Dressing up for Seasonal Death
(November 16th 2007) Just like lifestyle journalists, science writers have their seasonal themes, too. Instead of stories about celebrities on summer vacation or how-to's on christmas decoration however, we deliver news on tick-borne encephalitis or the science behind leaves turning red and yellow in autumn. Fall foliage has been a particularly fruitful topic in the last years as it has everything a seasonal research theme should have: emotional appeal, a high profile and conflicting theories with new findings every year. An overview from Brynja Adam-Radmanic.
They're the kind of questions children tend to ask: Mum, why do the leaves turn yellow or red in autumn? Dad, where has the green colour of the trees gone to? For years and years, textbooks for children and biologists alike have had an easy explanation: Trees digest the chlorophyll, transport the degredation products to branches and bole to recycle them. When the dominant green is gone, it no longer masks the yellow carotins and red anthocyans left in the leaves, so their colours can charm our senses on golden October and November days.
As we know now, this story has some flaws. One fact in particular made botanists doubt the old theory. In trees and shrubs that turn red, the anthocyans haven't been waiting in the leaves since spring. These pigments aren't in fact produced until autumn. Therefore, if trees invest in their production, anthocyans must have a function specifically for the autumn leaf.
It seems fall foliage is more than just a waste basket of leftovers, not worth recycling for next year. But what purpose could this pigment production serve? In the last few years several new theories have emerged attempting to explain why trees bother to dress up for their last appearance of the year.
One is the "leaf signal" hypothesis. It proposes the colour to have a repellent function, signalling to aphids and other insects to avoid the tree in their search for a place to lay their eggs. In analogy to mechanisms in sexual selection, the leaves' bright colours are thought to show the fitness of the tree.
Just as female birds choose to mate with the most impressively coloured male, for insect mums a tree's autumn foliage could be a similar, though converse sign. According to the leaf signal hypothesis it's a possible measure of the tree's ability to ward off the hatching larvae in spring.
In fact, it can be shown that aphids prefer green leaves over the yellow and red ones and - even more compelling, that trees with strong colours in the fall tend to suffer less damage from insects the next spring. Still, such a correlation between weaker colours and the susceptibility to insect damage could be caused by other, more general factors as well.
Another factor that has been shown to serve a function in the autumn leaf is the ability of anthocyans to shield the degrading photosynthetic apparatus from too much light, protecting the transport and recycling of its components. This 'sunscreen hypothesis' was substantiated by mutant studies. They show that trees lacking the pigments actually have difficulties shipping nutrients out of the leaves when preparing for winter.
As was announced this october, researchers at the University of North Carolina in the USA have found that trees in nutrient-poor soil produce more anthocyans, a finding supporting the sunscreen hypothesis. If the pigments are essential for the regaining of nitrogen and other essential elements from the plant's leaves in order to recycle them in the spring, their presence should be even more important in nutrient-poor soil.
In addition to the leaf signal and sunscreen hypothesis, there's another suggestion concerning the function of anthocyans in fall foliage. There is some evidence that the pigments can be of vital importance for the tree also after the leaves have fallen. In 2005, biology professor Frank Frey and a former student, Maggie Eldridge, showed extracts of red maple leaves to be an excellent form of weed control for the tree.
Extracts from green leaves don't affect the growth of lettuce, but when watered with extracts from scarlet-tinted autumn leaves, its germination and growth is dramatically reduced. This indicates a role for anthocyans in allelopathy, the plants' chemical warfare in interspecies competition for space and light. It is assumed that the anthocyans leak from the autumn foliage surrounding a tree, preventing the growth there of seedlings from other tree species.
These hypotheses and researchers' ongoing search for evidence to support them promise seasonal science stories for the many autumns to come. So tune in this time next year for an update, if you like...
