Planting Abies religiosa (Sacred fir) plants in the shade of pre-existing shrubs (Senecio cinerarioides, narrow grayish-green foliage) as protective “nurse plants”. The large trees in the background are adult Pinus hartwegii, the pine that reaches the treeline. Abies religiosa is completely absent from this site located at 3800 m altitude, on the northeastern slope of the Nevado de Toluca volcano, in central Mexico, because it is too high in altitude. The planters’ staff is made up of locals of Indian origin. Credit: Cuauhtémoc Sáenz-Romero, UMSNH
The monarch butterfly migration is one of the wonders of the natural world. Every fall, a new generation of monarch butterflies is born in the northern United States and southern Canada. Hundreds of millions of these butterflies then fly to the mountains of central Mexico, located between 4,000 and 4,800 km away. There they winter in the sacred fir Abies religiosa forests at high altitudes. Without these sacred fir trees, monarchs would not be able to survive their grueling migration.
But with global warming, these forests should slowly move up the slopes. Around 2090, they will leave the mountain. It will therefore be necessary to create new forests outside their current geographical area: for example on the mountains further east, which are higher.
“Here we show the feasibility of planting new sacred fir forests on a nearby volcano, Nevado de Toluca, at altitudes between 3,400 and 4,000 meters,” said Dr Cuauhtémoc Sáenz-Romero, professor at the Michoacana University of San Nicolás de Hidalgo in Mexico, and lead author of a study published in Forest boundaries and global change.
“We call this ‘assisted migration’: planting seedlings from existing sacred fir populations to new sites whose climate by 2060 is expected to become similar to that of current wintering sites due to global warming .”
Take a stand
In 2017, Sáenz-Romero and colleagues collected cone seeds from eight stands of sacred fir in the Monarch Butterfly Biosphere Reserve (MBBR) in Mexico, at altitudes between 3,100 and 3,500 meters. They grew seedlings from these, first for two years in a greenhouse at 1,900 meters above sea level, then for another year in a nursery at 3,000 meters.
In July 2021, they transplanted the seedlings to four sites along an altitude gradient on the northeastern slope of Nevado de Toluca.
Local foresters, graduate students and professors from the State University of Michoacán, Mexico, who set up the experiments in Nevado de Toluca. This is a site located at an altitude of 3,600 m, just above the maximum natural distribution limit of Abies religiosa (around 3,550 m). Credit: Cuauhtémoc Sáenz-Romero, UMSNH
The researchers chose this mountain because it is closest to the MBBR and has a peak 1,130 meters higher than the highest occurrence of sacred fir trees, at 3,550 meters. It is also a protected natural area.
They planted 960 plants at four altitudes: 3,400, 3,600, 3,800 and 4,000 meters. The latter is the treeline of the Nevado de Toluca and was included to find the highest altitude at which sacred fir trees can survive in today’s climate. Seedlings were distributed across 30 spatial blocks by elevation, taking care to include an equal number of each original stand in the MBBR.
Seedlings were always planted under “nurse plants” to protect them against excessive insolation and extreme cold. These were Senecio cinerarioides shrubs up to 3,800 meters, and Lupinus montanus shrubs and Pinus hartwegii trees up to 4,000 meters.
Every two months between September 2021 and December 2023, Sáenz-Romero and his colleagues (including graduate students and local foresters from the Matlatzincas Indian people) measured each seedling’s performance—that is, its survival, its height and diameter. Since the aim of the experiment was the conservation of sacred fir trees and not the production of wood, survival was considered the most important measure.
Planting Abies religiosa (sacred fir) plants at the treeline (4,000 m above sea level) of the Nevado de Toluca volcano, in the shade of pre-existing Lupinus elegans as protective “nurse plants”. This site is approximately 450 higher in elevation than the maximum natural distribution of Abies religiosa. Credit: Cuauhtémoc Sáenz-Romero, UMSNH
Colder and higher
The results showed that the performance of transplanted plants decreased as the “ecological distance” – the weighted difference between a series of climatic variables such as temperature, precipitation and drought – between the original site and the site of plantation increased.
Overall, survival and growth deteriorated when seedlings were transplanted to colder and higher sites than the original MBBR stand. At 4,000 meters above sea level, growth was almost zero, while many seedlings showed frost damage.
Between 3,600 and 3,800 meters, seedlings had 54% less vertical growth, 27% less biomass, and 27% less survival than at a reference altitude of 3,400 meters. The authors considered this survival rate “very acceptable”.
“These planted stands could ultimately serve as overwintering sites for the monarch butterfly in warmer climates,” Sáenz-Romero concluded.
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Seedlings of Abies religiosa (Sacred fir) produced in a communal nursery in Ejido La Mesa, at 3,000 m altitude (to induce hardening of the plants), on the border of the MBBR. The seeds came from MBBR and the plants were later planted in Nevado de Toluca. In the photo, Francisco “Don Pancho” Ramirez-Cruz, former chief of Ejido La Mesa, was in charge of producing the plants. Credit: Cuauhtémoc Sáenz-Romero, UMSNH
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Abies religiosa (Sacred Fir) plants (foreground) were planted in groups of eight plants in the shade of pre-existing shrubs as “nurse plants” (Senecio cinerarioides), to benefit from protective shade against the temperatures extremes (hot or hot). extreme cold), which is critical in the context of ongoing climate change. Each seedling planted came from seeds collected at MBBR, at lower altitudes than the planting site. Credit: Cuauhtémoc Sáenz-Romero, UMSNH
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Abies religiosa (Sacred Fir) covered in monarch butterflies at Ejido La Mesa, 3,340 m elevation, central MBBR area. Abies religiosa seeds were collected at this site to produce seedlings in a nursery which were then planted at Nevado de Toluca, at altitudes of 3,400 m, 3,600 m, 3,800 m and 4,000 m. Credit: Cuauhtémoc Sáenz-Romero, UMSNH
“In fact, in recent years, monarch butterflies have established new, large colonies in colder locations in the Nevado de Toluca, suggesting that they are already looking for new places to overwinter, as their historic sites in the interior of MBBR are now too warm our plants have reached full maturity, they will hopefully also discover our planting site.
“We emphasize that the creation of new areas for monarch butterflies is not mutually exclusive with continued efforts to conserve their current habitat in the Monarch Butterfly Biosphere Reserve. The two approaches should be complementary, with equal priority. “
More information:
Cuauhtémoc Sáenz-Romero et al, Establishing monarch butterfly wintering sites for future climates: Abies religiosa expansion of the upper altitudinal limit by assisted migration, Forest boundaries and global change (2024). DOI: 10.3389/ffgc.2024.1440517
Quote: Scientists create new wintering sites for monarch butterflies on a warming planet (October 18, 2024) retrieved October 18, 2024 from
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