Spectral separability of bark beetle infestation stages: a single-tree time-series analysis using Planet imagery

Bark beetles cause severe damage to European forests leading to impacts on many sectors, from the environmental to the economical. Timely mapping of the different stages of an attack is very important. Remote sensing has been widely used to map bark beetle damage using both airborne and satellite data. Newly available satellite multispectral data with a daily revisit time and high spatial resolution has the potential to monitor an attack in all its phases. This study explores the spectral separability of bark beetle infestation stages using the Planet imagery at individual tree level. Multi-temporal spectral analysis of 78 trees in different stages of a spruce bark beetle attack was carried out. Bands and vegetation indexes derived from 42 multispectral images were compared to eleven field surveys over a time span of approximately four months. The spectral separability analysis was done considering three criteria exploring: 1) the significance of the differences, 2) the magnitude of the differences and 3) the separability in a supervised classification context. The field surveys reported different effects depending on the season of the bark beetle attack - spring vs. summer. Spectral bands and indexes extracted from trees in the healthy and red-stage were significantly different. Trees in the green-attack stage at the end of the summer showed a statistically significant difference from healthy trees. The separability measured with a supervised classifier showed that it is possible to separate healthy, green-attack and red-stage trees with high accuracy values (kappa accuracy above 0.9)