How fall color works

Daylight

Shortening days start the process. As nights lengthen, a tree stops producing chlorophyll and begins closing off the leaf at the stem. The green fades, and the yellows and oranges already present in the leaf become visible. Day length repeats almost exactly from one year to the next, so a given place tends to change at close to the same time each fall, and the year-to-year variation is usually measured in days rather than weeks.

Elevation

Within a single state, a summit and the valley below it can be three or four weeks apart. In New Mexico, aspen high on the Sangre de Cristos change while the cottonwoods along the Rio Grande are still green, and the two are about an hour apart by road. Higher ground generally changes first and sheltered low ground last. In mountainous country, elevation usually accounts for more of the difference in timing than latitude does.

Latitude

Northern areas generally change before southern ones, and across a long state the gap is substantial. Michigan’s Keweenaw Peninsula is typically finished while the southern counties are only starting, and Georgia’s highest ridges change about a month before its coastal plain. Where a state has little relief, latitude accounts for most of the difference in timing, and the change moves gradually from north to south.

Tree species

Species set both the colors available and the order they appear in. Sugar maple produces orange and scarlet; aspen and birch turn gold; oaks and hickories turn bronze and russet and hold their leaves longer. A few deciduous conifers, among them western larch, tamarack and bald cypress, turn gold or rust before dropping their needles. Species also change at different times: black gum and sourwood redden early, maples follow, and oaks are often several weeks behind the trees around them. A slope of mixed hardwoods therefore colors over a longer period than a stand of a single species, and a pure aspen grove can go from green to bare in about ten days.

What changes the quality of the color

Weather affects how strong the color is more than when it appears. Warm sunny days followed by cool nights favor the red pigments, so those years tend to look more vivid; warm nights work against them. A dry summer can mute the display and shorten it, and a hard freeze or a windstorm can strip a slope in a day. Weather can move the timing as well, though usually by days rather than weeks. Most of the difference people notice between one fall and the next is in how strong the color is and how long it lasts, rather than in when it arrives.