March 22, 2016

There has been a lot speculation and hopeful remarks about the extent of mortality caused by the 10-11 March 2016 storm in Mexico.

We (Lincoln Brower, Isabel Ramirez, Pablo Jaramillo, Ernest Williams and Daniel Slayback) have now obtained and analyzed temperature data from our measuring devices:

(A) out in the open and

(B) beneath the Oyamel forest canopy on the Sierra Chincua.

The temperature in the exposed area dropped to about -4.5 degrees centigrade. Under normal winter conditions, the forest would have acted as a blanket and kept the forest a few degrees above freezing. However, this storm was unique in our experience over the past 39 years: the wind blew very hard (over 60 miles per hour) and homogenized the temperature of the air both in the open and beneath the intact forest, obliterating the usual blanket effect.

The storm also soaked the area with at least 2.85 inches of rain. We know from laboratory experiments that the lethal temperature for 50% of monarchs that have been wetted is between - 4 and -5 degrees centigrade. The data thus point to approximately 50% mortality.

We are gathering more details and will report again as the facts are assembled.

Lincoln Brower
Research Professor of Biology
Sweet Briar College
Sweet Briar, VA