Leaf season. I don't know of many two-word combinations that invoke such vivid imagery. No doubt you had a colorful picture come to mind as you scanned those words. Who can't help but marvel at the splendor of a brightly colored autumn vista?So, what's behind this annual event? Why do leaves change color every autumn, and why are some years more vivid in color than others?Although there is still a lot to learn about some of the nuances, we do know the primary reasons. This color transformation involves two important points. The first has to do with timing, or when leaves start to change. The second involves how weather signals the role of the color pigments and addresses why some years are more intense in colors than others.TimingWhat triggers color change in leaves pertains to the calendar or timing. As days become shorter, the amount of sunlight available for plants to photosynthesize is reduced. This is nature's way of signaling plants that winter is on the way. Energy begins to shift from food production into storage and reserves. And this is when color pigments begin to change. As the photosynthesis process slows down in response to shorter days, so does the production of chlorophyll, the dominant pigment responsible for all the green we see when plants are actively growing.But there are three pigments responsible for leaf color. In addition to chlorophyll, carotenoid pigments are also present during the active growing season. They're best-known for producing the yellows, oranges and browns. But because chlorophyll is dominant, it's not until the food-production process shuts down in fall that carotenoid pigments begin to become apparent. Eventually, photosynthesis ceases and all chlorophyll is depleted, allowing the carotenoid pigments to take center stage.The third pigment, anthocyanin, isn't even present in leaves until autumn. Warm, bright days of fall produce lots of sugars in leaves. But as the days shorten and the nights cool, these excess sugars are trapped as veins leading into and out of the leaves gradually close. The combination of bright light and trapped sugars stimulates the production of anthocyanin pigments. They produce vivid shades of reds and purples and many hues in between. So, in the absence of chlorophyll, carotenoid and anthocyanin pigments light up the fall landscape.WeatherThe second factor -- and the one most responsible for the intensity of autumn color -- is the weather. The most brilliant fall displays result from a warm wet spring, a mild summer, bright sunny autumn days and cool but above-freezing nights.So, as you enjoy this season's display of color, hopefully you'll be rewarded with exceptional views. And next spring, if it's warm and wet, be happy. It may be the makings of a spectacular fall.(Joe Lamp'l, host of "Fresh From the Garden" on the DIY Network and "GardenSMART" on PBS, is a Master Gardener and author. For more information, visit www.joegardener.com and www.DIYnetwork.com. For more stories, visit www.scrippsnews.com.)(Distributed by Scripps Howard News Service, http://www.scrippsnews.com)
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