Red Alert

At first blush, New Zealand appears a land without the vivid autumnal reds of the northern hemisphere. But Ann Graeme finds splashes of scarlet in our native forests.

 Red is the colour of blood. To mammals like us, red signals excitement, danger and lust. Red attracts birds and lizards too, for they see the same range of colours
that we do. The red flowers of pöhutakawa catch the tüï’s eye and lure the lizards. So do the red fruits of karamü, supplejack and strawberries.

 But red is less attractive to insects like honey bees. Indeed, it is scarcely visible. Bees see a different rainbow of colours beginning with ultra-violet (which we cannot see)
and excluding red. Blue is their favourite colour, which is why they are attracted to blue clothing.

The heath myrtle of Western Australia has masses of pink and white flowers, each of which dulls to rusty red after a bee has visited and pollinated it. How efficient! Now the bees’ activity is directed towards flowers awaiting pollination rather than wasted in revisiting fertilised flowers.

I used to wonder about red roses, some so dark bees would be unlikely to notice them. They don’t attract birds or lizards either, so why are they such a rich, velvety red? Then the simple answer struck me. Red roses are not selected by nature but by people. Most wild rose species are white or cream or tinged with pink, and it is selective
breeding that has created the red roses we admire.

Red plays other roles in the world of plants, especially in the northern hemisphere where in autumn the forests of deciduous trees become a riot of red and yellow. This delights the eye, but the show is not for us but for a much more mundane purpose – a housekeeping matter. It is the tree’s response to the coming of winter.

Leaves are green because they contain the green pigments called chlorophyll, essential for capturing the sun’s energy and transforming it by photosynthesis into plant food. That’s all very well in spring and summer but  as the dark, cold winter approaches, leaves may become a liability. Photosynthesis will slow down or stop and the leaves may be damaged by frost. The energy the plant uses to maintain the leaves may end up being more than the
energy the leaves can produce. It may be best to get rid of them, which is what deciduous trees do in autumn.

 But first, like a thrifty housewife, the tree saves what it can. From the leaves it trucks out sugar and amino-acids and stores them in the bark of its branches and the outer
sapwood of its trunk and roots. The green chlorophyll pigments are broken down and their constituents are taken away and stored, to be recycled into new leaves in the spring. It is at this time that the red pigments called anthocyanins are created. Their powerful antioxidants act like bodyguards, shielding the cells from UV damage and disarming rogue chemicals.

Now the leaf veins fill with corky tissue, shutting down the water supply to the leaf. With its green colour gone, the hidden orange and yellow pigments show through and so do the newly created reds. The coloured leaves flutter the ground, to decay and add what’s left of their nutrients to the soil.

 Red anthocyanins play many roles in the plant world. Masked in apparently green leaves, they protect the chlorophyll from too much and too strong light, which actually inhibit photosynthesis. They act as a sunscreen protecting the growing shoots from damaging UV light and frost damage. It is anthocyanin that colours young fern fronds pink. Anthocyanins help desert plants resist the stress of drought and their antioxidants repair wounded leaves.

 Anthocyanins serve so many purposes that Kevin S Gould of Auckland University calls them “nature’s Swiss Army knife”.

Useful as anthocyanins may be, on land green chlorophyll is still the best colour to capture most of the spectrum of sunlight, which is why the land is carpeted in green plants. Green works well in the surface waters of the sea too, fuelling the carpet of floating plankton, each plant so tiny and so short-lived that they are invisible.

But deeper down as sunlight passes through the water, some of the colours are absorbed, first the red, then orange, then yellow. As the red end of the spectrum diminishes, chlorophyll becomes less effective at capturing light energy. Now it needs the help of other pigments. This is why seaweeds that grow deeper in the sea are brown
and red. Their green chlorophyll is masked by these helpful pigments, which capture the energy from blue, green and yellow light waves and transmit it to the chlorophyll to use
for photosynthesis.

Nature’s signals do not always have such a rational explanation. It is tempting to think that the colour of the red-and-white spotted mushroom called fly agaric is warning us
of danger. And poisonous it is – though not as deadly as its innocuous-looking relative, the death cap fungus.

Fly agaric is hallucinogenic too. It is not a fungus to fool with but it is pretty and widely pictured in children’s story books. Elves and pixies are often seated unwisely on red-
and-white-spotted toadstools!

Thanks to botanist Shannel Courtney for his help with this article. 

 99.9% green

In New Zealand, our native forests don’t blaze with red and yellow and gold. Nearly all native plants are evergreen. We have few deciduous trees and shrubs. There are several explanations for this.

Our islands are temperate and our winters are not extreme. Dropping leaves is a sacrifice most of our native plants don’t have to make. Those that do – ribbonwoods, lacebarks, tree daisies, köwhai and fuchsia – grow in fertile places, often on valley floors where the frost drains down. They can afford to lose their leaves each autumn because there are enough
nutrients in the soil to grow a new crop in the spring,and frost gives them a good reason to drop their leaves and become dormant.

Another explanation may be that many of our broad-leaved plants belong to tropical families
whose members are evergreen. This is so for kämahi, rätä, pöhutukawa, quintinia, hïnau, tawa, pigeonwood, mähoe, pukatea, kohekohe and püriri. And our podocarps like rimu and tötara are evergreen, too, so green is the colour of our native forest all year round.