Trees that live fast, die young
The old saying “live fast, die young” isn’t just for rock stars and playboys - it also applies to trees. And climate change appears to be speeding up the rates at which trees grow and therefore potentially hastening their deaths, meaning some of the assumptions we make about the role of forests in mitigating against climate change might be wrong. Chris Smith spoke Roel Brienen, from the University of Leeds, about the lifespans of trees...
Roel - Tree ring records really tell us how old trees can become, simply by counting rings. From tree rings we can also calculate how fast they are growing; and we can do that simply by measuring the ring width, because the ring width of a tree reflects how fast the tree grows in diameter. And what we did in our study was we related the growth of trees, derived from tree rings, to the ages of the tree, to the tree's maximum lifespan; and by doing this what we found is that the slowest trees in a population are the ones that live the longest. This shows that this 'live fast, die young' principle really applies also to trees. And we did this using tree ring databases - that we scored from the internet, online - of more 210,000 trees of 80 different species; and in most species, we find that the fastest trees are the ones that live the shortest, and the slow ones are the ones that live long.
Chris - And this is comprehensive, you think, therefore, across the world and also across times, would you say? Because if we think that the climate is changing, would that effect be intrinsic to the data that you've looked at on your trees?
Roel - It would be, we think. Because what we did is we looked really globally. We included trees from the tropics, from temperate regions, from the Arctic regions; and we included trees that grow fast or slow for all kinds of different reasons. And we therefore think that because this occurs in all these different tree species growing in different biomes - and some in an open forest, others in closed forest - we think that this phenomenon, this 'live fast, die young' phenomenon, occurs universally across all species.
Chris - And the key question then Roel: if I take a slower growing species and I make it grow faster, does that mean then that it does actually die sooner, even though it's a slower growing species traditionally?
Roel - Yes, yes. What we really find is that within a species... previously Ángela was talking about having pioneers and having other species that are built to live much longer. But what we did is we looked within species. So we find that even within species... if we have an oak, for example, an oak that grows very, very slowly, that oak will eventually live much longer than an oak that grows very fast. So even within species we find exactly the same phenomenon.
Chris - And is it just the tree growing fast that undermines its longevity, or is it that some other factor that makes the tree grow fast also makes the tree die young?
Roel - Well this is an interesting question, and we have tried to look at this; and we cannot pinpoint exactly what the mechanism is, why these fast growing trees live shorter. But there are various hypotheses. One could be that they simply make wood that is of lower quality, or that they are less resistant to survive droughts, for example. And that could be one reason. Another possible cause - and this is a much simpler mechanism - is that trees simply die once they reach a certain maximum diameter, a maximum size.
Chris - Is it also possible it's something as simple as: if you've got warmer climates that favour tree growth, but warmer weather also encourages more tropical storms, you're going to have more winds that are going to knock your tree flat even though it's growing beautifully?
Roel - That could be a possibility, but we find it really across all climates and all climate zones; and in areas where the major tree death is storms, but also in areas where the major causes of tree deaths are different, insect attacks for example, or droughts. So because we find it occurring across such a wide variety of habitats and different forest types, we believe it's not driven by one simple factor.
Chris - Given that we have used the amount of carbon dioxide that trees use to grow in predicting how climate change is likely to play out into the future, does the fact you found this really make that much harder to predict then, or does that throw some flies into the ointment?
Roel - This does have some consequences for our predictions of CO2 in the future, I believe. One important thing to realise and understand is that in the past - because trees have been growing faster due to increases in atmospheric CO2, due to increases in temperature - forests and vegetation globally has been taking up roughly one third of our CO2 emissions over the past two or three decades. And many climate models predict that this continuation of CO2 and the further warming into the future.... that as a result of that, we will still see an increase in carbon uptake by forests globally. That is what we call a carbon sink. However, if trees that grow faster also die younger, that will mean that a forest in the future may not be able to provide such a service as they have been doing in the past.