Hydrocarbon pollutant impact on spectral and biophysical properties of willow and grass species
Vegetation is a potential indicator of land contaminated by hydrocarbons. Remote sensing holds great potential for detecting stress in vegetation caused by the presence of hydrocarbons, but as yet this potential has not been realised, largely due to a lack of understanding of the biophysiological impacts of hydrocarbons on plant growth, and the specific spectral expression of them. Previous research has demonstrated success in laboratory simulation of hydrocarbon contamination and the analysis of the spectral and biophysical properties in plants, but has tended to always focus on scenarios with high concentrations of hydrocarbons (> 5 g/kg). It was assumed, that vegetation growing in hydrocarbon polluted areas would show a general decrease in plant health condition and show associated changes in reflectance. However, it is not known if lower concentrations of hydrocarbon in soils (< 5 g/kg) have the same effects as high concentrations and if the impacts observed at leaf scale would be observed at a canopy scale. The aim of this research has been to develop understanding at the leaf / plant scale initially, to then scale up to hyperspectral camera and simulated satellite-based scales to develop operational understanding that will benefit industry, regulators and environmental agencies alike.Two experiments were performed, one at plant scale with willow (Salix viminalis var. Tora) and maize (Zea mays var. Lapriora), and the other at canopy scale with turf grasses. At leaf scale, a pot-based experiment was designed with plants growing in pots containing a hydrocarbon (crude oil or refined oil) layer at the base of the pot with different concentrations levels (0.5 g/kg, 5 g/kg and 50 g/kg). At canopy scale, natural oil seep environments were simulated in 1 m by 0.5 m trays with a hydrocarbon layer with two different concentrations (50 g/kg and 0.5 g/kg) present only in the half of each tray and located at the base. Biophysical, reflectance and fluorescence measurements were made in both experiments using different portable spectrometers (MultispeQ, ASD Fieldspec Pro and Specim IQ camera).Biophysical variables (chlorophyll concentration, biomass and growth) decreased in the presence of high concentrations of hydrocarbon in the soil, showing a general reduction in all the studied parameters and in all plant species with respect to the non-polluted plants.26On the other hand, the presence of lower hydrocarbon concentrations (0.5 g/kg and 5 g/kg) revealed an increase in chlorophyll concentration, biomass of the entire plant (even in roots) and the growth rate, with respect to the non-polluted plants.To analyse the effect of different hydrocarbon concentrations on vegetation, changes in spectral response were analysed using continuum removal analysis and vegetation indices. Absorption features were identified related to pigment concentration (chlorophyll, carotenoids and anthocyanin) and biomass (starch, cellulose, lignin and water). Variations in absorption feature characteristics (band depth, band area and band width) depended on the hydrocarbon concentration in the soil, with increases in band depth and band area for low polluted treatments and decreases in the same characteristics for high polluted treatments. Vegetation indices in both plant and canopy scale experiments were able to discriminate between high pollution and low pollution in soil, as did the red edge. The red edge position showed movements towards longer wavelengths (improvements of health status of the plant) with the presence of low pollution in the soil and towards shorter wavelengths (reduction of health status) in high polluted treatments. At canopy scale a greener ‘halo’ surrounding a high polluted zone was seen, creating a spatial pattern previously observed in field campaigns in natural oil seep areas.To evaluate these outcomes at landscape scale using satellite imagery, reflectance data were convolved to different spectral resolutions related to three different sensors (WorldView- 2, Sentinel-2 and EnMap). The simulations showed that at the moment efforts need to continue to improve the sensor resolutions in order to monitor hydrocarbon polluted areas, or that a combination of sensors with varying resolutions may be required.In conclusion, the results of this study demonstrated biphasic behaviour of plants in hydrocarbon polluted environments and the potential of remote sensing in environmental monitoring and exploration of these areas. Thesis full text embargoed until 31/03/23
Remote sensing, vegetation, hydrocarbons, pollution, spectral analysis, vegetation indices, absorption features, reflectance, Hyperspectral spectroscopy, Soil contamination
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