How do trees know winter’s coming?

Trees keep track of what season it is by measuring the length of the nights. This sycamore tree (Platanus occidentalis)  is bare everywhere except where the street light shines on it at night. The night time illumination has fooled those branches into thinking it’s still summer, so they’ve held onto their leaves. (Photo taken March 20, the spring equinox)

We humans can look at a calendar to know what time of the year it is, so that we can prepare for the weather of the upcoming season. But we also have the luxury of being able to go indoors if the weather doesn’t suit us. Our fruit trees (or any trees) are completely exposed to the weather, 100% of the time. It’s even more important for them to keep track of the time of year – keeping their annual cycles of growth and dormancy in sync with the season is critical for them to survive and reproduce. Trees don’t have a calendar. How do they keep track of what time of year it is?

One of the most important ways trees track the time of year is by measuring the day length, or photoperiod. More specifically, they measure the length of the night. Trees may not have a calendar, but somewhere in their cells they do have a 24-hour clock, which measures how long the nights are.

Many plants around the globe use this technique to prepare for changes in seasonal types of weather. Closer to the equator, those changes tend to be swings in rain fall, alternations between rainy seasons and dry seasons. In the mid-latitudes, further from the equator, the important weather shifts for plants to keep track of are alternations in temperature, between warm summers and frigid, icy winters.

Trees in those mid-latitudes use the shortening days of autumn as a cue that winter is coming, so they stop making tender new growth, and many trees prepare to throw off their leaves in autumn so they can be completely dormant and bare all winter.

Years ago, when I was in college, a professor explained how you can sometimes find a tree where some of its branches have been “fooled” by a streetlight shining at night into thinking the nights are still long. The branches under the street light will retain their leaves long after the rest of the tree has shed its leaves and gone bare.

A few days ago, I spotted one of the best examples of this phenomenon I’ve ever seen, shown in the accompanying photo. The tree is a sycamore tree (Platanus occidentalis, not to be confused with the sycamore fig, Ficus sycomorus, mentioned elsewhere on this blog). Most trees around here have already been pushing out new growth for a number of weeks now, but the sycamore is from more northern latitudes than Florida, so it’s accustomed to longer winters, and it sleeps later before waking up in spring. It’s kind of fun when you get to see something like this, which gives us a bit of insight into how the plants around us orchestrate their lives.


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