Season's Change

Another shot from Cambridge.  So here is what I learn: three factors influence autumn leaf color-leaf pigments, length of night, and weather, but not quite in the way we think. The timing of color change and leaf fall are mostly regulated by the calendar, that is, the increasing length of night. None of the other environmental influences-temperature, rainfall, food supply, and so on-are as unvarying as the steadily increasing length of night during autumn.  The intensity of colors related to weather conditions that occur before and during the time the chlorophyll (which gives leaves their green pigment) in the leaves is dwindling. Temperature and moisture being the main influences.


A succession of warm, sunny days and cold - but not freezing - nights bring about the most spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions-lots of sugar and lots of light-spur production of the brillian anthocyanin pigments, which tint reds, purples, and crimson. Because carotenoids are always present in leaves, the yellow and gold colors remain fairly constant from year to year. Soil moisture also plays a role - a sever summer drought, for instance, may delay autumn by several weeks. 


Every season hath its pleasures;
Spring may boast her flowery prime,
Yet the vineyard's ruby treasures
Brighten Autumn's sob'rer time.
--Thomas Moore


Fall is my favorite season in Los Angeles, watching the birds change color and fall from the trees.
--David Letterman