- 2018年01月12日15:39 来源：互联网
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Dendrochronology——Not always the ring of truth
The use of wood to assess past environments needs care
TREE rings are Mother Nature's history books.
Researchers often look at chemicals stored in them when trying to reconstruct past environments.
Like all history books, though, they can be unreliable.
And in the case of one particular chemical of interest, mercury, Jose Antonio Rodriguez Martin of the National Institute for Agricultural and Food Research and Technology, in Madrid, and his colleagues have just shown how unreliable they are.
Dr Rodriguez Martin was studying pines on La Palma, in the Canary Islands.
He knew that Hoyo Negro, one of the island's volcanoes, had erupted violently in 1949, and believed that the clouds of ash and explosive ejections of rock from the eruption had destroyed all of the pines nearby.
However, as he describes in Naturwissenschaften, when he and his team examined the area, they were stunned to discover a revenant from this cataclysm just 50 metres from the crater.
Like a battle-scarred soldier, this now-great pine had many half-healed wounds in its trunk.
It had survived those. It did not, however, survive Dr Rodriguez Martin's axe.
He reluctantly felled it and, for comparison, collected a tree of similar age that was growing 400 metres from the eruption site, and another that was 16km away.
One unpleasant element often ejected in volcanic eruptions is mercury.
That was certainly the case in 1949, as soil-sampling showed.
The question was, how much of it had got into the trees·
When he analysed the survivor's scar tissue Dr Rodriguez Martin found high levels of the metal in it:
more than 22 micrograms per kilogram of wood.
That was not surprising, given that this tissue would have formed soon after the eruption.
What was surprising was that the wood of the survivor tree's rings had only 2.3 micrograms of mercury per kilogram in it.
More surprising still, the rings of the other two trees were mercury-free, despite there being mercury in nearby soil.
Tree rings are made of tissue called xylem.
This is a set of channels that carry water and minerals from the soil to the leaves, so its composition might be expected to reflect what is in the soil.
A second set of channels, the phloem, found just under a tree's bark, carries the products of photosynthesis to the roots.
This tissue, however, is not preserved from year to year.
When Dr Rodriguez Martin sampled the phloem of the trees he had chopped down, he found it much richer in mercury than their xylem.
Phloem from the tree at the eruption site had 9.5 micrograms of the metal per kilogram.
The two more distant trees had 2.9 and 2.0 respectively.
The explanation for the discrepancy lay in the trees' bark.
This had similar mercury levels to the phloem.
What Dr Rodriguez Martin reckons is going on is that trees are using their bark, which they shed continually, as a waste-disposal system.
Mercury is carried there by the phloem and selectively deposited in it, rather than in the plants' more permanent parts.
The metal is then got rid of when the bark is sloughed off.
That makes it invisible to researchers who come along later. What happens to mercury, moreover, might well happen to other toxic materials.
That could throw the calculations of palaeoecologists severely out of kilter.
All of which is a caution to those who read the Book of Tree Rings for clues to what happened in the past.
Trees, it seems, are as good as rewriting history as people are.