Getting to the root of a tree’s habits

THERE'S quite a lot going on in the world of trees at present. The leaves of deciduous trees in Ireland change colour and drop to the ground quite late in the autumn.

The timing varies from species to species and, to a lesser extent, it depends on the weather. But it's usually well into October before things really start happening.

But trees have to be able to anticipate events and make advance preparations. These preparations are happening right now.

So why do deciduous trees drop their leaves in autumn? Why do the leaves change colour? Why are autumn leaves less brightly coloured in Ireland than in eastern North America? There are various answers to these questions — some simple and some rather complicated.

The reason that the tree drops its leaves is really quite simple. It's a strategy adopted by some tree species for surviving cold winters in the temperate zone. There are no true deciduous trees in the tropics.

The tree extracts all the useful chemicals from the leaves in the autumn and stores them underground until they're needed the following spring. It then drops what's left of the leaf, a sort of sucked-out husk, and, unless you sweep it up and take it away, this dead leaf eventually decomposes and is broken down into other chemicals which will help feed the tree in future seasons.

What makes a leaf green in summer is chlorophyll, which is a protein, and this is the main chemical that the tree sucks out of the leaf and breaks down for underground storage. Without the chlorophyll to make it green, the leaf goes brown or yellow.

The complicated bit is what makes the leaves of some species go various shades of red in autumn. The glory of the fall colours in New England comes largely from the red leaves of species of maple, oak and sumac. Interestingly some species also have red leaves for a week or two after the buds burst in spring.

Recent work by scientists in New Zealand and the United States seems to provide an answer to the riddle. The science isn't simple but the basic idea is that leaf fall, and bud-burst to some extent, are stressful events for a tree that leave it open to attack from things called free radicals.

OXYGEN is the vital fuel that the tree uses to power the processes that break down nutrients. But oxygen is strong stuff and if it's not controlled can run riot and produce these free radicals which attack the tissues and the DNA of the tree.

The same thing can happen to our bodies and we combat it by producing anti-oxidants from various vitamins that we eat. Trees don't use vitamins, they use things called flavones. In particular they produce a flavone called anthocyanin which is bright red in colour. The anthocyanin turns the leaves red in autumn, and sometimes in spring.

But I have American maples, oaks and sumacs growing on my land in Co Kildare. Some of them do go red in autumn, others don't. What's going on here?

It seems again to have to do with anticipation. Anthocyanin is expensive to produce, from the tree's point of view. It has to commit a lot of resources to it. It will only do so if it anticipates the need for protection from free radicals. In North America it has long been noticed that the fall colours are brightest when the weather is particularly cold.

Trees can't foretell what the winter will be like. But they do have a kind of ‘memory’ that tells them that Irish winters tend to be milder than New England ones. Like I said, there's more going on in the world of trees than meets the eye.

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