Note: this project has been updated. See 2024-2025 Update & Findings below.
With MBF’s support, Ambiomas, a Mexican non-profit established by Salvador Huitrón (a 2022 Lincoln Brower Award recipient!) is leading a drone flight project at the monarch overwintering sites. The purpose of this project is to develop a forest health monitoring and alert system using remote-sensing technology.
In 2023 during months monarchs are not present, drones flew over sites where monarchs typically roost, using historical areas of overwintering sites. Images of the forest were obtained from these flights and a Digital Surface Model (DSM) was developed to estimate the total number of trees present and tree density per hectare in each site.
The Ambiomas team, along with FOCEN (Neovolcanic Axis Conservation Fund), who also supported this project, analyzed the information collected in each flight area to determine a Normalized Difference Vegetation Index (NDVI) range for these oyamel trees, using NDVI as a measure of tree vigor. The monitored sites revealed different degrees of tree vigor, ranging from "Healthy" (NDVI=0.74-0.90) to "Extremely Devitalized" (NDVI=0.46-0.56). Two monitored areas showed apparent favorable conditions with high densities of trees and a high proportion of trees in the Healthy Tree Category. Other sites, however, showed signs of stress and decay and a higher proportion of trees in the "Severely devitalized" category.
Because NDVI values can change after the rainy season, possibly related to the greening of the understory, it is important to assess how much of this difference may be an actual change in tree health over time. A comparison of NDVI values in dry seasons of consecutive years was recommended to help identify processes of recovery or decline in the vigor of the trees (see 2024-2025 Update & Findings below).
While tree vigor categories allow comparisons to be made, there is a need for fine tuning to determine more precise indicators of devitalization processes in Oyamels.
2024-2025 Update & Findings
In May and October of 2024, before and after the rainy season, from the same polygons of 2023, Ambiomas repeated drone flights collecting over 11,000 new aerial photographic images. An additional 10.3 hectares were flown over the ejido Jiguipilco, State of Mexio. The new images were processed to obtain NDVI values, and these were contrasted with NDVIs found in 2023 to document changes in forest density and tree vigor and identify critical conservation and restoration sites within the MBBR.
During the analyzed period, the monitored trees gained vigor after the rainy season, but were unable to maintain this gain after the following dry season. This dry-season loss poses a significant challenge for the conservation of wintering sites in the face of longer and more intense droughts.
Ambiomas also found that different sites experienced different degrees of vigor loss: in some locations (such as El Rosario and La Mesa), vigor loss was abrupt, elsewhere it was relatively moderate (such as Piedra Herrada), and in others it was slight or almost nonexistent (such as Carpinteros). This raises the question of what local factors could be contributing to these different responses and could be the starting point for longer-term monitoring.
This project represents a first effort in the construction of a Baseline Tree Vigor database at Monarch Butterfly Overwintering Sites in Mexico, and ultimately will contribute to the development of the forest health monitoring and alerting system. This is an excellent example of how using the latest technology can help conservation efforts!
