Bristlecone pines and climate change
Bristlecone pines are famous as a candidate for the title of the oldest living things (it depends on what you count as a lifetime). The oldest is over 4,500 years old. That’s an awful lot of tree rings, but by measuring the width of each ring, we can see how much the tree grew that year. A group of US scientists did that, and found that they’re speeding up.
There are a few different species of bristlecone pine, so they used the Great Basin bristlecone pine, or Pinus longaeva (the scientific name means ‘long-lived pine’). And specifically, they found that trees on or just under the treeline had markedly wider rings in recent decades, particularly since 1950. And we all know what to suspect, especially if I put it in the title: climate change.
It’s no good jumping to conclusions, though, and the researchers didn’t. They collected data from a number of different sites, to check that it wasn’t just at one place. They neatly confirmed that it wasn’t a result of the mathematical techniques used to analyse tree ring sizes, by using a very simple analysis on a lot of data. They checked that it wasn’t something automatic in the way the trees grow—in some of them, only sections grow, which could conceivably grow thicker, but it seems they don’t. They also considered the direct effects of atmospheric pollutants such as CO2.
Finally, they used two different temperature records, one from thermometer readings, and one calculated from other tree rings, to show that the ring widths up near the treeline correlate with temperature. Why only near the treeline? Well, it seems that bristlecone pines growing near the treeline are most dependent on warmth (which makes sense, as above the treeline, it’s too cold for trees to grow), but lower down, it’s the amount of rainfall they get that’s the key.
So, climate change is affecting the bristlecone pines. What will happen? The authors don’t really discuss it, but I’d guess the warmer temperatures will let the treeline move uphill, albeit rather slowly, as new trees take a long time to grow. And what of their famous lifespan? Growing faster might seem to be good for them, but often long-lived species manage it by being very slow and tough—giant tortoises are another example. If they can catch up with their potential treeline, though, those on the treeline could be expected to once again grow very slowly.
Salzer, M., Hughes, M., Bunn, A., & Kipfmueller, K. (2009). Recent unprecedented tree-ring growth in bristlecone pine at the highest elevations and possible causes Proceedings of the National Academy of Sciences, 106 (48), 20348-20353 DOI: 10.1073/pnas.0903029106