Date of this Version
Hartman, J.C. 2015. A desert in disguise: The resilience of the Nebraska Sandhills. PhD dissertation. University of Nebraska - Lincoln.
The Nebraska Sandhills are the largest sand dune system in the Western Hemisphere, and are unique because they remain relatively undisturbed from row crop agriculture. Research in the past two decades demonstrated that the Sandhills are dynamic on millennial timescales, switching between stabilized, vegetated states to non-vegetated, mobilized states. The Sandhills are currently stabilized, but understanding how ecological processes are altered as sand dunes transition from stabilized to mobilized states, provides insight into the thresholds, stability, and resilience of this grassland ecosystem. My research investigated the impacts of vegetation disturbances on ecological processes and the sand dune surface stability. For ten years (2005-2014), Press, Pulse, Grazed, and Control treatments were applied to stabilized sand dunes. Press treatments killed vegetation and prevented re-growth, while Pulse treatments killed vegetation in one year and then allowed two years of recovery. I measured changes in key ecosystem processes (aboveground biomass, nitrogen cycling, soil respiration, and total soil C and N pools) as sand dunes transitioned from stable to mobile states. I found that Press vegetation disturbances surpassed the grassland’s resilience and led to mobile sand dunes after 4-5 years. However, there was a time lag between complete loss of aboveground vegetation and increased erosion, which indicated that belowground ecological functioning buffers disturbances and is important in controlling sand dune surface stability. The Pulse treatment reduced aboveground biomass and shifted the system to an annual and C3 grass dominated plant community, but belowground functioning continued at reduced rates and erosion was minimal. However, in the 11th year of the study, the spatial variance in erosion activity increased, potentially foreshadowing a state shift in the Pulse treatment. A severe drought in 2012 produced similar impacts on ecosystem processes as the Pulse treatment, reducing aboveground biomass and increasing the contribution of forbs. The 2012 drought also increased available soil nitrate and ammonium, reduced net N mineralization and soil CO2 flux, but did not cause measurable erosion. My results indicate that the Nebraska Sandhills are resilient to climate and vegetation disturbances on decadal timescales, and that continued belowground functioning is critical to the stability of this ecosystem.
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