Driving around town over the past couple of weeks, I have been noticing some reds, yellows, gold, and pinks in trees one expects to be clear green at this time of year in Napa Valley. After all, it is still summer and the first day of autumn is a week away.
The phenomenon is premature fall coloration.
It is a common, but erroneous, notion that fall color develops in trees only when the nights turn freezing cold. Yes, temperatures have a lot to do with fall color but there is a lot more to it.
As Kramer and Kozlowski term it in “Physiology of Woody Plants” the color of the leaves can be telltale signs of “water strain.”
I noticed this in a young coral bark Japanese maple planted in a front yard a couple years ago. I suppose the tree was on a drip irrigation system, but the thin, rocky soil and too-limited irrigation probably prevented adequate root growth. The tree went off-color in summer and eventually died. It was replaced, and now the matching tree near it is doing the same thing.
The examples that caught my attention this year are hybrid “red-silver” maples. They come in named varieties like Autumn Blaze and Autumn Fantasy. These hybrids have some of the beauty of red maples plus some of the aggressiveness of silver maples.
“Trees for Urban Landscapes” (Edward F. Gilman, Delmar Publishers, 1997) describes red maples as “growing best in wet places.” Silver maple, he says “grows best along stream banks and flood plains… Save them for planting in areas where nothing else will thrive.”
Depending on one’s point of view, hybridizing the two species produced a more refined silver maple, or a less elegant, but tougher, red maple.
Still, both “parents” are water-loving trees, not dry summer-adapted California natives.
There are quite a few in my neighborhood, growing in the city “right-of-way,” the curb strip, now showing a range of colors from fairly green to mostly red. The root zone conditions tend to correlate with the foliar color. Those in dry gravel curb strips are showing more premature fall color while those in irrigated curb strips, or adjacent to lawns are still showing fairly normal summer foliage color.
In deciduous trees like these, which develop red autumn coloration, formation of the red pigments, anthocyanins, is favored by drought. But many trees do not have that capability. Their fall colors may be various shades of yellow, gold, or even brown. The yellows appear as the greens fade out, unmasking the yellow that was already present in the leaves.
This is similar to the yellowing of older leaves on a stressed plant. The plant surrenders the oldest leaves first. The green chlorophyll pigment fades out, revealing the yellow color of xanthophyll.
Other pigments are present in leaves, depending on tree species: carotene for orange and tannins for brown.
Weather and soil moisture can affect the onset and intensity of color and duration of leaves remaining on the tree but nothing in nature will make trees of certain species turn red in autumn.
Chinese hackberry (Celtis sinensis) is a good example. It can succeed in hot, dry locations, but will show yellow, early fall coloration when stressed.
On a hill where I know the topsoil is very thin over a rocky base, a few are growing well, but not uniformly. Some are showing yellow and gold fall colors, which correspond to root zone conditions.
As the days grow shorter and nighttime temperatures cooler, and in trees that are stressed by summer dryness, photosynthesis slows and the chlorophyll in the leaves disintegrates. The green fades out and colorful pigments appear.