Rising Temperatures, Falling Leaves

KEY CONCEPTS

• As peak leaf-peeping season spreads across the U.S. each fall, it brings billions of dollars in tourism to some states.

• The timing and brilliance of fall color are influenced by temperature, sun exposure, and rainfall—along with a range of other conditions. 

• These conditions are shifting with climate change, disrupting the ecological and economic value linked to fall foliage. 

• Fall plant cycles are complex, but our understanding of their response to climate change is growing. 

Not all about looks

Golden aspen groves in the Rockies. Orange sassafras slopes along the Blue Ridge Parkway. Scarlet maples dotting New England’s lakeshores. 

As peak leaf-peeping season spreads across the U.S. each fall, it brings billions of dollars in tourism to some states. The fall display also signals the end of the growing season—a critical time for forest ecosystems and their carbon budget.

But the conditions that trigger fall color each year are shifting with climate change, potentially disrupting the ecological and economic value linked to fall foliage. 

Foliage factors

Some of the key factors that affect the timing, duration, and brilliance of fall color include:

• Temperature: Fall temperatures have increased by 2.7 °F on average across the U.S. since 1970, delaying the cool trigger that plants need to start shutting down. Warmer autumn temperatures generally (though not always) lead to later and shorter peak fall foliage periods.

• Sun exposure: As fall days shorten, the reduced sun exposure cues plants to ramp down photosynthesis. This process reshuffles leaf pigments and starts the colorful displays of fall. When tree species shift northward in response to warming, shorter days at higher latitudes can cause earlier leaf color and drop. 

• Rainfall: Abundant summer rain and soil moisture can cause later, brighter color. But too little or too much rain can both stress trees and cause leaves to change and drop early.

Trees are sensitive to these three factors—but that’s not all. A range of other conditions (which vary across tree species and regions) can also influence the timing and color of peak fall foliage. Here are a few:

Fall back: factors linked to later leaf fall

• Higher October temperatures were most strongly linked with later leaf fall in the Northern Hemisphere. And October warming comes along with fall warming seen in all U.S. regions. 

• Nighttime warming during the fall can contribute to a delayed start of leaf color and drop. And October nights have warmed since 1970 in hundreds of U.S. locations.

• Declining winds lead to later leaf fall in higher latitudes (>50°N). 

• Lower latitudes (25° to 40°N) are generally more sensitive to the effects of warming temperatures and tend to experience delayed leaf fall.

Early arrival: factors linked to earlier leaf fall

• Drought and water stress can trigger earlier shedding of leaves. Higher latitudes (> 50°N) may be especially sensitive to drought effects with projected warming this century. 

• Extreme drought and heat stress can cause leaves to shrivel before displaying their fall colors, dulling the seasonal color palette. And many U.S. locations have experienced longer heat streaks and rising drought risks with warming. 

• Greater cooling degree days were significantly linked with earlier leaf fall in temperate Northern Hemisphere locations. And cooling degree days have increased since 1970 in over 200 U.S. locations. 

• Higher latitudes (50° to 70°N) are generally more sensitive to sun exposure and tend to experience earlier leaf fall.  

Dull or dazzling: factors linked to leaf color

Shorter days and less sun exposure cue plants to ramp down photosynthesis for the season. Doing so reduces chlorophyll (a green pigment used during photosynthesis), revealing orange and yellow leaf pigments that were previously masked by green. At this stage, some species (such as red maples) start producing red or purple pigments, enhancing the vibrance of fall forests.

Once this process starts, the balance of light intensity, temperature, and moisture can affect color displays:

• Bright and cold days can intensify the production of red pigments. 

• Cool nights and bright, sunny days can further enhance vivid color displays, which favor a 9-12°F difference between daytime and nighttime temperatures. 

• Early frost can damage leaves and limit production of red pigments. 

Climate connections

If the long list of factors above gives the impression that fall plant cycles are complex, that’s because they are. Some other seasons are better understood. 

For example, earlier leaf emergence in the spring is a broadly-observed response to warming winters. By comparison, the effects of climate change on fall foliage are more variable and less well understood.  

While some studies observe a delay in fall leaf color of several days per decade in the U.S., others indicate no significant change in the timing of leaf color in Europe from 1982-2011. 

But this is an active area of research and our knowledge is growing. A 2020 study found that higher plant productivity during spring and summer can lead to earlier leaf fall in temperate trees. 

Earlier leaf fall means shorter growing seasons, which in turn could limit the amount of carbon that can be stored in forests. As we increasingly rely on forests and other ecosystems to soak up excess CO2 from the air in an effort to mitigate climate change, it’s critical to understand how rising temperatures affect falling leaves.

POTENTIAL LOCAL STORY ANGLES

How is fall foliage progressing in your local area?

Explore the National Phenology Network’s Land Surface Phenology satellite-based maps with coverage for the contiguous U.S. Check the Smoky Mountains predictive fall foliage map for 2022 for a rough estimate of the timing of peak color in your area. Northern Arizona University’s PhenoCam provides daily photos of foliage around the world.

LOCAL EXPERTS 

The SciLine service, 500 Women Scientists or the press offices of local universities may be able to connect you with local scientists who have expertise on fall foliage and climate change. The American Association of State Climatologists is a professional scientific organization composed of all state climatologists. 

NATIONAL EXPERTS 

Amanda Gallinat, PhD
Visiting Assistant Professor
Colby College
Contact: agallina@colby.edu
Related expertise: Plant and animal phenology

Yingying Xie, PhD
Assistant Professor 
Northern Kentucky University
Contact: xiey2@nku.edu
Related expertise: climate change on plant phenology


Christine Rollinson, PhD
Forest Ecologist
The Morton Arboretum
Contact: crollinson@mortonarb.org (cc: Tyler Prich, tprich@mortonarb.org)
Related expertise: Climate effects on trees, forest communities, ecosystem functioning, and phenology

Richard Primack, PhD
Professor of Biology
Boston University
Contact: primack@bu.edu
Related expertise: impact of climate change on plant phenology

Alyssa Rosemartin
Partner and Application Specialist
USA National Phenology Network
School of Natural Resources and the Environment, University of Arizona
Contact: alyssa@usanpn.org
*Available for interviews in Spanish and English

Howard Neufeld, PhD
Professor
Department of Biology
Appalachian State University
Contact: neufeldhs@appstate.edu
Related expertise: plant ecology, fall foliage