Spectral reflectance characteristics of eucalypt foliage damaged by insects

Variables related to foliar damage, leaf morphology, spectral reflectance, chlorophyll fluorescence and chlorophyll content were measured from leaves sampled from mature eucalypts exhibiting symptoms of crown dieback associat ed with bell miner colonisation located in Olney State Forest, near Wyong, New South Wales. Insect-damaged mature leaves and healthy young expanding l eaves of some species exhibited a conspicuous red coloration caused by the presence of anthocyanin pigmentation. For the mature leaves, the level of r ed coloration was significantly correlated with insect herbivory and leaf n ecrosis. Significant correlations were also found between the level of red pigmentation and the following four spectral features: maximum reflectance at the green peak (550 nm); the wavelength position and maximum slope of th e red edge (690-740 nm) and the maximum reflectance at 750 nm in the near-i nfrared portion of the electromagnetic spectrum. While it has been shown th at anthocyanin pigments are synthesised in some eucalypt species in respons e to certain abiotic stresses causing photoinhibition and activation of pho toprotective mechanisms, this work proposes that biotic agents such as leaf damaging insects and fungal pathogens may induce a similar response in euc alypt foliage resulting in increased levels of anthocyanins. The potential of anthocyanin levels to be related to leaf ontogeny for some eucalypt spec ies was also illustrated in the reflectance spectra. Thus, it is essential that leaf age be considered. This work demonstrates that the identification of a number of key features of leaf spectra can provide a basis for the de velopment of a robust forest health indicator that may be obtained from air borne or spaceborne hyperspectral sensors.