Moreover, there was evidence of positive feedback between trees and soil.
They measured leaf calcium in 1995 and again in 2001, and found that trees with low-calcium leaves stayed the same, but the calcium content of calcium-rich leaves actually increased. So calcium in leaves raises soil pH, which further increases calcium in leaves, which raises soil pH and so on. Finally, all this had a predictable effect on earthworms: in the soil beneath the high-calcium broadleaves, there were lots, of several species. Beneath pine and larch, there were essentially none.
There are lessons here for gardeners, even if they're not quite obvious.
Don't imagine you can render a chalky soil more rhododendron-friendly by planting a few pine trees – it simply takes too long. Remember, too, that an acid soil is a fragile thing, convertible overnight into an alkaline one by the addition of a few buckets of lime, so even the soil under an old pine tree may or may not be acid, depending on what else you, or a previous gardener, have been adding to the soil.
There's a lesson for composters too. For reasons that remain mysterious (to me anyway), the pH of compost made from general garden waste often turns out to be surprisingly high. But even if it takes ages to compost conifer needles, I think we can safely predict that the finished product will be acid.
* Ken Thompson is a plant biologist with a keen interest in the science of gardening. He writes and lectures extensively and has written four gardening books, including Compost and No Nettles Required. His forthcoming book (March 2014) is Where do Camels Belong? The Story and Science of Invasive Species.