Colour and movement

Contrary to some overseas perceptions, the Australian 'spring' is a colourful season. This mish-mash - and yes it really is a bit mished and mashed - gives a sense of the colour out in the Ku-ring-gai bush this morning, in mid-August. The pink flowers of the boronias were particularly intense.

Speaking of colour and seasons, but admittedly not of Australia and spring, why are autumn leaves more often red in America and yellow in Europe?

Fully functioning leaves are mostly green because they have more green chlorophyll (needed to process the sun's energy) than any other coloured pigment. In deciduous trees, the leaves change colour in autumn as the chlorophyll is recycled - either yellow as a default, or red if a new pigment, anthocyanin is produced. As I explained previously, anthocyanins may provide sun or pest protection, or both, or neither...

Researchers from the Universities of Haifa (Israel) and Kuopio (Finland) suggest we should look at evolution to see why there are more red autumn leaves in America than Europe. There study was published in the latest issue of the scientific journal New Phytologist.

They evoke a long evolutionary war between trees and the insects that use them as hosts. In autumn, insects such as aphids feed of leaves and lay their eggs. Aphids are attracted to yellow leaves so red leaves are a deterrent to this pest. But why do some leaves remain yellow?

Professors Lev-Yadun and Holopainen theorise that until 35 million years ago, much of earth was covered in evergreen tropical forest. A few ice ages and droughts led to the evolution of deciduousness as a way of coping through difficult times. It's proposed that the evolution of red autumn leaves, to ward of pests, also began during this time.

In North America, the trees and their insect pests migrated together along the long north-south mountain ranges as the climate changed. The evolutionary 'war' continued unabated.

In Europe the mountain ranges extend east-west and tree species that could not survive severe cold were more likely to die out, with their insect companions. At the end of the ice age cycle, many of the insect species were extinct and the surviving tree species has less need to spend energy on producing red leaves.

So the scientists hypothesise. Evidence for the theory includes dwarf shrubs that grow in Scandinavia which still produce red leaves. Smaller trees are more likely to survive ice ages becauase they can be protected from more extreme conditions under a layer of snow. The associated insects were also relatively snug and survived to fight another day.

As I said, all this has nothing to do with Australia or spring. My weak connection is that with accelerated climate change, it is important to have corridors for plants to relocate and to evolve. The beautiful bush at Ku-ring-gai is relatively extensive and has at least some connections to the north, although there isn't much mountain height (moving up and down is another way to cope with temperature changes).

And all this begs the question of why the sundew in the photo above is red. Obviously it doesn't repel the kind of insects it likes to digest. I suspect it's more to do with sun protection given the highly exposed places they live. Ah, evolution!