Dating drought in Nebraskaâ€™s Sandhills
By NET News
August 13th, 2014
Lincoln, NE — The Sandhills are a lush and complex grassland ecosystem sitting atop the massive Ogallala aquifer, supporting many cattle ranches and species of wildlife. So itâ€™s quite a contrast to visit the research sites of David Wedin, an ecology professor at the University of Nebraska-Lincoln. On a warm summer day, Wedin lead the way to one circular plot heâ€™s put through what he calls the â€œdeath and destructionâ€ treatment, killed once every three years.[audio:https://kvnonews.com/wp-content/uploads/2014/08/sandhillsdrought8_12_14KVNO01.mp3]
â€œWe actually herbicide it with Roundup, glyphosate,â€ Wedin said. â€œSo weâ€™re simulating a severe disturbance of some kind.â€
Wedinâ€™sÂ Grassland Destabilization ExperimentÂ has been going on for about a decade on university property in north-central Nebraska. Several test plots hold only patchy vegetation and lots of bare sand. In addition to herbicides, heâ€™s used an agricultural disk to scrape off the grass in some places.
â€œWeâ€™re trying to understand what happens when the system goes past its point of resilience and loses its stability, and kinda how the Sandhills falls apart,â€ Wedin said. â€œWhat happens ecologically in that process.â€
During the last 10,000 years, data point to four periods of â€œmega-droughtâ€â€” one lasting nearly 3,000 years. That drought, possibly exacerbated by strong winds, wiped out vegetation across the Sandhills. Wedin said during the most recent mega-drought 800 years ago, â€œyou would have been standing at this point and looking at the largest set of moving sand dunes in the western hemisphere.â€
UNL scientists have spent the last 15 years dating the Sandhills through a process called optically-stimulated luminescence, which measures the energy held in sand grains. UNL Professor Paul Hanson, a geologist with theÂ Nebraska Geological Survey, uses this technique to study how and when the sand dunes last moved.
Hanson said to think of each sand grain as an individual rechargeable battery. â€œThe sand grains are exposed to sunlight, they lose their electrical charge. And when those sand grains are buried in the ground theyâ€™re gaining that electrical charge again. So the length of time theyâ€™re buried dictates how full that battery is,â€ Hanson said.
Hanson and his colleagues drill down 60-80 feet to take core samples of the dunes, then return to their lab to study them.
On the UNL campus, Hanson leads the way through a rotating door of darkness to enter the lab lit by red and amber lights. Inside, Hanson and his students remove the outer edges of the core sample to work with the unexposed sand grains in the middle. After sieving the sand down to the right grain size, they treat it in several different acids to concentrate the quartz. Then they load the individual grains of sand onto disks into a machine that reads the luminescenceâ€”like what you see from fireflies.
â€œSo if your eyes were more sensitive and could actually see smaller quantities of light, you could actually see the sand grains give off the light under the right conditions,â€ Hanson said. But since our eyes arenâ€™t that sensitive, this machine runs 24/7, dating sand one individual grain at a time. By averaging the useful data from enough grains of sand, â€œyou can use that as a clock to tell how long the grains have been buried,â€ said Hanson, and thus the last time the dune was bare and moving.
The geologic history Paul Hanson constructs provides context for the ecological research Dave Wedin is doingâ€”trying to understand what conditions are necessary to make a healthy grassland destabilize and fall apart, and how long it takes to recover. Wedin said heâ€™s been quite surprised by the plots killed every three years, particularly given the extreme drought in 2012.
â€œI never would have thought you could completely kill the vegetation out here and it could go another four years before you started to see erosion. And by saying there wasnâ€™t significant erosion I mean less than an inch,â€ Wedin said.
Even with surface vegetation killed, the stressed grasslands proved far more resilient than Wedin imagined, retaining their root systems, sand and organic matter for years. Heâ€™s now trying to revegetate the plots, which he said has been even harder:
â€œItâ€™s a situation where the professors come out and learn something that everybody that lives out here already knew: that itâ€™s easier to destroy the stability out here in the Sandhills than it is to restore it once itâ€™s lost,â€ Wedin said. â€œAnd that was humbling.â€
Ultimately, Wedin and Hanson agree droughts capable of making the Sandhills resemble the Sahara are very longâ€”probably on the order of decades or a couple hundred years. Theyâ€™d have to be far more severe than what weâ€™ve seen in the last two hundred yearsâ€”but not in the last millennium.
â€œIf we know that in the last 1,000 years this landscape produced droughts that destabilized the whole landscape, it seems prudent to think that can do that again. And that discussion is without human-caused climate change. But we donâ€™t even need to invoke human-caused climate change to say, itâ€™s a reasonable prediction sometime in the next couple hundred years weâ€™re going to see droughts out here that the whole thing falls apart,â€ Wedin said.
And long before things reach that point, he pointed out a few hard years with no grass could be devastating for the Sandhills ranching economy.
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