. . . too frequently.
There is a bit of a push on about planting trees because we are woefully behind on the 2030 2050 and 2100 targets for reducing our collective carbon foot-print. If you ignore Prochlorococcus [prevococcus] - easily done because it's much smaller [2 x 10-22 kg each] than the mighty redwoods [1 million kg each] of California - then the most effective way to lock up yer carbon is to plant loadsa trees. Trees are mostly cellulose, but also lignin, amino acids, DNA all of which are carbon rich 'organic' chemicals.
Now a small but perfectly formed group from Dept Geosciences U Edinburgh and Dept Geography UC London have looked, critically and numerically, at this simple view of depleting the carbon overload that is broiling up the planet and published it in Nature. Their conclusion, contra Gertrude Stein's rose, is: a tree ain't a tree ain't a tree - some trees are more equal to the task of sequestering carbon for long enough to make a difference.
At a meeting in Bonn in 2011, 43 countries agreed collectively to plant 350 million hectares of land with trees by 2030. That has the potential to lock up 42 billion tonnes of carbon. But only if you write off any immediate, or even medium term, return on your investment. IF you plant huge tracts of Sitka spruce Picea sitchensis (like the ESB has just done up our mountain) because it's easy and you can start thinning for fence-posts in ten year time THEN you're going to bank only a tiny fraction of that amount - maybe 1 billion tonnes.
The most recent IPCC Intergovernmental Panel on Climate Change report requires us to de-gas 200 billion tonnes of carbon this century, so the 42bn is pulling its weight. Sitka ain't, because it will all too soon be burned off into the atmosphere. But dissing high turn-over cannot be universally appropriate: think Prochlorococcus - that microbe is locking up 5 Gtonnes of carbon each year; is doubling each day; but reports the same biomass all the time. Contributing 50% of its numbers as food for others is a far higher rate of turn-over than any plantation of Sitka.
This reminds me of James "Gaia" Lovelock beating himself up for planting trees on his little farm in Devon: what he should have done is just lock up his scythe and walk away. Natural succession would have given him a forest (before he died) that was a natural fit for the local climate. By natural fit, I mean one that was as diverse and inclusive as possible and so far more robust to the slings and arrows of outrageous epidemics and weather disasters. I was talking to hill-walker Mick the Mountain= last night. He has noticed that mountain ash / rowan Sorbus aucuparia saplings wilding up the West face of the Blackstairs. This despite the current inducements to put more sheep up there. Natural regeneration is more likely to 'take' and support more diversity (from rabbits to Rhodococcus) than planting hectares of alien monoculture.
And that's something that's missing from an equation that contains only a single "C" variable. Real forest à la Lovelock is a lot of stable carbon, yes, but it is also a fantabulous genetic, chemical and pharmaceutical
One final calculation. Between 2007 and 2011 we planted a short 50% of a hectare with trees that are more à la Lovelock than à la ESB. It cost us a mort o'money. 350 million hectares between 7 billion people is 5% of a hectare each. We are punching slightly above our weight, therefore, and can feel a tiny bit tree-smug.