Evolution of tolerance : its influence on the ability of remote sensing to detect contaminated ground via vegetation stress

The detection of contaminated ground, such as metal contamination is desirable to locate contamination events, determine land use and for mineral exploration using geobotany. Remote sensing users currently attempt to use spectral indicators in plants to locate contaminated ground via stress effects on vegetation. However, up to now this approach has ignored the phenomenon of evolution ofmetal tolerance in some plant species. Some species are metal tolerant, as well as some populations within a species. Tolerant plants may dominate the community on contaminated ground, and may not exhibit the conventional stress spectral signatures, thus limiting the applicability of the approach. This study investigated the problem through an analysis of reflectance data from experimental plots. Metal tolerant and non-tolerant plants from two grass species (Festuca rubra and Agrostis capillaris) were grown in standardised conditions with control, copper, zinc and salt (NaCl) treatments. Leaf reflectance properties (400 1100 nm) of the different treatments were measured using an integrating sphere. Pigment concentration analysis was also undertaken. Differences between treatments were examined using the raw reflectances, as well as a number of published and novel vegetation indices and red edge position. Non-parametric statistical tests were used. Tolerant plants showed different spectral responses to non-tolerant plants irrespective of treatment. No spectral analysis technique consistently showed a stress response in all non-tolerant treatments, although different techniques did show different stress responses. In the second part of the study canopies ofmonocultures and mixtures of tolerant and non-tolerant ecotypes ofFestuca rubra were grown in plots with control and zinc treatments. Reflectance measurements (400 2500 nm) were made under artificial lighting using a spectroradiometer mounted inside a light proof tent. Pigment analysis was also undertaken. Spectral analysis techniques were the same as those used in the first part of the study. No vegetation index gathered from the literature distinguished the control from the metal treatments for the monoculture plots, but many were successful for the mixture plots. Indices developed during this study based on the green-red region were successful. Red edge position bore no useful relationship to treatment. Non-tolerant stressed plants showed an early senescence, which may be crucial to their detection. The addition of metal contamination to the natural environment will typically stress non-tolerant plants that are present, and tolerant plants may become dominant in the community. Leaf and canopy reflectance models were used to simulate reflectance changes following community responses to metal stress. These showed that the detection of contaminated ground via vegetation stress requires that the area being surveyed is either comprised of a non-tolerant ecotype which is stressed (i.e. before being outcompeted by tolerant plants) or that the contamination level is so high the tolerant ecotype shows stress. The results indicate that the use of remote sensing to detect contaminated ground via vegetation stress is far from straightforward. To be successful the measurements have to be made during a community transition from non-tolerant stressed plants to tolerant plants (although this transition could vary from weeks to years). The measurements should also be made over the senescence time period, to locate areas senescing early. As many methods of analysis (indices, red edge position etc.) as possible should be used. The users also have to accept that false negative results will occur where tolerant populations are on contaminated ground. The cause of stress will not be identifiable.