Sand Hills: A Story to be Told

What can a grass-stabilized sand dune tell us about the future? What can it tell us about the past? That depends on where you look and how long you look at it. For Dr. Dave Wedin from the University of Nebraska at Lincoln, studies of the Nebraska Sand Hills might help explain why some arid, sandy landscapes are blowing desert-like dunes while others are productive, vegetated grazing lands.

Recent paleo-climatic research revealed that within the last 10,000 years, the Nebraska Sand Hills have transformed into vast, blowing sand dunes, and then back into stable grasslands at least six times. What caused these destabilization events? Scientists believe they were the result of "mega-droughts" - droughts so severe that the dust bowl era looks like noise in a busy signal.

The Grassland Destabilization Experiment (GDEX) was undertaken to answer several ecological questions at the Barta Brothers Ranch, a University of Nebraska research facility. Dave and other scientists set out to describe how evapotranspiration, drainage, and energy balance change as uplands change from continuous grasslands to bare sand.

In addition, they sought to determine how the stability of the dunes changed when grasslands were lost. They also wanted to characterize the rate of change and measure the resilience of the belowground ecosystem to short and long-term disturbance. Since the Nebraska Sand Hills are the largest grass-stabilized dune in the western hemisphere, they were an ideal place to test these hypotheses.

Using the LI-8100 Automated Soil CO2 System, they measured baseline soil respiration, and then assessed how respiration changed over time and with different treatments to the plant community. Using a survey chamber and long-term chamber, Dave and his team characterized soil CO2 flux. To their surprise, soil respiration remained higher than expected, even after all the above-ground vegetation was gone. Soil respiration in upland dunes was still roughly 25% of the level seen in control plots four growing seasons after all vegetation in the dunes had been experimentally killed.

How do they explain this observation? Though the autotrophic organisms were killed, heterotrophs continued to respire in the soil ecosystem. 20% of the root biomass still remained in the soil after four growing seasons in the experimentally disturbed plots. Thus, the below-ground legacy of the grassland kept the dunes stable for four or more years. The root structures kept wind erosion at a minimum, though water erosion was visible within two years of disturbance. They observed that in this ecosystem, soil respiration is an indicator of below-ground ecological function - from healthy grasslands to destabilized dunes.

Among the many interesting things learned from this study, one thing is certain: it takes a very severe drought to destabilize the Sand Hills - a drought more severe than that of the dust bowl era. While the Sand Hills resisted destabilization during recent droughts, they are vulnerable to severe climatic events. With uncertain climate predictions, the Sand Hills could become North America's largest blowing dune field, but this seems unlikely in the short term. In the very least, the Sand Hills will continue to provide scientists with opportunities to gain insight into the resiliency of ecosystems in semi-arid, fragile landscapes.

Thanks to Dr. Dave Wedin, Professor, Applied Ecology Faculty Area Leader, University of Nebraska at Lincoln for contributing to this article. Photographs courtesy Dave Wedin and David Loupe (University of Nebraska at Lincoln, Geosciences).