It’s October, and October should mean the beginning of rainy, cozy days with sweaters and books and tea, but instead we’re still having 80 degree weather (plus a 90-degree heat wave last week), and it definitely doesn’t feel like fall. And yet…. the leaves on some of the trees seem to be turning to orange and red around here….hmmmm… It feels so anachronistic.
I assumed that leaves wouldn’t change color until the days get cooler. Apparently, this is a common misconception. Leaves changing color actually has more to do with shortening day length than the temperature dropping.
Leaves typically change colors around the time of the autumn equinox. The equinox is when the daylight hours equal the nighttime hours: equi– means equal, nox means night. (This isn’t strictly true. It’s currently a couple of weeks past the equinox and our daylight hours are still a bit over 12 hours, but this is because the sun still provides some sunlight even after it dips below the horizon. Technically, the equinox is when the sun crosses the celestial equator, and is therefore above the horizon for roughly 12 hours. Or so I gather…. It’s a lot of celestial mumbo jumbo that I don’t really understand. Details details…)
As the days shorten, the plants are triggered to stop making chlorophyll. Thus, they lose their green color. The leaves become shades of red, orange, and yellow because of carotenoids, xanthophylls, and anthocyanins in the leaves. Carotenoids and xanthophylls are always present in leaves. They make leaves yellow, but when there’s also a lot of chlorophyll in the leaf, the green color of the chlorophyll overpowers the color of the carotenoids or xanthphylls and we perceive the leaves to be green rather than yellow. As the chlorophyll disappears, the underlying yellow color is revealed. Anthocyanins are produced only in the fall. They make leaves red. Trees produce more anthocyanins when when days are warm and sunny, and nights are cold. In this situation, leaves can photosynthesize and produce fuel for the plant during the day, but the fuel (sugars) can’t travel down the leaf veins at night because it is too cold, so excess sugars build up. The excess sugars are turned into anthocyanins (the reasons for and mechanisms of this are “complex” according to multiple sources, so I am probably oversimplifying things here). Years that have an abundance of warm early fall days with cold nights result in more anthocyanin production and thus a better fall foliage display.
According to Farmer’s Almanac, the Big Sur Coast Highway is prime fall foliage location in California. Who knew? Peak season is expected to be Oct 15 – 31, so I might have to make a trip out there one of these weekends. If only it would get cool enough so I could wear a sweater and drink some tea while admiring the leaves too…