Why leaves change color in fall

Three pigments

A leaf’s autumn color is decided by three groups of pigments, and only one of them is new in the fall.

• Chlorophyll gives leaves their green and powers photosynthesis. Through the growing season it is constantly produced and broken down, which keeps the leaf green.
• Carotenoids make yellow, orange and brown. They are in the leaf all summer, hidden by the green — which is why yellows and golds are fairly constant from year to year.
• Anthocyanins make red, purple and crimson. Most of them form in autumn, in response to bright light and surplus sugar in the leaf cells. This is the part of the display that varies most.

The trigger is the length of the night

As nights grow longer, chlorophyll production slows and then stops, and the green fades to reveal the other pigments. Night length is the most dependable cue a tree has; temperature, rainfall and food supply vary far more from one year to the next. At the same time the veins at the base of each leaf gradually close off, trapping sugars in the leaf and feeding the production of anthocyanins.

That is why the calendar of color is so regular in broad outline — and why the typical windows are useful — even though no two years are alike in detail.

What the weather adds

According to the Forest Service, a run of warm, sunny days and cool, crisp but not freezing nights produces the most spectacular reds: sugar builds up during the day and the cool nights keep it from leaving the leaf. Other patterns work against the display:

• A severe summer drought can delay the start of fall color by a few weeks.
• A warm spell in fall lowers the intensity of the colors.
• Cool, wet summers can bring color on early, as Minnesota’s DNR notes.

Which tree turns which color

Species matter as much as weather. A hillside of sugar maple and a hillside of oak look nothing alike in October. These are the Forest Service’s descriptions:

• Red maplebrilliant scarlet
• Sugar mapleorange-red
• Black mapleglowing yellow
• Aspen and yellow-poplargolden yellow
• Hickoriesgolden bronze
• Oaksred, brown or russet
• Dogwoodpurplish red
• Sourwood and black tupelocrimson
• Beechlight tan

Some trees barely take part. Striped maple turns almost colorless, and elm leaves shrivel and fall with little more than a drab brown.

Conifers that turn gold

Most conifers stay green, with one well-known exception in the North Woods. Tamaracks lose all their needles every year, and before they do, they turn a bright golden yellow.