The National Trust wants to plant 20 million trees, but will it achieve anything?
Anyone who can remember as far back as the last election will recall the various parties trying to outbid each other with how many trees they were going to plant.
Many others have similar ideas – the National Trust is planning to plant 20 million over the next 10 years, and TSB Bank is promising to plant a tree for every mortgage sold. Whether any of these trees need planting is a question for another day (trees are very good at planting themselves, given the opportunity), but what will all these trees achieve?
One of the main motives is to absorb and lock up carbon from the atmosphere, thus helping to slow the rate of climate change. How good are trees at doing that? Before we can answer that, we need to think about where all those trees might go. In a crowded country like Britain, many would have to go in uninhabited (or sparsely populated) uplands; it’s estimated, for example, that fully a third of Scotland could be suitable (biologically, if not politically) for establishing new woodlands.
At least some of this potential new woodland would replace heather moorland. And we don’t have to guess what happens if you do that, because a variety of experimental plantings, of varying ages, already exist. Researchers have looked at four of these, planted with either birch or Scots pine, and aged from 12 to 39 years, to see how carbon storage is getting on. Their results are published in Global Change Biology.
The trees, naturally, store carbon as they grow, so the effect of planting trees is always to increase above-ground carbon storage. Unfortunately, there is a corresponding reduction in the amount of carbon stored in the soil, which more or less exactly matches the increase above-ground, so the net effect is zero.
The underlying reasons for loss of soil carbon are complex, but there is certainly a big increase in soil respiration, i.e. in carbon lost from the soil as carbon dioxide. More dissolved organic carbon may be lost in drainage water too, but that hasn’t been measured.
Disappointing as this result is, the reality is probably even worse. These results come from fairly small experimental plots and water flowing in from the surrounding moorland means that their effect on soil water content is negligible. But if whole landscapes are afforested, we know that trees have a powerful drying effect on the soil, which further increases soil respiration, so we can reasonably expect that planting trees on heather moorland would lead to an overall loss of stored carbon.
Of course, there are lots of other good reasons for planting trees, and planting trees in the right place can undoubtedly contribute to carbon storage. But concern about loss of soil carbon means that planting trees on peat more than 50cm deep is already prohibited, and these results suggest that if we want to maximise the benefits, we need to be even more careful about where we plant trees in the uplands.
Incidentally, this work illustrates the importance of long-term research into issues like sustainability and climate change, where the “obvious” answer is not always the right one. Visit ecologicalcontinuitytrust.org for more details on this.
Should any of this affect what you do in your garden? No, not at all. Your garden doesn’t have the kind of soil where this would be a problem, and anyway the other impacts of trees, on biodiversity, temperature and flooding, mean that planting a tree is still one of the best things you can do for the environment.
Ken Thompson’s most recent book is Notes From a Sceptical Gardener. Visit books.telegraph.co.uk.
