Gypsum ecosystems ecohydrology: a remote sensing approach
Gypsum soils impose stressful conditions to arid zone vegetation, which results in edaphic restriction processes that favor the occurrence of specific flora. In addition to the hypotheses that rely on the chemical nature of gypsum , the hydric characteristics of gypsiferous soils have also been considered for the explanation of gypsophily. The contradictory evidence found so far, has limited the formulation of a clear hypothesis on how the presence of gypsum affects soil water availability. While some authors indicate that gypsum soils show poor water conditions for plants, others suggest water advantages over surrounding soils. Here we develop a landscape-level approach based on remote sensing to elucidate the ecohydrological processes associated with gypsum vegetation. For this purpose, we used the LSWI (Land Surface Water Index) and NDVI (Normalized Difference Vegetation Index) spectral indices, derived from the processing of the 2016-2019 time series of Sentinel-2 satellite images. These indices are related to land cover water content and vegetation primary productivity, respectively. Our goal was to evaluate the differences in the annual dynamics of these two indices in 13 gypsum outcrops and their periphery along an aridity gradient in Andalusia, and for the structural vegetation types that develop in each of them. Results show that in most arid outcrops the water content of the ground cover was higher than in the periphery throughout the year, or at least in the summer drought months. Although we could not identify a clear segregation in ecohydrological behavior among all vegetation types studied, the gypsum soils covered only by annual grasslands, and the gypsiferous tomillares seem to show differences in this sense compared to the others. Despite the possible water advantages of gypsum soils in arid areas, we found no evidence that this leads to differences in primary production of vegetation, suggesting that the chemical nature of gypsum imposes a low water use efficiency in gypsum vegetation.
Keywords: aridity, ecohydrology, ecosystem functioning, hydrological niche, gypsophily, NDVI, LSWI.