Health Assessment of Avicennia spp. Forest Through an Unmanned Aerial Vehicle Located at the Upper Gulf of Thailand

Abstract

The health of trees requires a visual assessment of symptoms or more elaborate measurements of chlorophyll fluorescence and the spectral reflectance of foliage. A lack of reliable and feasible health indicators severely limits the current ability to assess the vitality of mangrove trees. In this study, the crown recovery of Avicennia spp. was monitored after a severe insect defoliation event in December 2017 using UAV-based surveys. The objective was to identify the reliable indicators of a tree’s health status that can be assessed from the ground.

Two aerial surveys were conducted in the Mahachai mangrove forest (Samut Sakhon), in January 2018 and April 2019, respectively. In addition, a ground-based survey of health symptoms, specifically the presence of stem wounds, fungal fruiting bodies, and signs of woodborers, was conducted in 28 sample plots (20 m × 20 m) which were distributed along five line transects. Aerial images were used to delineate and calculate the area of crown defoliation (gaps) in each plot. The relationships between the presence of certain health issues and the recovery of defoliated crowns, within each plot, were analyzed using an ordinary least squares (OLS) model.

The results showed that the Avicennia trees were able to recover rapidly after an initial defoliation, indicating a high resilience to such disturbances. However, only stem wounds should be used as vitality indicators as their presence was associated with a significantly reduction in crown recovery, whereas the association with the presence of wood borers and fungal fruiting bodies was insignificant. In conclusion, the monitoring of mangrove health should not only rely on a visual ground-based assessment but should be accompanied by aerial surveys which can provide more reliable vitality indicators.