Trees do all of their growing in the spring. The new branches and the buds for next years leaves are already set by the end of June; the leaves you see now were set back in the spring of 2013. This year’s leaves are still making and storing carbohydrates to fuel next spring’s growth spurt, and now their work is almost done and it’s time for a show.
The vibrant leaves typical to New England and Northeast Asia occur because the transition from hot to cool weather is fairly quick, taking place over a few weeks instead of a few months. Those regions also have large forests of deciduous trees that make the colors more picturesque.
You might have noticed that some years the colors are brighter than others, and that depends on the weather. Cool sunny days make for brighter colors than warm rainy ones. Even within a single tree, the branches that get more sunlight with have brighter leaves, just as plants produce more flowers in the areas that get more sun. That’s why some trees may have red leaves on one side and yellow leaves on the other.
The process of making food for the trees to last the winter is what leaves do all summer long, called photosynthesis, in which light energy converts carbon dioxide and water into carbohydrates. Green leaves contain lots of chlorophyll, a chemical that allows plants to absorb energy from light to fuel the process. The longer, sunnier summer days mean that the chlorophyll is active enough to trump all other pigments present in the leaves. But when the days grow shorter the chlorophyll loses its spunk and other chemicals that create different pigments get their day in the sun, so to speak.
Scientists theorize that anthrocyanins are brightest during sunny dry autumns because, in the absence of chlorophyll, they protect the leaves’ ability to keep producing sugars as long as possible before winter (which is why the sunnier side of the trees that contain anthrocyanin tend to be red). Seen most often on: maples, oaks, sourwood, sweetgums, dogwoods, tupelos, cherry trees and persimmons.
These pigments also help to absorb sunlight for sugar production further into the season, and they don’t require the trigger of dry weather and bright sunlight that anthrocyanins do. Seen most often on: hickory, ash, maple, yellow poplar, aspen, birch, black cherry, sycamore, cottonwood, sassafras, and alder.
You might have noticed that some leaves just turn brown (that’s the tannin) and that oak trees, in particular, retain their leaves much longer than others. The scientific term for the leaves falling is abscission (a weakening of the connection at the base of the stem of the leaf) and when leaves fail to disconnect from the tree in the autumn it’s called marcescent foliage. Some scientists think it is way for those trees to gather more water from the winter snow that collects to the leaves, while others think that waiting until spring to drop leaves provides better compost for the roots.
Five useful online guides to the prettiest fall foliage:
- The U. S. Forest Service Fall Colors Guide
- The Weather Channel’s Fall Foliage Map
- Popular Science Guide to Fall Foliage (Infographic)
- The Farmer’s Almanac
- Forbes 10 Best Fall Foliage Destinations