A few years ago, I spent a lovely week in Fort Collins, Colorado. Two great things came out of my time there. First, I was introduced to one of my all-time favourite beers, Fat Tire Amber Ale. Yum. While Fat Tire was immediately gratifying, the second has only borne fruit today, in the form of this paper in Nature. Yay.
The Fort Collins workshop, held at the USGS’s John Wesley Powell Center for Analysis and Synthesis, was convened to tackle a big question in forest ecology: what is the relationship between mass growth and tree size? And more specifically, how fast do the biggest trees grow and how do they sequester carbon? Surprisingly, no one had looked at this before, apart from the odd study on a handful of tree species. This was probably due to a combination of few data (there just haven’t been that many really big trees measured) and the fact that the answer seemed obvious. The prevailing wisdom was that big trees slow down their growth. The bigger they are, it was thought, the slower they grow, until eventually they just stop growing and die. Evidence for this assumption can be found in studies showing that old forests, and the leaves of old trees, are less productive. But until enough people with enough data had been stuck in the same room together, no one could really know if this assumption was true for most trees or most of the world’s forests.
All together, the Fort Collins group had access to growth measurements from a staggering 673,046 trees, belonging to 403 tree species from tropical, subtropical and temperate regions across six continents. Our analysis of these data showed that, for most tree species, not only did the growth of the biggest trees not slow down, but mass growth rate increased continuously with size. In fact, some trees were growing so fast that in one year they could put on the mass of a 20cm-diameter tree. In other words, if they had started growing this fast as seedlings it would only take a year for their trunks to reach 20cm wide! Of course, these trees are already so big that we just don’t notice the extra mass they are putting on. But it is amazing nonetheless, and not what anybody would have expected. But perhaps increasing mass growth shouldn’t come as a complete surprise.
Though individual trees may be growing faster than we thought, their mass growth doesn’t necessarily translate to increased forest-level productivity. Older forests have fewer trees in them because many trees die. So while the biggest trees are getting heavier quicker, the forest itself may not be.
Still, I expect this paper might cause a bit of a stir and may have future ramifications for forestry and carbon policy. As carbon sinks, big old trees might suddenly have got a whole lot more valuable.