Leaf Senescence: More Than a Color Show
Featured Image Caption: Autumn’s changing light levels and temperatures impact when trees senesce. What other factors might play a role? (Credit: 0x010C, licensed through CC BY-SA 4.0, via Wikimedia Commons.)
Source Article: Maschler J., Keller J., Bialic-Murphy L., Zohner C.M., and Crowther T.W. (2022) Carbon Source Reduction Postpones Autumn Leaf Senescence in a Widespread Deciduous Tree. Front. Plant Sci. 13:868860. doi: 10.3389/fpls.2022.868860
Changing Seasons and Changing Colors
Happy September! As summer comes to a close, many of us in the Northern Hemisphere are looking forward to autumn. Personally, I’m excited about pumpkins, apple picking, and cooler days. For many, trees changing colors from green to bright red, orange, and yellow is a staple of fall.
Leaves typically look green because they contain chlorophyll, which is needed for photosynthesis. As chlorophyll production slows down in the fall and remaining chlorophyll breaks down over time, leaves lose their green color and red, orange, and yellow pigments become obvious instead.
In addition to looking amazing, leaves changing colors at the end of their growing season are evidence of a process called leaf senescence. This is essentially trees shutting down until spring, and reabsorbing nutrients from their leaves before they fall off.
What is Senescence?
So why do we care about autumn leaf senescence? This process marks the end of a tree’s growing season, and the point at which it stops photosynthesizing for the year. Photosynthesis (plants’ use of light, carbon dioxide, and water to make oxygen and sugars) is a major way forests remove carbon from the atmosphere. In fact, it is estimated that forests have taken up the equivalent of over 25% of carbon emissions from fossil fuel combustion and land use change (for example, when a forest is developed or converted to farmland). Since trees stop photosynthesizing when they lose their leaves, figuring out when and why trees senesce is important for estimating a forest’s annual carbon uptake.
Autumn leaf senescence is triggered by changes in temperature and day length, and recent research further suggests that factors like nutrient and water availability or even air pollution may impact the timing of senescence. Scientists are also exploring the carbon sink limitation hypothesis. Trees use carbon in many ways, such as to grow and create fruits and seeds. They primarily import carbon through photosynthesis via their leaves. This hypothesis suggests that once the tree has imported all of the carbon that it needs for the season, it is more likely to begin to senesce. By this logic, decreasing a tree’s ability to photosynthesize (such as by removing leaves) would mean more time before the carbon requirement is fulfilled, so the tree would be more likely to senesce later. If a tree requires more carbon (if, for example, it produced more seeds than usual) then it would also be likely to senesce later.
As a comparison, imagine that trees are large tanks for water (with the water representing carbon), and that leaves are water pumps. Over a season, the pumps must fill the tanks. If the pumps are efficient and fill the tanks earlier than expected, then they can be put away. In contrast, if a tank is larger than usual, the pumps would likely take longer to fill them.
Tracking the Timing of Trees
A recent study tested the carbon sink limitation hypothesis by removing different amounts of leaves from birch trees to see if this would impact senescence time. Different amounts of buds were also removed, with the intention of reducing the trees’ carbon requirements because these trees would be producing fewer fruits and seeds.
The researchers found that bud removal did not significantly impact senescence time, but leaf removal did. Specifically, removing leaves delayed senescence, as hypothesized. For a tree with 75% of its leaves removed, the point at which half of its leaves were senesced was delayed by an average of 4.31 days. This may not seem meaningful, but the alteration in growing season could have larger effects on carbon uptake when applied to a larger area (for example, if a disease caused a whole forest to lose most of its leaves). In such a situation, a forest may take up substantially more or less carbon from the atmosphere, a factor that could be incorporated into models to predict the effects of climate change.
While further research is needed to determine the relative importance of different factors on leaf senescence time, it’s exciting to get a peek at how complicated the process might be. So as the seasons begin to change, admire the changing leaves if you can… and consider the natural forces that might be at work.