October 06, 2022

Why do trees shed their leaves?

Written by TreeUtah
Colorful leaves that have dropped from their branches Colorful leaves that have dropped from their branches Photo by Jem Ashton

Each year, the landscape is painted with a vibrant array of reds and yellows as the trees lining the horizon change color and eventually drop from their branches. It’s an eye-catching spectacle, but also commonplace, allowing the routine marvel to cycle through each year without raising questions. Questions like “how do the leaves change color?” or “why do they do it?” If you’ve ever wondered why it happens, read on!

Rooted in place, trees clearly can’t migrate or seek shelter from the harsh winter conditions that will soon be upon us, so they’ve adapted to make do with their circumstances. As the days become shorter and temperatures drop, production of chlorophyll, the chemical responsible for the green color of leaves, stops and other chemicals dominate. Carotenes, the chemical responsible for yellow pigments, has been in the leaves the whole time, but chlorophyll’s brilliant green outshines the yellow for most of the year. Anthocyanins, chemicals responsible for reds and pinks, will enter production as long as temperatures stay above freezing. Drier, warmer Autumns will bring more red leaves (and a shorter period of time before leaves turn brown and drop). The balance of chemicals throughout the tree does not occur uniformly, which is why you might see greens, yellows, and reds all on one tree.

The color change is a side-effect of the shedding process, which involves the weakening of leaves’ abscission layer. The abscission layer is formed where the leaf grows out from the branch and is its primary point of connection. This point of connection is preserved throughout the Summer by the production of a hormone called auxin. Production of auxin slows as days get colder and shorter, causing the connections that hold leaves onto the branches to weaken and eventually break.

Bare branches in the Winter have three primary benefits. First, it allows the tree to expend far less energy in harsh conditions. The energy it would take to maintain leaves throughout a harsh Winter would be too much for a tree to handle. Second, it helps the tree preserve moisture in its trunk. During the Winter, almost all the water usually available to a tree is frozen, so the tree can no longer access it. By dropping its leaves, a tree can halt the process of evapotranspiration (how trees keep cool in the heat – think of it kind of like sweating) and the tree also saves water by not needing to send any to its leaves. Preserving moisture in a tree’s trunk is critical for its survival – it ensures the primary structure of the tree lives and it reduces risk of breakage. The third primary benefit is that the lack of leaves allows forceful Winter winds to pass through the branches with minimal strain. With leaves, the increased surface area would lead to increased wind resistance and more strain on the branches, which would increase the chances of detrimental breakage.

The vast majority of trees go through this process each year, keeping them safe and healthy throughout the Winter, but what about evergreens? Why don’t they go through this process? Well, tune in next week to find out!