Monitoring At-Risk Trees Using Spectral Reflectance Data from Satellites

A desktop early warning method for arborists and tree management to carry out migitating actions.
Uses data from two commercial satellites: WorldView-2 and WorldView-3.
Satellite data processed by remote-sensing software to generate spectral reflectance of individual trees, and compared with the data of previous year(s).
Have to be using cloud-free images, although software could compensate for other atmospheric effect on images.
Spectral reflectance relates to chlorophyll content and leaf cellular structure, which correlates to tree stress and/or structural stability. These symptoms could not be readily observed by the naked eye.

For the following analysis, data of the spectral bands of red, red edge, and near-infrared 1 and 2 on 25 May 2010 and 17 April 2015 (five years apart) were acquired from WorldView2 satellite.
- Three Banyan trees (Ficus microcarpa) in Hong Kong.

Over the five years, the drop in the spectral reflectance of the near-infrared band for Tree #1 was 4%, Tree #2 was 3.5%, and Tree #3 was over 7%.
The author believes that there is a direct correlection between the drop in spectral reflectance and the closeness to the date of collapse.
Based on the indicators, all three trees were stressed and structurally unstable, leading to their failure after 96, 94 and 2 days, respectively.

In a back-dated analysis for a tree that has already fallen on 10 August 2016, data dated 2 September 2014 and 8 September 2015 (one year apart) were acquired from WorldView2 satellite.
- Three Elm trees (Ulmus sp.) in Oslo, Norway.

In just one year, the drop in the spectral reflectance of the near-infrared band for Fallen Tree #1 was 5%. There was not much variation in spectral reflectance in the adjacent Alive Tree #2 and Alive Tree #3.

Conclusion:
This method is not meant to diagnose the cause of stress and structural instability.
Presents more objective and quantitative means to measure the health of trees.
Could be combined with field inspection to enhance the management of trees by implementing timely mitigating actions.

Reference:
International Society of Arboriculture's Arborist News, Volume 26, Number 2, April 2017