One of the important aspects of conserving monarch populations is protecting their overwintering grounds in Mexico. Every year, monarchs migrate thousands of miles to seek out a specific tree – the Oyamel fir (Abies religiosa, also named Sacred fir). These trees, which can be found at high elevations (2,800 to 3,350 meters) in cold climates and moist soils (Sáenz-Romero et al., 2024), protect monarchs from extreme temperatures and precipitation during the winter. Because of their specific habitat requirements, climate change is having a significant impact on Oyamel forest survival. With temperatures increasing and precipitation becoming more variable, existing Oyamel forests are expected to disappear inside the Monarch Butterfly Biosphere Reserve (MBBR) by 2090 (Sáenz-Romero et al., 2024).
In response to this predicted loss, scientists are beginning to look into alternative options for navigating future climate change scenarios. Recently, scientists from the Universidad Michoacana de San Nicolás de Hidalgo in Morelia, Mexico, have begun testing the possibility of establishing new Oyamel forest habitat in the surrounding mountains with promising results.
Currently, Oyamel firs grow at a maximum elevation of approximately 3,500 meters, but with future climate projections, these habitats would need to shift to higher elevations by about 500 meters. In order to test the feasibility of expanding Oyamel forests into higher elevations, Dr. Cuauhtémoc Sáenz-Romero set up an experimental design to attempt to establish Oyamel firs on a nearby volcano, Nevado de Toluca, located 75 kilometers southeast of current monarch overwintering habitat with substantially higher elevations than the summits of the MBBR.
Dr. Sáenz-Romero and his team planted Oyamel seedlings at four different sites along an elevation gradient of 3,400 – 4,000 meters to test seedling survival and growth. After two years of monitoring, the research team found that at 3,800 meters seeding survival rate was 68% while at 4,000 meters survival was 44%.
After assessing the status of the transplanted seedlings, Dr. Sáenz-Romero concluded that the establishment of Oyamel fir trees is feasible at both 3,600 and 3,800 meters of elevation (up to 300 meters above current maximum Oyamel fir forest elevations). Above elevations of 3,800 meters, the mortality rate of seedlings and significant decline in growth rate become more pronounced.
To Dr. Sáenz-Romero, testing plant growth out in the field was key to making this research project a success. Without testing tree growth in the field, computer models could show that Oyamel firs “will have their suitable climatic habitat at and beyond the 4000 m of elevation timberline by year 2090,” said Dr. Sáenz-Romero. However, what these computer models don’t tell you is “if a seedling that is planted today beyond 4000 m, would survive” tonight’s frost. “That is why we planted at 4000 m, and measured the survival mortality and growth rate” Dr. Sáenz-Romero concluded.
Dr. Sáenz-Romero determined that these results demonstrate that Oyamel firs can successfully be transplanted to higher elevations and have the potential to establish new overwintering habitats for monarchs. He noted that the trees planted as a part of this experiment should reach an acceptable size for monarch use by 2060.
One important aspect to establishing new overwintering grounds is that they are located in protected areas. The sites selected by Dr. Sáenz-Romero and his team were located within forests owned by the town of Calimaya, an Indigenous community known for their excellent forestry management. Dr. Sáenz-Romero’s paper highlighted how working with local communities is key to successful conservation as Indigenous communities help protect and monitor forest activity, provide local expertise, and enable long-term conservation success. Local communities “might never read our English academic papers,” noted Dr. Sáenz-Romero. However, “if they see our results on the field tests, they could be convinced or not of shifting the elevation of the Oyamel fir reforestation.” This hands-on experience can help make conservation efforts more widely accepted and strengthen long-term collaborations for monarch protection.
While these findings are an exciting step for monarch conservation in Mexico, Dr. Sáenz-Romero noted that there is no guarantee that monarchs will relocate to newly established forests. However, recent surveys provide a hopeful outlook on the situation. Historically, Nevado de Toluca, where the study took place, has been relegated as a marginal overwintering site for monarchs, but during the winter of 2023/2024, areas along the southwestern slope housed the largest colony of overwintering monarchs. This increase in monarch presence highlights the opportunity to establish monarch habitats in new locations that will eventually be utilized by current populations.
Overall, Dr. Sáenz-Romero’s research highlights the importance of combining current restoration efforts with new habitat establishment initiatives. Dr. Sáenz-Romero pointed out that these two ideas are not mutually exclusive and should benefit each other. He hoped that the two options will work in tandem as top priority goals for monarchs in Mexico, working to protect existing habitat while preparing for a time when climate change deems that new locations are required.
Full article:
C. Sáenz-Romero, Osuna-Vallejo, V., Herrejón-Calderón, P., Pérez-Cruz, L.A., Joaquín-Juan, M.G., Cruzado-Vargas, A.L., O’Neill, G.A., Zacarías-Correa, A.G., Manzanilla-Quijada, G.E., Lindig-Cisneros, R., Blanco-García, A., Endara-Agramont, Á.R., and Lopez-Toledo, T. “Establishing monarch butterfly overwintering sites for future climates: Abies religiosa upper altitudinal limit expansion by assisted migration”, Frontiers in Forests and Global Change, 2024
Photo credits: Cuauhtémoc Sáenz-Romero, UMSNH