Elyse C. McCormick | November 18th, 2025

Picture this – it’s a crisp autumn day, the sky is bright blue for once, sunshine is pouring over you, and you’re caught in a gust of wind that makes a full cascade of red and orange leaves flutter down like confetti from the nearest tree. You have hot apple cider in your hands, and life feels good. In this moment, you have perfectly encapsulated the magic of why we call this autumn wonderland “fall” – most deciduous trees give us vivid colors right before their leaves fall off (Figure 1).  

Figure 1. A maple tree works on getting rid of its leaves. (Source: Elyse McCormick)

But…why do they do this? After all, not all trees lose their leaves. That’s part of how coniferous trees got their common name, “evergreen” – they stay green all year long, lose less water because they have needles, and only lose their needles as needed. Not even all broad-leafed trees lose their leaves; many rainforest trees maintain their greenery all year long. What makes our autumn beauties shed their leaves once a year in temperate climates? 

Scientists have been interested in this question for a long time. Leaves are essential to a tree’s survival; they spend all their time making food for the tree via photosynthesis and helping with gas exchange [1,2]. But that’s an energetically costly process and maintaining this much metabolism can be a life-or-death situation for the tree. If a tree is caught with leaves during the winter on an unusually warm day, the leaves will immediately start to photosynthesize in response to light.

This might seem like a great thing, after all, why wouldn’t the tree want to keep making food through the year? But, photosynthesis creates a winter enemy for the tree: water. Trees have internal “veins” just like you and I. Vascular tissues called xylem and phloem transport water and liquid sugars from the roots through the tree. [3]. These tissues are delicate, and the cells that comprise these tissues could burst easily if ice crystals are formed within these “veins”. If these crystals form within the xylem, the tree can die from collapsed vascular tissue or embolisms, where excess gas gets trapped inside this “vein”. When a tree sheds its leaves, it can “winterize” itself to protect its xylem from these dangers. Removing leaves during these conditions has an additional water-related benefit – it also helps trees lose less water as they undertake minimal photosynthesis and transpiration during a very dry period [2,3]. 

The final big issue may seem a little more understandable – insects. Insect damage to leaves is a problem even during the summer, when leaves are at their most delectable and nutrient-dense. Were these trees to maintain their leaves through the winter, they would sustain more damage from herbivory and parasites during an already sensitive time. Scientists have found that insect stress has been a major driver of evolutionary change for leaf loss, going back as far as 97 million years to understand how tree leaves have adapted to defend themselves over time [4]. 

So, now that we know why trees lose their leaves, how do they do it? Trees are not passive, letting the wind take the leaves where it may. Trees have actually developed specialized cells to help lose leaves easily. They’re called “abscission” cells — from the same root word as “scissors” — because they are specially structured to build up and then give the leaf a gentle “snip” off the branch (Figure 2) [1,5].

Figure 2. Abscission layer (stained maroon) of Maple seen in longitudinal section. These cells build up and “cut” off the base of the leaf tissue (light blue), from the base of the stem (dark blue/purple). (Source: Gary Weidemeyer, University of Wisconsin Botany Teaching Collection.)

Wind can help take leaves down, but it’s really a process initiated by the tree to ensure it gets rid of its leafy liabilities before winter hits. 

References:

1. NPR. “Why Leaves Really Fall Off Trees”. Robert Krulwich. [October 30, 2025]. https://www.npr.org/2009/10/30/114288700/why-leaves-really-fall-off-trees
2. Taiz, Lincoln, Ian Max Møller, Angus Murphy, and Eduardo Zeiger. “Plant physiology and development.” Oxford University Press. (2023).
3. Press, Malcolm C. “The functional significance of leaf structure: a search for generalizations.” The New Phytologist 143, no. 1 (1999): 213-219.
4. Labandeira, Conrad C., David L. Dilcher, Don R. Davis, and David L. Wagner. “Ninety-seven million years of angiosperm-insect association: paleobiological insights into the meaning of coevolution.” Proceedings of the National Academy of Sciences 91, no. 25 (1994): 12278-12282.
5. Taylor, Jane E., and Catherine A. Whitelaw. “Signals in abscission.” New Phytologist 151, no. 2 (2001): 323-340.