Tuesday, 3 May 2011

Stormy weather.

The tornado that tore through parts of Auckland yesterday, and last week’s strong winds in Wellington, got me speculating about plants and storm damage.  In the tornado, whole trees were uprooted, and some were smashed into houses and cars.  In the Wellington storm, I noticed that some tree species were easily damaged, while others seemed immune.  Most of the small leafy twigs that littered the footpath the following day were mahoe (Melicytus ramiflorus) and large-leaved species of Coprosma (like C. grandifolia).
Coprosma grandifolia
Mahoe (Melicytus ramiflorus)

Pretty much every man-made structure in the path of the Auckland tornado was damaged in some way.  There’s no doubt architects and engineers could build houses and shopping malls that can withstand a tornado, so why don’t they?  The answer is in the cost: tornados are rare in New Zealand and the chance of one striking your town is so small that it’s simply not worth building a reinforced concrete bomb shelter for a house.  In parts of the USA where tornados are more common, many houses have underground shelters in their basements.
I suppose it’s possible that plants could resist tornados too.  They’d need stronger and deeper roots, which might not be as efficient (per kg of root tissue) at taking up water; wood with fewer vessels and more fibres, which wouldn’t conduct water so well; and thick strong leaves, which probably wouldn’t be so efficient at photosynthesis.   
So why do trees get flattened in a tornado, just as houses do?  When tornados are rare, over-strengthened individuals are handicapped every day by the extra resources needed to produce those strong roots, stems, and leaves.  The carbohydrates and proteins they have to invest in strength can’t be used to make flowers and seeds, so in most years, such plants leave fewer offspring, and so their traits get rarer in the population.   True, those trees that survived the Auckland tornado of 2011 might leave more seedlings next year than those that were damaged, but unless there’s a tornado there every year, the over-strengthening costs will kick in and those stronger offspring will again be at a disadvantage. 
There seems to be an analogy here between the design of a house and the structure of a plant that some people claim is proof that living things have a designer, just as houses do.  The triumph of evolution by natural selection is that it explains instances of apparent design without the need to invoke an unseen designer.
Additionally, if the features that allow some plants to survive a tornado aren't heritable, then they can't be passed on and they'd be unable to evolve.  Plants grown in windy sites like Wellington often grow stronger than those in more sheltered places.  That’s why a 120 km/h gale in Wellington does less damage to trees than the same wind in Hamilton.  Most of this is due to plasticity, where a plant is capable of responding to different environments by producing different growth forms, but the specific response itself isn't inherited.  Such plasticity might itself have evolved if a plant’s dispersal range includes diverse environments, such that offspring that can respond to whatever environment they land in will be able to leave more descendents than those whose growth form is fixed.

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