Autumn is such a beautiful and even bittersweet time of year; beautiful, in that the leaves of hardwood trees and deciduous vines turn flaming colors of crimson, russet, and gold; and bittersweet, in that while losing their glorious leaves, they are simultaneously embracing the needed protection provided by their now barren trunks and branches from the impending dark, and the bitter cold.
How is it that the turning of the leaves is so predictable? While we ‘Humboldtians’ are out enjoying the last music festival or camping trip of the summer; how is that the trees are already quietly and with increasing Technicolor brilliance, heralding the seasons change?
The answer is decreasing day length. After the summer solstice in June, our day lengths are getting shorter, and delicate, broad leaves take notice. Slowly, the production of the light capturing, green pigment, chlorophyll, wanes…and the resulting sugar production comes to a stop. As the green chlorophyll pigments becomes less and less, other pigments present in the leaves are unmasked and begin to show. Carotenoids are responsible for orange, yellow and brown, and anthocyanins are responsible for reds and purples.
The palette of colors that ultimately show every autumn varies with the climate preceding the decline in chlorophyll production. Temperature and moisture are the most important variables at play.
The ideal temperatures that lead to the most fiery autumnal shows of ‘reds’ first require warm, sunny days with plenty of sugar production. Since chlorophyll levels are still high in August and September, there is plenty of photosynthesis still occurring. (Photosynthesis is the process by which sunlight is turned into sugar). Second, nights must be very cool, but not freezing, so that veins in the leaves constrict somewhat, preventing the sugars from leaving. Lots of sugars combined with pulses of bright light are the perfect ingredients for anthocyanin production and our favored cardinal fall show of arboreal, scarlet fire.
Moisture levels have the ability to delay the onset of fall color by several weeks; and also play with levels of pigment production. The combination of temperature and moisture together, cause the greatest swings in variability. Not varying too much are the yellows and golds produced by carotenoids. Carotenoids are always present in the leaves, regardless of climate variability, and so serve as a baseline palette of autumnal beauty. Our Humboldt autumn shall always, at least, wear gold.
Meanwhile, while most of us are finally declaring fall is here based on the evidence of colorful leaves we noticed on the drive home or the fact that we fired up the woodstove for the first time in a long while, the hardwood trees and deciduous vines have made preparations to bring down the curtain for the year. In response to the chronic, declining day length and intensity of light, the veins carrying fluids into and out of leaves gradually close off as a layer of cells form at the stem base of each leaf. Once this separation layer is complete, their leaves fall.
Autumn, Fall, whatever her name. She is beautiful and bittersweet, and a gentle, annual reminder of the necessary, circle of life.
Draft by Jennifer Wheeler 10/12/12
Jennifer Wheeler is a botanist for the BLM Arcata Field Office with a passion for managing grassland and coastal dune landscapes as well as for eradicating invasive weeds, particularly French and Scotch broom. She resides in McKinleyville with her family and her 2,000 square foot garden and hopes to someday clear 1,000 pounds of produce if she can successfully convince the gophers to let her. (so far this year 808 pounds and counting!)