Project Overview - Hypotheses Three

Project Overview

Hypothesis Three

Subregions of the Sand Hills differ in their proportions of interdunal wetlands; thus, we expect the subregions to respond differently to both historic and modern droughts.

We will look for subregional patterns in the geologic record by an iterative process of OSL dating and mapping of dune generations on aerial photographs to determine the extent and number of wind-blown sand deposits less than 2000 years old. The response of the Sand Hills to recent and current droughts will be examined using spatio-temporal analyses of vegetation greenness (NDVI) from Landsat (30m), MODIS (500m), and AVHRR (1km resolution), providing an independent picture of subregional behavior. Finally, our MM5 simulations will indicate the spatial scale at which either the "wetland" or the "desertification" climate feedbacks may operate (if indeed the model supports their existence).
Finally, the ability of the Sand Hills to recover from "ecosystem collapse" (i.e., widespread dune destabilization) appears to have been remarkably high, at least for the last several millennia. Not only are interdunes underlain mainly by peat, but also well-developed paleosols lie buried within the dunes, indicating long periods of landscape stability. The simple water balance equation again suggests the mechanism for this resilience. Even if precipitation (P) decreases because of a "desertification" feedback caused by extensive bare sand, as long as ET remains low, a high proportion of P contributes directly to D, leading to the recovery of depleted groundwater, and the recovery of the "wetland" climate feedback. Grassland recovery is also predicted to be rapid when soil moisture and microclimate conditions become favorable. The rhizomatous C4 grasses that recolonize and stabilize bare sand in the Sand Hills have very high growth rates because of their high photosynthetic rate and high water and nitrogen use efficiencies.

(right) Subregions in the Sand Hills revealed by PCA on image time series of AVHRR NDVI biweekly composites for the decades of the 1990s (excluding 1994). Gradients of temperature and precipitation (decreasing SE to NW) as well as differential land surface phenology and regional hydrological flow networks give rise to the subregions. R,G,B displays PC3,PC1,PC2. (Lincoln County sand field omitted.)