June 3, 2010

Leave it to Beavers

In recognition of the recent discovery of the world’s largest beaver dam, we take a look into the activities of past beavers to see what they may be able to tell us about previous climate changes, and we speculate on the impacts that on-going climate changes may be having on the likes and dislikes of current beavers.


World’s largest beaver dam in found in Northern Alberta, Canada.

It turns out that beavers and their activities can a lot to tell us about the history of past droughts and floods.

Our first article featured focuses on beaver damming and climate variability in Yellowstone National Park. The article was written by two scientists at the University of New Mexico and the work was funded by the National Park Service, the Geological Society of America, the University of New Mexico, and the National Science Foundation.

We all know that beavers build dams and once their dams are constructed, sediment builds up behind the structure. Studies have shown that up to one meter of sediment can be deposited behind their dams in a single year. Beavers may occupy a dam site for up to 50 years, and their structures can significantly alter the landscape. Where conditions are ripe, their activities can be seen over thousands of years in the sediment and a well-trained eye can rather easily spot beaver activity in sediment cross-sections. Furthermore, the organic material in the sediments provides for accurate radiocarbon dating thereby allowing us to see when beavers were active in an area in the past.

Persico and Meyer focused their research on beaver dams of Yellowstone National Park (YNP); they note that “Previous studies indicate that beaver-pond sediments have distinctive sedimentologic features, including beaver-gnawed wood fragments, that allow their recognition in fluvial stratigraphy.” They hunted around the Park and found evidence of beaver dam activity over the past 4,000 years. They state that “Thirty-nine radiocarbon ages on beaver-pond deposits fall primarily within the last 4000 yr, but gaps in dated beaver occupation from ~2200–1800 and 950–750 cal yr BP correspond with severe droughts that likely caused low to ephemeral discharges in smaller streams, as in modern severe drought.” Beaver activity turns out to be related to drought in the area, and a figure in their article was particularly of interest to us at World Climate Report.

The figure (Figure 1) not only shows when beavers were active, but it shows a reconstruction of the Palmer Drought Severity Index (PDSI) for YNP over the past 2,000 years. We cannot help but notice that over the past 500 years, the PDSI values have hovered near normal with little evidence of extreme droughts or wet periods. By contrast, the unstable climate with high variability occurred 600 to 1000 years before present, and the beavers apparently didn’t like it and shut down the damming activity. Over the past 1,000 years, the trend in YNP has been away from drought and toward wetter conditions, and the beavers seem quite happy about the situation.

Figure 1. Chronology of beaver-pond deposits over the last 2000 yr. Major peaks in fire-related debris flows ~800 and 2200 cal yr BP indicate severe late Holocene droughts and correspond to minima in beaver-pond sedimentation probability. Less relation is evident with probable fire-related sedimentation. Palmer Drought Severity Index (PDSI) reconstructed for grid point 110°W, 45° N from tree-ring records in the Western USA; negative values indicate drought (from Persico and Meyer, 2009).

Since beavers in Yellowstone Park seem to prefer wet periods over dry ones, we’ll assume that the preference is held by their Eurasian cousins as well.

With this in mind, two recent articles in the Journal of Hydrology bring news that should be welcome to beavers over there. The first article is from a pair of hydrologists in Switzerland who note “It is expected that global warming will influence the precipitation regime. In the last decade, several studies have investigated changes and trends in streamflow to test whether such changes can already be seen in measured time series.” Schmocker-Fackel and Naef state that other scientists “detected upward and downward trends in annual maximum flow series in a worldwide dataset, but most of the series showed no significant trend.” In their own study, they collected streamflow data from throughout Switzerland with some of the time series extending back to 1850. They conducted a trend analysis and concluded “Only few negative trends could be found, a slight increase could be detected for the period 1936–1965 with a maximum of 16%. Contrary to this, up to 42% of the stations show a significant positive trend for periods starting before 1966 and ending after 2000. Especially high values were obtained when the two last periods of 2001–2005 and 2006–2007 were included as well. There are only few positive trends for periods starting 1966 or later or for time series ending before the 1980s.” Sounds perfect for the beavers of Switzerland who are trying to make a comeback there after being eradicated by hunting!

And although beavers aren’t found as far east as Korea, if they were, they probably would find recent moisture trends to their liking.

Precipitation measurements are available for Seoul, Korea going back to 1778 (pretty amazing). Three scientists from Korea used this long time series to evaluate various drought indices (Figure 2). Kim et al. note that a mega-drought period existed from 1882-1910; furthermore, they state “The 10 years when droughts were the most severe for the 171 years other than the mega-drought period were 1950, 1943, 1939, 1944, 1837, 1951, 1949, 1940, 1952, and 1982. Most of these severe droughts occurred from 1939 to 1952.”


Figure 2. Historical diagram of the drought events represented by the AEDI from January 1, 1807 to December 31, 2006. Each bar was plotted at the start date of the dry period (from Kim et al., 2009).

So from Yellowstone Park to Switzerland to Korea, we find good news for critters that don’t like periods of drought. As noted repeatedly, the build-up of greenhouses should warm the Earth as well as increase precipitation. So far, we seem to be witnessing fewer droughts and greater streamflow.

References:

Kim, D.-W., H.-R. Byun, and K.-S. Choi. 2009. Evaluation, modification, and application of the Effective Drought Index to 200-Year drought climatology of Seoul, Korea. Journal of Hydrology, 378, 1–12.

Persico, L., and G. Meyer. 2009. Holocene beaver damming, fluvial geomorphology, and climate in Yellowstone National Park, Wyoming. Quaternary Research, 71, 340–353.

Schmocker-Fackel, P., and F. Naef, 2009. More frequent flooding? Changes in flood frequency in Switzerland since 1850. Journal of Hydrology, 381, 1–8.




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