Autumn Leaf Color: What's Elevation Got to Do with It?

September may be the first month of the fall season, but you don't have to wait until the month is underway to steal a glimpse of fall colors in the trees overhead. Beginning as early as late August in some places, all you have to do is look up at the trees on surrounding mountains.

It's true -- the first hints of fall color start off at the highest vistas first, then week after week, sweep down to the lower elevations and valleys. The reason why has everything to do with the cooler temperatures found at these higher elevations.

Temperature Decreases with Altitude

If you've ever taken a hike on a crisp, fall day, you know firsthand that air temperatures can start off mild at the base of the mountain yet quickly turn cooler as you climb the summit. In fact, an increase in elevation of just 1000 feet can equate to a temperature decrease of roughly 5.4 °F on a clear day (3.3 °F if it's cloudy, raining, or snowing). In meteorology, this relationship between elevation and temperature is known as a lapse rate.

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Cooler Temperatures Tell Trees to Prepare for Winter

Cooler temperatures (cool, but above freezing) cue trees that it's time for their winter dormant period. Instead of manufacturing sugars for food, cool temperatures lead chlorophyll to dwindle faster, meaning that other leaf pigments (which are ever-present but otherwise masked by chlorophyll production) have a chance to overpower the green machine.

Once peak leaf season has arrived, having several days of cooler weather can also lead to a good burst of color over a short period of time. Here's what other weather conditions can lead to good fall colors...

Trees Change Color from the Crown, Down

Not only do the highest trees change color first, but the highest leaves in a tree do too. As the season cools down, a tree's growth cycle equally slows. Since the leaves at the tip-tops of trees are furthest from the roots, nutrients stop reaching them first (less nutrients = less chlorophyll = bye bye green). And since these lofty leaves are the most exposed to light, by that same respect, they're also the first to respond to fall's lessening daylight hours -- another event which results in the slowdown of chlorophyll and the promoting of color change.