The first day of autumn. Fall 2021 in the northern hemisphere begins tomorrow at 3:21 PM.

Back in March, we wrote about the vernal equinox, calling it a time of anticipation, while the autumnal equinox is a time of introspection. It is weird how I’m asking hard questions of myself these past few days. Where am I going, and what did I do that was meaningful this summer? So far, I don’t have any good answers.

While fall is a time for looking back, we also tend to live in the moment. The “Goldilocks” time of year. The weather isn’t too hot or too cold.

The most striking characteristic of fall is the emerging colors of the leaves that transform the landscape. Leaf viewing is a huge tourism boom for rural communities. Backcountry roads will soon be clogged with “leafers” from the suburbs headed out to look at leaves

Art in the trees

The changing of fall leaves defines every feature of the fall season, even in areas that don’t have defined seasonal changes. The colors of autumn; yellow, orange, red, maroon, and purple, derive from the colors of a typical fall landscape.

Phenology is the study of seasonal events that occur throughout the life cycle of an organism. What seems like nothing more than a tree changing colors in the fall, involves some complex science.

The standard green of a summer forest comes from the chlorophyll pigment in the leaves. Plants use and replace chlorophyll throughout the summer. As the days grow shorter and the temperatures cool, plants send less green chlorophyll pigment into leaves. The yellow-pigmented carotenoids begin to peek through.

Carotenoids are always in leaves; chlorophyll just drowns them out. Xanthophylls are the yellow pigments that give egg yolk and squash their yellow hue. Beta-carotenoids are the better-known orange pigments of carrots and pumpkins.

The other pigments, the red anthocyanins, on the other hand, are made by plants as the trees are growing. There are conflicting theories regarding their function, but one suggestion is that they aid in disease prevention. We’ll see more on that below.

Different trees-different colors

There is a nifty consequence to these leftover color pigments. Different tree types can be identified by their leaf fall leaves. This leaf guide published on a Pennsylvania news site is a great little reference.

Our woods here in north-central Maryland is home to hickory, beech, poplar, maple, and oak, hence the overwhelming gold hue in the above image. The northern tier of North America and Europe tend to have redder leaves. Maybe there are more red maple, sugar maple, and red oaks. The dominant leaf color in different tree species seems to reflect different carotenoids or anthocyanins.  For example, a plant that retains more anthocyanins will be redder. One that has a high concentration of both will be brighter orange.

Autumn and climate change

There they are, those two words that make people cringe. Seriously, human-driven climate change is screwing everything up.

Three things control leaf change in trees: day length (technically, night interval), temperature, and water availability. Unless an asteroid hits the Earth, the day length isn’t going to change. But we have seen the devastating effects of drought and flooding. We know that temperatures are fluctuating.

A recent (like last week) study looked at the leaf change rates in Switzerland over a period between 1961 and 2018. The authors noted a delay in leaf change by 4 days per decade. That’s 18 days later than leaf change was 35 years ago! 

OK, so, big deal. The leafer season is pushed back a few weeks. Aunt Edna and Uncle Donnie have to wait two more weeks to break Nanna out of the home and hit Skyline Drive. But see, the color change is only part of the process.

The timing of nuts and berries could be altered, leaving squirrels, deer, and other woodland fuzzies looking for food when they should be hibernating. Those red pigments, the anthocyanins, may play a role in preventing pests from infesting trees. The red color may function as a deterrent. If the trees don’t turn at the right time, it might be too late to ward off the bugs.

Finally, the later the leaves change, the more it may alter bird migration patterns. There will still be places to roost and food to eat later in the year. I still have three hummingbirds hanging out around the feeders.

The take home

Fall is the favorite time of year for most people. Wine festivals, harvest festivals, corn mazes, Halloween. There are endless opportunities to get outside and surround yourself with the colors, smells, and atmosphere of Autumn. A poll from last year showed 51% of people said fall leaves were their favorite part of the season, and of the top 30 activities, about half involved outside activities.

The mood of Autumn is set by the colors of nature. Those colors come from biology and chemistry, science, composing the most beautiful backdrop for living,