Monarch butterflies: can 'assisted migration' save this important species?

For several weeks in 2015, the sound of chainsaws cutting down trees was incessant in the Angangueo municipality in the Mexican state of Michoacán. When a group of people tried to stop the deforestation, it was already too late: 10 hectares (25 acres) of forest had already been destroyed on hillsides in the central area of the Monarch Butterfly Biosphere Reserve.

The land on which the illegal logging took place belongs to the government of the state of Michoacán. There are no communal lands nor Indigenous communities that conduct surveillance in the area, which exist in other areas of the reserve that are collectively owned.

In order to recover the area that was affected by illegal logging, many public officials initially proposed reforestation, regardless of the fact that it was during the dry season and, was therefore an unfavorable time to plant trees. On the other hand, the directors of the reserve preferred to consult scientists.

However, among scientists, there were also conflicting opinions. Some argued that reforestation was the right method. Others advocated that natural regeneration was the way to go by not disturbing the area and waiting for the forest to recover on its own.

“It was a complicated discussion among academics,” said Dr. Cuauhtémoc Sáenz-Romero from the Natural Resource Research Institute (INIRENA) at Michoacan University of Saint Nicholas of Hidalgo (UMSNH).

“However, when we went to the region, it was clear that the conditions for natural regeneration did not exist: the trees that were still standing were gravely damaged and the crowns were destroyed. In the case of oyamel firs [Abies religiosa], the highest part of the crown is where seeds are produced. With a tree like that, you have to wait five to 10 years for the crown to recover and begin producing seeds.”

Dr. José Arnulfo Blanco García, the coordinator of the Restoration Ecology Research Laboratory at the same university, recalls that when they visited the area, “Cuauhtémoc [Sáenz-Romero] and I said: ‘We cannot be sure this will regenerate on its own.’ So we advocated for an active restoration plan for the area.”

For a decade, many groups have discussed whether reforestation or natural restoration was the better method for forest recovery. The debate exists because, on a global scale, it is acknowledged that many reforestation initiatives conducted are not successful; The percentage of trees that survive are often very low, and it is common to use very few tree species in these projects. This creates “very simplified forests,” which means that they do not contain a variety of tree species.

The final decision in this case was to combine active restoration with natural restoration, while at the same time conducting an experiment known as “assisted migration”.

The two scientists from UMSNH, Sáenz-Romero, a specialist in the genetic improvement of forests, and Blanco García, who specializes in ecology, decided to work as a team to restore the Monarch Butterfly Biosphere Reserve and other forested areas. They also decided to implement actions such as the “assisted migration” of trees. This method, which has been used for some time in other countries, including Canada, aims to secure a future for ecosystems and forest species that are threatened by climate change. The oyamel fir (Abies religiosa) is among those species.

In 2016, the scientists began their work restoring the 10 hectares of land affected by illegal logging.

As part of the restoration plan, the team divided the space into different sections of land. The strips on the borders and closest to the nearby forest were not touched. There, they put their faith in natural regeneration. About 70 percent of the remaining land underwent soil conservation and active reforestation work. This project involved more than just planting trees. Each step of the process had to be carefully implemented, which does not always happen and is part of the reason why reforestation projects often fail.

“Reforestation is often unsuccessful because some species planted are too small, planted at unfavorable times, or incorrectly selected for the area,” said Blanco García. “There are several factors that influence the reduced success of reforestation initiatives.”

In the case of this reforestation project, the team of ecologists traveled through the adjacent forest to document the trees they could find and their proportions relative to other species. They found that for every pine tree, there were three oyamel firs. They then used this ratio to select their seeds. Seeds were chosen very carefully and seedlings that were at least one and a half years old were used. This is in contrast to other projects that often choose seedlings that are six months old. Planting took place only during July and almost 200 residents from nearby communities were invited and trained for the reforestation activity.

Five years later, the results from the project surprised the scientists themselves; They achieved a survival rate of 83 to 84 percent. This rate is very high when taking into account the fact that some government reforestation programs, conducted over six-year terms in the past, have had survival rates between 10 and 35 percent.

“When high-profile reforestation initiatives are conducted, it is very probable that they will not have good results and [officials] only want to take photos,” says Blanco García.

The last time the team visited the restored site, the pine trees they had planted were on average three meters tall (10 feet) . The oyamel firs, a species with a slower growth rate, were approximately one and a half meters tall (5 feet).

In the sections where natural regeneration was chosen, the results were also unexpected.

“Five years later, we did not record a single seedling of an oyamel fir, pine tree or any other species from the area,” said Blanco García. “It would have been an error to expect that the entire site would regenerate on its own.”

Through their work in these forests, the researchers have corroborated that natural restoration is not always the best course of action, especially when the forested area is highly degraded and is under increasing pressure from climatic phenomena, such as droughts.

“Nature cannot run its course in a normal way when, instead of having 260 parts per million of carbon dioxide, you have 417,” says Sáenz-Romero. The climate, he insists, is totally removed from its normal course.

“So everything we learned in our ecology classes no longer occurs. There are two permanent alteration factors: climate change and social pressures changing land use. You are recurrently faced with grazing, illegal logging, and timber extraction,” explains Sáenz-Romero. “In nature, things are not how they are in books. It’s because of this that we have to [test ideas through] action.”

Blanco García agrees; “[For] sites that are too degraded, the truth is that there is no way to expect nature to do something.”

Since 2007, Sáenz-Romero has created climate models in a joint effort with Gerald Rehfeldt from the Moscow Forestry Sciences Laboratory in Idaho, which is affiliated with the USDA Forest Service. The goal was to understand how climate change affects forest species.

These models show Sáenz-Romero that, in the case of Mexico, cloud and coniferous forests are the climatic habitats (the space occupied by the favorable climate for a biome or a species) that are most vulnerable to climate change.

Cloud forests are located in a very narrow strip and require lots of humidity. Temperate forests of coniferous trees are in the high parts of the mountains. Because of climate change, these areas will be subject to higher temperatures.

“The space available with a favorable climate for these forests is decreasing,” says Sáenz-Romero.

In terms of forest species, those that are likely to encounter problems are those that are already undergoing the process of extinction. These include three species that are endemic to Mexico, such as the Chihuahua spruce (Picea chihuahuana), the Martinez spruce (Picea martinezii) and the Mexican spruce (Picea Mexicana).

“These will go extinct unless there is radical action on our part,” says Sáenz-Romero.

Other forest species at risk due to climate change are those that grow in the high parts of the mountains, like the Hartweg’s pine (Pinus hartwegii). This tree is present around Mexican volcanoes like Popocatépetl, Iztaccíhuatl, Nevado de Toluca, and Pico de Orizaba. Another species at risk is the oyamel fir (Abies religiosa).

Climate change models show that, if current tendencies continue, the Monarch Butterfly Biosphere Reserve will not have a suitable climate for the oyamel fir forest in the future.

“This is not to say that the oyamel fir will not exist,” says Blanco García. “It will probably be a mixed community of vegetation in which the pine tree is the most abundant, but the oyamel fir is not.”

What climate models have shown for several years is already beginning to be seen in the region. “All the hillsides that are in the Monarch Butterfly Biosphere Reserve, and that face south, are becoming more susceptible to climate events. And the saddest news is that this is where the butterfly colonies have been historically located,” says Blanco García.

In 2016, for example, a strong winter storm caused severe damage to about 20,000 trees throughout the reserve. Many trees fell, and others lost their highest branches. Because the seeds of the oyamel fir are produced in its highest branches, very few seeds fell during the next few years. This affected the trees’ natural regeneration.

Just over 10 years have passed since scientists from UMSNH began documenting the effects of climate change on the trees in the region. In 2010, for example, researchers were contacted by residents of the Indigenous community of Nuevo San Juan Parangaricutiro, in Michoacán, to help them understand what was happening with their trees.

On a national level, this Indigenous community stands out for being one of the most successful in terms of community forest management. The sustainable use of their forest has allowed them to create jobs, have a tourist center, and carry out several projects in the community. For this reason, when their forest began to show changes, they looked to scientists for help.

“They showed us a site, Los Volcancitos, where there had been unusual defoliation of Pinus pseudostrobus [the smooth bark Mexican pine]. The trees were losing foliage, many were already dead and already debilitated. They had infestations, like that of the bark beetle,” says Sáenz-Romero.

“But it was evident that the cause was not the beetle, but that the trees had been severely weakened. They told us: ‘This never happened to us.’”

Sáenz-Romero suspected that it may be an effect of climate change. In order to be sure, he shared photos and data with his colleagues, including scientists from Canada who are studying the effects of climate change on forest species.

In Canada, the researchers documented that summers are becoming increasingly hotter and drier, which is causing tree defoliation. Sáenz-Romero says that, in the case of Mexico, this does not happen in summer, but instead in spring: March, April, and May. There is an increasing amount of heat, and less rain. Therefore, the trees must rely on less water until June, when precipitation increases.

“There is a much hotter and drier environment, which weakens trees,” says Sáenz-Romero.

What happened in Nuevo San Juan Parangaricutiro, according to what Sáenz-Romero has concluded, is a result of the combination of a very dry year with low humidity and the stony conditions of the soil in the Los Volcancitos site.

According to the researchers, this case, and what happens in the Monarch Butterfly Biosphere Reserve, may be examples of a process that is being seen around the world with increasing frequency. This process has been nicknamed “forest decline,” which describes unusual woodland mortality.

Normally, as part of a forest’s natural renovation, about three percent of trees in forests are dead or in the process of dying.

“There are increasingly more sites in the world where there are mortality events above 15 percent, which are very unusual and are related to periods of drought that are hotter. The combination of droughts and temperatures hotter than ever before is becoming deadly for several species of trees,” says Sáenz-Romero.

Blanco García added that it is even more difficult for forests to regenerate naturally and to adapt to these new events.

“What are we going to do?” asks Sáenz-Romero. “Sit back and wait under the belief that nature is intelligent and will regenerate on its own? Or accept that we live in a changing world, [which is] pretty degraded, and that much of what is said in ecology books no longer applies?”

Upon observing the effects of climate change on different forest species, Canadian researchers have begun what they call “assisted migration”; Helping trees, such as conifers, migrate to areas of higher altitudes, where they can best resist changing climatic conditions.

In Mexico, Sáenz-Romero, Blanco García, and others have started to implement the “assisted migration” of oyamel firs in areas of the Monarch Butterfly Biosphere Reserve. They have used this technique, for example, in the areas that were damaged during the 2016 winter storm.

According to Sáenz-Romero, assisted migration involves “collecting oyamel seeds, producing plants in nurseries, and reforesting areas with higher altitudes and where it is predicted that a favorable climate will occur in the future for that particular population.”

The researchers are also using nurse plants, which are bushes that serve to generate protective shade for the seedlings that have been planted nearby.

The researchers acknowledge that several of the techniques they use are already known in communities in an empirical way.

“What we are doing is quantifying things that they have already noticed,” says Blanco García.

The researchers’ experiments are being conducted at sites like Las Palomas in the La Mesa community, which sits at 3,440 meters in altitude in the municipality of San José del Rincón. Las Palomas and Los Ailes, which has an altitude of 3,360 meters, are both within the center of the biosphere reserve. The oyamel fir seedlings that were used for these reforestation projects were produced in nurseries that have an altitude of 3,000 meters.

In Las Palomas, already-existing nurse plants were used to provide shade. The scientists also used more conventional reforestation techniques, like planting in a grid, with a distance of one and a half meters between each tree. With these techniques, the survival rate was 65 percent.

In Los Ailes, the scientists decided to break the norm of planting in a grid. They used nurse bushes there too, but these were plants from a lower altitude, rescued from the forest and planted in circles, “to optimize the usefulness of the shade from the bushes.” The result was a tree survival rate of 92 percent, even after the recent extreme drought that occurred as a consequence of the presence of La Niña (a cold current from the Pacific) in November 2020 to April 2021.

The achievement of these high survival rates was not a small feat, especially with a species like the oyamel fir, which does not prosper easily in reforestation projects. The tree species is located in the Trans-Mexican Volcanic Belt, which spans from the Nevado de Colima on the western border to the Pico de Orizaba in the east. It can be found in mountains with altitudes of 2,800 meters and higher.

The oyamel fir is vital for the preservation of the climatic conditions in the forests of central Mexico. During each winter, it receives monarch butterflies and is part of the process refilling aquifers that supply water to cities in the center of the country.

Studies conducted by Lincoln Brower, a researcher who dedicated a large part of his scientific interest to oyamel fir forests, showed that when these trees are healthy, among other things, they allow a “blanket and umbrella” effect. On cold nights in winter, their crowns function as blankets that help prevent all the heat from escaping from the microsite. When winter storms arrive, the trees’ crowns serve as umbrellas to keep butterflies’ wings from becoming wet and freezing. This process is affected when there are fewer oyamel firs or when they are not in optimal condition.

Sáenz-Romero and Blanco García highlighted that conserving and rescuing the oyamel fir forests is not only vital for the migration of the monarch butterflies and water security for Mexico’s large cities, but also for the future of the communities that live in and around the biosphere reserve. Part of its economy is supported by tourism and, in several cases, by sustainable forest management.

Sáenz-Romero stresses that “if we cannot save the [oyamel fir], we cannot save almost any species, because with it holds the interests of society, of authorities, and of the three countries that the monarch butterfly visits.”

This article was translated by Sarah Engel