Bill Pramuk, Trees and People in Napa Valley: Stressed trees show early fall colors
Sept. 22 is the first day of autumn and some of our trees are beginning to show it. We tend to associate fall colors and falling leaves with chilly nights and shorter daylight hours, but that is not the half of it.
I have been noticing some reds in the red-silver maples, and yellow leaves from black walnut trees. These are trees one expects to be clear green until our main fall color comes on. Here in Napa Valley, the best fall colors tend to appear in late October and November.
The color we are seeing now is premature fall coloration brought on by a long hot summer, drought stress and, in many cases, sun injuries to tree trunks. Having a close look at a red-silver maple showing premature fall color in an unirrigated curb strip in my neighborhood, I noted the sunny side of the exposed trunk badly sun injured, or "sun burned" if you prefer. Direct afternoon sunlight impacting it had killed the bark, and split-gill fungus had colonized the dead and dying tissue.
A good reference on this for arborists is "Physiology of Woody Plants" (Kramer and Kozlowski). It says the color of the leaves can be telltale signs of "water strain."
I see this constantly in young trees on drip irrigation. Planted in compacted or thin topsoil with one ring of drip irrigation tubing, no reality-check monitoring of root zone moisture, and few or no branches shading the bark, the trees go off-color in summer. The trunk and branches show discoloration and injury where direct afternoon sunlight impacts them.
There is an example in a nearby front yard where this happened to a pair of 'Sango Kaku' coral bark Japanese maples. In this very upright variety the branches do not spread enough to shade the trunk. One declined and died a slow death of drought stress and sun injury. It was replaced. The matching tree near it did the same thing and later both replacements died.
The examples that caught my attention this year are hybrid "red-silver" maples. The named varieties include '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."
Both "parents" are water-loving trees, not dry-summer-adapted California natives.
There are quite a few in my neighborhood, growing in the city curb strips, 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 tend to show premature fall color while those in irrigated curb strips, or adjacent to lawns are still showing fairly normal summer foliage color.
When autumn comes to California there is not much red coloration to be found in our native trees. We see red in poison oak going summer — dormant in dry woodlands, and a beautiful red in wild grape vines (Vitis californica), festooning deciduous trees along the creeks. And that's about it. The red-fall-color genes are absent in our native trees.
Yellow and gold are common here. Lately I've been noticing it in early shedding of leaves scattered on Shurtleff Avenue, from the row of black walnuts along the edge of Camille Park. The color appears as the greens fade out, unmasking the yellow 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 xanthophyll.
In maples and other species that have the genes for it, the red comes from anthocyanins that develop in cool — not freezing — sunny weather. Other pigments are present in leaves, depending on tree species: carotene for orange and tannins for brown.
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 colors appear.
Photos: Take a look inside Napa's new (and old) Pine Street MarketPine Street Market 7
Pine Street Market 6
Napa's Pine Street Grocery 1967
Napa's Pine Street Grocery 1967
Pine Street Market 4
Napa's Pine Street Grocery 1967
Pine Street Market 5
Pine Street Market 2
Pine Street Market 3
Pine Street Market 1
Bill Pramuk is an ASCA registered consulting arborist and an ISA certified arborist. Visit his website, www.billpramuk.com , email questions to , or call him at 707-363-0114.
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