November 4, 2007

Great Fall Color for Trees is not Surprising


Once again the experts got it wrong. Fall color doom sayers were fueling the fire of arboreal panic earlier this year, looking into their silvicultural crystal balls and predicting a disappointing season for leaf viewing aficionados. The drought, the regional 'experts' said, would cause trees to shorten their color display and bring fall senescence much earlier than usual. Of course, that hasn't happened.

The fall color around town is actually rather good. In fact, this fall seems to be unusually long. In previous years, even those with adequate rainfall, leaves have tended to fall much quicker. For example, by the end of November in zones 7 and 8, Tulip poplar Liriodendron tulipifera has all but completely dropped its leaves. But that just isn't the case in 2007. Zelkova trees have been showing great uniform color in parking lot islands. Red maple trees have been more colorful, on average, than recent years. Sweetgums are holding on to their leaves and are just now showing reds and purples.

Why our drought has caused normally knowledgeable people to fall off the cart and predict the unpredictable is baffling. Water availability is, at best, a tiny consideration when it comes to fall leaf color. Fall color is governed by the differential between day and nighttime temperatures, and the presence of pigments in the tree leaf. Anthocyanins, the pigment that produces red in the tree leaf, can actually be heightened by poor soil and deficits of nitrogen.

Fall leaf color is not a survival function of the tree. It is simply a visual artifact during the leaf shedding process, as chlorophyll disappears at the end of the growing season.