Featured Image: Autumn colors in the White Mountains of New Hampshire. Source: Lindsay Green-Gavrielidis
Reference: Fu, Y.H., Piao, S., Delpierre, N., Hao, F., Hänninen, H., Liu, Y., Sun, W., Jansenns, I.A., Campioli, M. 2018. Larger temperature response of autumn leaf senescence than spring leaf-out phenology. Global Change Biology 24: 2159-2168. https://doi.org/10.1111/gcb.14021
What causes leaves to change color?
Every fall, people all over the globe flock to temperate and boreal forests to gaze at the forests that have been transformed from lush green to vibrant shades of yellow, orange, and red (i.e. leaf peeping). Leaves change colors because the green pigment, chlorophyll, that plants use for photosynthesis, breaks down, triggered by the changing environmental conditions that fall brings. The breakdown of chlorophyll signals the end of the active growing season, a process called leaf senescence. Chlorophyll is broken down in an effort by the plants to recycle the nutrients contained in the pigments—these nutrients can be used again the next spring to produce new leaf growth. With the green pigment gone, leaves appear different colors—brown, yellow, orange, or red—depending on the other pigments present in the leaves.
Summer and autumn heat delay autumn leaf changes
Exactly what specific environmental conditions trigger the change in leaf color in the autumn is highly variable, depending on tree species and location. Previous studies have relied on laboratory-scale manipulations or large-scale analysis of satellite data to try to determine the driving factors. Recently, a team of researchers led by Yongshuo Fu at Beijing Normal University and the University of Antwerp conducted a controlled study to determine the effects of warming during the summer and fall on the timing of leaf color change in the European beech tree. The findings, published in the journal Global Change Biology, show that when trees have proper water and nutrients, warmer than normal summer and fall temperatures delay the onset of fall leaf change.
In this study, scientists exposed two-year-old European beech tree saplings to warming during the summer or fall in outdoor chambers. Due to the unique experimental design, the researchers were able to keep the temperature in the chamber consistently higher than the ambient temperature, while also allowing for natural temperature fluctuations during the day and night. Overall, the change in leaf color change was delayed by 6-8 days per 1° Celsius (3° Fahrenheit) increase in temperature. The researchers also found that summer and autumn warming sped up the time of leaf-out in the spring, the time when trees begin to sprout new leaves for the growing season. Therefore, warming temperatures extend the growing season by advancing spring leaf-out and delaying autumn leaf color change.
Conclusions and future directions
While warmer summer and fall temperatures may delay the onset of leaf color change in the European beech, it is important to consider other environmental factors that could alter these results. In the present study, the authors provided adequate nutrients and water to the tree saplings. In nature, these conditions are not always the case, and they too might impact the timing of leaf color change. It is also important to remember that different species respond differently to environmental conditions.
While future research is needed to figure out how complex environmental conditions will affect fall colors, resources are widely available to determine when fall foliage will be in peak color in your area. Check out your local leaf tracker, pack up your car, and head out to enjoy the fall foliage and marvel at the amazement of our natural world. For New Englanders, foliage trackers are available for Maine, New Hampshire, and Vermont.