Is there a heart in the northern hemishpere that doesn't swell at the first sign of leaves changing color in the fall? For many of us it is the season of greatest nostaligia - of sweaters and boots and hot tea in cold hands, of new beginnings, great hikes, travel and honking geese - of costumes, pumkpins, harvest and all manner of colorful hues beset upon the landscape!
Why do the colors change? Is it the final stand of a dying leaf? Is there an evolutionary purpose to it? Will it always be as dramatic and brilliant as it is now?
Let's start with how it happens.
The simple answer is that trees can do one of two things with their leaves to deal with freezing temperatures. They can become tough (like needles of evergreens) or they can drop their leaves. For the trees that evolved the latter strategy, a few things happen as they are getting ready to drop their leaves. In response to the shorter days and decreased sunlight in early fall, a layer of cells forms at the base of each leaf that closes off the veins that carry fluids into and out of the leaf. These clogged veins trap sugars in the leaf and promote production of anthocyanins, which prodcues the red pigment you see in many fall trees. The other colors you see are a bit diffferent. During the summer, the leaves are chock full of chlorophyll - which hides the the yellow and orange colors of other chemical compounds that are also there (which serve purposes related to photosysnthesis). But these pigments get exposed as the chlorophyll begins to decrease with the oncoming cold season. The browns you see are tannins (a waste product of the tree's life processes).
The amount of moisture in the soil as well as temperature affect the timing and brilliance of the leaf color. It's pretty hard to predict it, but generally speaking a warm period during fall decreases the intensity of autumn colors. Too much rain lowers the intensity of fall color as well because cloudy skies and low light reduce photosynthesis and production of those anthocyanins (the pigments responsible for the reds and purples mentioned above). A warm wet spring, mild summer weather, and warm sunny fall days with cool nights should produce the most brilliant autumn colors. A late spring or a severe summer drought can delay the onset of fall color by a few weeks.
So, how will climate change affect fall color? The trend seems to be that fall color change will continue to be delayed as the climate warms and that increased precipitation will likely dull the intensity of color. A recent Princeton University study used data from the National Phenology Network (the same data set that our New York observers are contributing to) and data from the Harvard forest in Massechusetts to build a model to predict these changes. This research has implications beyond the leaf peeping economy (which totals more than 8 million dollars a year for New England!). The timing of fall color change and leaf drop is relevant for carbon cycling, predicting growing seasons, agricultural productivity and ecosystem productivity.
We need more eyes on the ground to validate these prediction models and ascertain what is happening during the fall season. What could be a better excuse to go leaf peeping than needing to collect your fall phenology data?