To drop or not to drop: How trees decide when to lose their leaves

Whether we like it or not, summer is coming to an end. Soon windows will be shut, kids will be back in school and the post-Labor Day respite from tourist-traffic will be punctuated by camera-wielding leaf-peepers. Picking the right time for leaf-peeping can be tough: if you head out too early, colors can be underwhelming; arrive too late and you end up missing the show. It turns out trees face an equally difficult problem in “choosing” when to drop their leaves.

Fall on Penobscot MtnFall colors on Penobscot Mountain

Deciduous trees lose their leaves as a way of saving valuable nutrients. The green pigments (called chlorophylls) we see in most leaves allow trees to absorb energy from the sun, like a solar panel. These pigments are nutritionally expensive for trees to create. In this sense, frost is a tree’s biggest enemy – frost can kill leaves, causing trees to lose huge quantities of expensive pigments.

To avoid such a loss, trees employ one of two strategies: They can fill their leaves with antifreeze compounds, which is the main strategy of conifers. The other strategy is to break down and absorb the green pigments before frost sets in, storing their ingredients for next year. As the green pigments are broken down and their constituent nutrients absorbed by the tree, other leaf chemicals become visible, giving us our beloved fall colors.

The onset of fall presents trees with an important decision: when to absorb their leaf pigments and drop their leaves. If a tree pulls in its pigments too early in the season, it misses out on valuable light energy. If it waits too long, it risks losing the pigments to frost, as I mentioned above. Trees must therefore pick the “Goldilocks” moment – losing their leaves at just the right time.

So how can trees tell when the time is right to drop their leaves? If you used only your senses, you would probably notice a few key indicators that fall is coming (besides this blog): days are getting shorter, temperatures are dropping and the air just feels different. Plants have been known to respond to all of these factors: they grow towards light, alter their gas exchange as temperatures change, and are sensitive in growth and survival to humidity and moisture availability. So how would we know if these factors influenced trees’ decisions to drop their leaves?

It turns out there is an entire field of biology dedicated to studying seasonal events in nature, known as phenology (not to be mixed-up with phrenology, which is a 19th century pseudoscience). Phenologists have long been interested in the cues trees use to drop their leaves. Part of the difficulty in teasing apart the mechanisms behind fall leaf drop is the fact that temperatures and day lengths both decrease at similar rates as fall sets in.

So how did phenologists ultimately work out which information trees use when deciding when to drop their leaves? There are several interesting methods. One method is to compare dates of fall leaf drop with weather data over many years. If leaves drop earlier in colder years and later in warmer years, then trees are probably using temperature to decide when to drop their leaves. However, if trees drop their leaves at relatively similar times each year even when temperatures are vastly different, then this suggests that trees are responding mostly to something else, like day length.

Though such weather studies are informative, the only definitive way to confirm which factor causes trees to drop their leaves is to directly manipulate that factor and see how the forest responds (i.e. to run an experiment). Amazingly, this has been done in experiments on entire forests, in which either the whole forest was warmed with heaters or the light regime of the forest was altered using lighting arrays. Some studies even used irrigation or tarps to change how much water the forest received. It turns out that all our factors of interest: temperature, day length, and moisture, together tell trees when to drop their leaves in the fall, though the relative importance of each factor varies across species. The combined use of all these factors may be a way for trees to gather as much information as possible from the environment.

Why should we be interested in phenology in Maine? Temperature and moisture will both be different in a changing climate, and the degree to which trees respond to both in timing their leaf-out and leaf-drop will affect the composition, stability, and profitability of Maine forests in years to come.

Zachary Wood

About Zachary Wood

I am a hiker and kayaker turned research scientist. I work as a PhD student at the University of Maine in Orono, where I research fish evolution, teach biology and take every excuse I can to escape to the University Forest. I spend the rest of my time roaming the state of Maine, searching for interesting ecology. There is an amazing amount of research that happens in Maine’s outdoors. My goal as a writer is to seek out and find the most exciting natural science that hides just beyond our sight – underwater in our lakes, up high in our trees and swimming through our rivers. I have studied ecology from Costa Rica to Greenland, but I feel most at home living and working in Maine. My work has appeared in news publications, scientific articles, and several science symposia. I need your help! If you see something interesting or have a question about science in Maine, please get in touch via email or Twitter @wacharyzood