it is now.

Summer droughts

According to Arthur DeGaetano, Professor of Earth and Atmospheric Sciences at Cornell University, the climate crisis could delay the arrival of vibrant fall leaf colours and shorten their peak.

The expert explains that as climate change causes warmer temperatures in the fall, the tree's signals to initiate abscission could come later and later.

Another risk for autumn colours associated with the climate crisis is the increase in the intensity of summer droughts, which could wither the leaves of deciduous trees months before the arrival of autumn.

Temperature plays a crucial role in influencing vegetation growth in high latitude regions. The global mean temperature, which has risen by 0.18°C per decade since 1981, likely drives an earlier spring onset and later autumn arrival.

However, the impact of temperature on the growing season can vary based on the type of vegetation. In forest-dominated ecosystems, a warmer climate can enhance photosynthesis and boost vegetation productivity.

Conversely, a warmer climate can lead to increased evaporation, drying out the soil and potentially hindering the growth of plants with shallow roots, such as grasses and herbaceous plants.

Climate change also increases the frequency of droughts during the peak growing season. These conditions can stress plants, causing early leaf shedding or color change, often referred to as a “false autumn”. The UK experienced such conditions in August 2022 during an extreme heatwave.

A longer and drier growing season can also heighten the risk of forest fires. A 2006 US study linked a surge in wildfire activity in the northern Rockies from the mid-1980s to increased spring and summer temperatures and earlier spring snowmelt.

Other factors

Within the modification of the colour of the leaves of the trees in autumn due to temperature, we find more factors such as the duration of daylight and the quality and quantity of light that the vegetation receives. This factor is important since it is the main mechanism for photosynthesis. Another factor is chlorophyll, from which plants draw nutrients and give their daughters their green colour. When the chlorophyll degrades, the green colour begins to disappear and the others we are present at this time appear, such as yellow and red.

If climate change is going to mean significant drought, that means trees are going to shut down and a lot of trees are just going to drop their leaves. Severe droughts really mean the tree just can't function, and that doesn't improve the colour.

Climate change also poses longer-term threats that could disrupt the display of leaves. The spread of invasive diseases and pests and the advance of tree species northward are factors related to rising temperatures that could make fall colours less vibrant, said Andrew Richardson, professor of ecosystem sciences at Northern Arizona University.

The onset of fall colours, which has been shifting into fall, could also continue to come later, said Jim Salge, foliage expert for Yankee magazine.

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