Physiological and spectral alterations resulting from mechanical girdling simulating Oncideres spp. damage on Acacia mearnsii: Understanding responses and implications for remote sensing monitoring

In Brazil, twig-girdler beetles of the genus Oncideres severely compromise the productivity of black wattle ( Acacia mearnsii ) plantations, a tree species cultivated in many countries to produce tannins, charcoal, and pulp. Despite the significant damage caused to plantations, the physiological and spectral bases of these insect attacks remain unknown. Therefore, this study aimed to evaluate the effects of mechanical girdling simulating twig-girdler damage on the physiological and spectral properties of one-year-old black wattle plants, as well as to determine when spectral changes become detectable. Five treatments were applied: control and one, nine, 18, and 25 days after girdling. Growth, chlorophyll a fluorescence, photosynthetic pigments, oxidative stress, and leaf reflectance were evaluated, the latter being used for multispectral simulations and vegetation index calculations. Significant reductions were observed in root biomass, photochemical efficiency, and pigment contents, with more pronounced effects in the treatments corresponding to 18 and 25 days after girdling. Conversely, an increase was detected in H 2 O 2 concentration and in the carotenoids to total chlorophyll ratio. Spectral properties differed among treatments, with more evident changes in the near-infrared region. These alterations were reflected in vegetation indices, allowing identification of those most suitable for detecting girdled saplings (DVI, GNDVI, NDVI, NLI, RVI, and SAVI). The findings contribute to understanding the physiological and spectral changes induced by mechanical girdling in black wattle. Although derived from leaf-level measurements, the spectral results might provide a basis for the development and testing of monitoring strategies in forest plantations using sensors mounted on remotely piloted aircraft.