Crown-scale evaluation of spectral indices for defoliated and discoloured eucalypts

Remote sensing for evaluation of canopy health in plantation eucalypts is a realistic option for forest managers in the near future if reliable and robust methods of spectral analysis can be developed. Pot-grown eucalypts of three species important to the Australian plantation industry were used for crown-scale spectral (400-1000 nm) evaluations of vegetation indices as indicators of common symptoms of stress. When defoliation treatments (in <i>E. globulus</i>) or exposure to cold and nutrient deprivation (in <i>E. pilularis</i>) resulted in large differences in leaf cover, the red edge position and slope indices, two normalized difference vegetation indices (NDVIs), modified chlorophyll absorption ratio index 2 (MCARI2) or modified triangular vegetation index 2 (MTVI2) were most strongly correlated to leaf cover. However the NDVIs were significantly affected by soil background in a study with <i>E. globulus</i>. The percentage of red leaves resulting from stress treatment was most strongly correlated with the anthocyanin reflectance index (ARI) and red-green index (RGI) in both <i>E. grandis</i> and <i>E. pilularis</i>, however the RGI was affected by background type in the <i>E. globulus</i> study while the ARI was not. Exposure to cold and nutrient deprivation led to marked changes in leaf cover for <i>E. pilularis</i> but not in <i>E. grandis</i> and a much more reduced level of chlorophyll in <i>E. pilularis</i> than is suspected in <i>E. grandis</i>. In <i>E. globulus</i>, defoliation from the upper crown was easier to detect with spectral data than from the lower crown. Results were generally comparable to studies of eucalypt crown condition from native forests.