By Erin Hayes-Pontius
To add to the seemingly never-ending list of proxies paleoecologists use to study the past, here’s another one: trees. If you have been following this blog for a little while, you probably read about what pollen can tell us, but using the trees themselves, rather than their pollen, can tell an entirely different story. Studying the pollen that accumulates in lake sediments has the distinct advantage of providing a record that is as old as the lake itself, which can sometimes be up to millions of years old. However, because of how long it takes lake sediments to accumulate, we cannot always be very confident of the date a particular layer of sediment represents. In contrast, trees generally provide a much shorter record than pollen, but because of their annual rings, give us an annual record to work with.
To extract cores from trees, you use a tool to twist or drill into the tree and then pull out, yielding a thin sample representing all of the rings from the bark to the very center of the tree. You might be worried that this hurts the tree, but it should not if it is healthy. A healthy tree will ooze resin out of the wound in the following days or weeks, both sealing up the hole and flushing out any harmful fungal spores that might have made their way in. At this point, I should also note that dendrochronologists core living trees as well as dead trees and even log cabins! If this intrigues you, you should definitely check out dendroarchaeology articles.
Once in the lab, rings are counted, measured, and used to infer a number of past conditions over time, such as drought, precipitation, temperature, or even fire. The reason we can do this is because trees grow seasonally (though this is more difficult with tropical trees), leaving rings showing annual growth; variation in ring width can tell us about past climate.
The topic I would like to tell you about today concerns a fairly recently published study entitled, “The unusual nature of recent snowpack declines in the North American Cordillera (Pederson et al. 2011). They used areas that fulfilled a couple of different criteria: first, the areas had to be in places that received most of their precipitation from snowfall, and second, the cores had to be taken from trees sensitive to changes in snowpack, such as Douglas fir and Ponderosa pine (Woodhouse et al. 2006). For their sites, the authors selected the headwaters of three major rivers important for water resources in the US, listed from north to south: the Columbia (“Upper Columbia Region”), the Missouri (“Greater Yellowstone Region”), and the Colorado (“Upper Colorado Basin”). Using trees in each of these three areas, they reconstructed snowpack variability over the last 800 years by inferring what is called snow water equivalent (SWE).
Before I go any further, I do not have permission to reproduce any of the figures in the article. Fortunately, however, this article is available for free from the US Geological Survey’s website. I’ll be referring to figures by number and letter, so be sure to follow along.
Interestingly, there were a few prominent trends in SWE in each region. There were a number of times when the SWE across a north to south gradient was antiphased, meaning that when SWE was unusually high in the Upper Columbia Region, it would be unusually low in the Upper Colorado Basin (check out Figure 1 [right]). Fast forward a few hundred years, and the trend switched: low SWE in the north and high SWE in the south. Depending on the time period, the Greater Yellowstone Region, situated in between the other two regions, either showed similar trends to the Upper Columbia or the Upper Colorado. These SWE trends would persist for a few decades at a time, look a bit noisy, and a hundred or so years later, would appear the opposite as they did last time. In Figure 2b, you can see that from 1511-1530, the Upper Columbia and the Greater Yellowstone had unusually low SWE, while the Upper Colorado had unusually high SWE. In the interval from 1845 to 1895 (see Figure 2e), the trend there was the opposite.
While the decadal antiphasing trend is interesting enough, the authors uncovered one other result that was startling and perhaps a bit alarming: snowpack has declined precipitously (pardon the pun!) and uniformly in all three regions over the course of the 20th century. The authors found that temperature and precipitation have been closely linked in the records, with low temperatures corresponding to high SWE and higher temperatures being associated with low SWE. This, combined with another study showing that humans have altered the climate to the extent that it is affecting water resources in the Western US, is troubling (Barnett et al. 2008).
According to others, however, the worst is yet to come. Schwalm et al. (2012) produced a study showing past, present, and future drought severity in the Western US, and the picture is not pretty. From 2000 to 2004, the Western US experienced the most severe drought of the past 800 years in its history. The authors used that as a starting point to compare that event to both the state of water resources over the past 100 years and to the future state of water resources over the next 100 years. The future precipitation estimates come from a group of models in the Intergovernmental Panel on Climate Change’s 2013 report.
As you can see, the state of water resources is expected to get much, much worse over the next hundred years even relative to the worst drought we had experienced in the last 800 years. It is definitely worth noting that the portion of the above figure covering 1900 to present was produced using- you guessed it- tree ring data. Although it may sound clichéd, understanding the past helps inform the future; it gives context and provides a bigger picture.
Barnett TP, Pierce DW, Hidalgo HG, Bonfils C, Santer BD, Das T, Bala G, Wood AW, Nozawa T, Mirin AA, Cayan DR, Dettinger MD. 2008. Human-induced changes in the hydrology of the Western United States. Science 319(5866):1080- 1083.
Beentree. 2006. File:Pressler drill 5 beentree.jpg. Accessed 20 March 2014. <http://commons.wikimedia.org/wiki/File:Pressler_drill_5_beentree.jpg>
Pederson GT, Gray ST, Woodhouse CA, Betancourt JL, Fagre DB, Littell JS, Watson E, Luckman BH, Graumlich LJ. 2011. The unusual nature of recent snowpack declines in the North American Cordillera. Science 333:332- 335.
Schwalm CR, Williams CA, Schaefer K, Baldocchi D, Black TA, Goldstein AH, Law BE, Oechel WC, U KTP, Scott RL. 2012. Reduction in carbon uptake during the turn of the century drought in western North America. Nature Geoscience 5:551- 556.
Woodhouse CA, Gray ST, Meko DM. 2006. Updated streamflow reconstructions for the Upper Colorado River Basin. Water Resources Research 42:1-16.