Photos by Christine Loberg
Leaves turn green each spring as they use pigments (chlorophyll, xanthophyll, and carotenoids) to harness the sun’s energy. The chlorophyll absorbs red and blue wavelengths but not green ones that are reflected back to our eyes; hiding underneath the chlorophyll are the other compounds for photosynthesis. As temperatures cool and days get shorter, leaves on deciduous trees stop producing new chlorophyll, and the familiar green color breaks down to reveal the other pigments that have been masked all season. Chlorophyll is expensive to make (for the tree); so when there isn't enough light to make it worth expending energy on, the green fades and the colors beneath show. This is a photoperiod process, not a temperature-dependent process.
How is the Red Pigment Made? Continued sunny days and cool nights trap sugars in leaves, and some leaves will form the red pigment anthocyanin, which colors trees like dogwoods or the non-native sugar maple near the Yosemite Chapel in the Valley. Vines like the poison oak along the El Portal Road also turn a brilliant orange, pink, or even purple.
What Colors Do Leaves Change to in a Variety of Yosemite’s Tree Species?
Quaking aspen: Yellow
What Weather is Needed for Good Color? The best autumn colors occur under clear, dry, and cool conditions but not freezing weather. The degree of color may vary from tree to tree and even leaf to leaf. Leaves directly exposed to the sun may turn red, while shaded leaves may be yellow. Leaves on marcescent trees, like some California black oaks, will wither but linger all winter and only fall next spring when new leaves emerge.
Do Some New Leaves Grow during Winter? Live oaks, tanoaks, bay laurel, and the conifers keep their newest leaves throughout the winter to get a head start on food production next spring.
Where in Yosemite Promises Good Color?
A search for quaking aspen, for instance, can be found in some favorite glades within the park. Easy for leaf-peepers to drive by are roadside stands along the Tioga Road near the Tuolumne Grove trailhead and the Yosemite Creek picnic area, while more isloated aspens grow along the Glacier Point Road past Summit Meadow. Plus, consider a gentle walk along the old road from Badger Pass ski area to Bridalveil Creek Campground that leads to aspen groves. Read on for more suggestions on where to see fall color:
When Will Color Be at its Height in Yosemite This Year? Hard to predict, depending on light and weather. Usually in late October and, generally, lingering until the first heavy winter storms or hard frosts often in early December.
Did You Know?
In Wawona and downstream, the South Fork Merced River provides habitat for a rare plant, the Sierra sweet bay (Myrica hartwegii). This special status shrub is found in only five Sierra Nevada counties. In Yosemite, it occurs exclusively on sand bars and river banks along the South Fork Merced River downstream from Wawona and on Big Creek.