Leaf Mass per Area of Wetland Vegetation under Water Stress Analyzed with Imaging Spectroscopy
Plant and community traits of wetland vegetation show a high intra-specific plasticity, originating from the high variability of environmental conditions. Remote sensing approaches promise to be able to retrieve some of these traits and their plasticity from the spectral reflectance signal of the canopy. In the present study, we evaluate a remote-sensing based approach for an analysis of spatial patterns of leaf mass per area (LMA), a key trait for ecosystem functioning and good negative correlate of potential growth rate. The test was conducted in Las Tablas de Daimiel, a National Park in Central Spain. This wetland was affected by a long-term drought, which introduced pronounced trait plasticity as part of the adaptation mechanisms of the vegetation to reduced water availability as well as a decrease in photosynthetic activity. Imaging spectroscopy (HyMap) data of the wetland were acquired in 2009 at peak drought intensity. At the same time, a field campaign was conducted. We applied an inversion of the PROSAIL model on these data to map the LMA distribution across the wetland. PROSAIL is a radiative transfer model that simulates the physical principles of light absorption and scattering in a vegetation canopy. The inversion enables the retrieval of trait information from the spectral signal. Furthermore, we assessed trends in photosynthetic activity and changing species composition across the wetland by analyzing time series of the normalized difference vegetation index (NDVI) as determined from various multispectral sensors. The mapped LMA values were analyzed within and between stands of different species and communities along a gradient of changing photosynthetic activity and species composition. LMA values retrieved for stands of species with high photosynthetic activity at peak drought intensity closely met values reported in trait data bases. The observed intra-specific LMA variability is in line with the expected plasticity of this trait along a moisture gradient that is reflected in a change in photosynthetic activity and species composition. We thus conclude that remote sensing approaches provide sufficient detail to trace the LMA-response of wetland vegetation to long-term drought stress.