Monday, October 22, 2007

Poor showing for fall colours

Warmer weather has been blamed for less than impressive Fall foliage in New England. According to Dave Gram, writing in the Buffalo News:

Forested hillsides usually riotous with reds, oranges and yellows have shown their colors only grudgingly in recent years, with many trees going straight from the dull green of late summer to the rust-brown of late fall with barely a stop at a brighter hue.

"It's nothing like it used to be," said University of Vermont plant biologist Tom Vogelmann, a Vermont native.

He says autumn has become too warm to elicit New England's richest colors.

While I'm no expert on this, the idea seems intuitively appealing. Brilliant Fall colours are a feature of the Great Lakes region - down here, leave tend to go from green to brown, often with only the slightest nod to yellows and reds. Unfortunately, "intuitively appealing" arguments can be misleading - after all, many people are attracted to creationism for just that reason.

It shouldn't be too difficult to answer the question of whether there is a correlation between Fall colours and climate change. The more interesting question is why there is such a correlation, why might drive such a change. The Gram article attempts to address this:

Warming climate affects trees in several ways.

Colors emerge on leaves in the fall, when the green chlorophyll that has dominated all spring and summer breaks down.

The process begins when shorter days signal leaves to form a layer at the base of their stems that cuts off the flow of water and nutrients. But in order to hasten the decline of chlorophyll, cold nights are needed.

In addition, warmer autumns and winters have been friendly to fungi that attack some trees, particularly the red and sugar maples that provide the most dazzling colors.

"The leaves fall off without ever becoming orange or yellow or red. They just go from green to brown," said Barry Rock, a forestry professor at the University of New Hampshire.

So where do the pigments come from? Why do the plants produce these pigments? While I didn't know the answer to that question, it didn't cross my mind that this was in any way a mystery. Turns out that it is a mystery. There are several hypotheses, including
  1. to attract fruit-dispersing birds (this only works for species with bird-disperse fruit and is, at best, an incomplete explanation; Hoch et al. 2001)
  2. anti-herbivore or anti-microbial defenses
  3. protection against drought or freezing tolerance
  4. protection against photoinhibition that might otherwise interfere with the recovery of nutrients from leaves (Holopainen and Peltonen 2002)
  5. as a means of signaling to aphids that a tree has invested heavily in chemical defenses (Hamilton and Brown 2001)
If Fall pigments play a protective role, it's reasonable to suggest that a shortened period of leaf senescence (triggered by a warmer Fall) might lead to reduced production of these compounds. It is, as I said before, an interesting story. Sadly, it seems like it's still just that - a story.
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  1. Hoch, W. A., Zeldin, E. L. and Mccown, B. H.. 2001. Physiological significance of anthocyanins during autumnal leaf senescence. –Tree Physiol. 21: 1–8.
  2. Holopainen, J. K. and Peltonen, P. 2002. Bright autumn colours of deciduous trees attract aphids: nutrient retranslocation hypothesis.Oikos 99 (1), 184–188.
  3. Hamilton, W. D. and Brown, S. P. 2001. Autumns tree colours as a handicap signal. –Proc. R. Soc. Lond. B Biol. Sci. 268: 1489–1493.

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