Oyamel fir forest trunks provide thermal advantages for overwintering monarch butterflies in Mexico

Abstract. 1. Survival of overwintering monarch butterflies following severe wet winter storms in Mexico is substantially higher for butterflies that form clusters on the oyamel fir tree trunks than for those that form clusters on the fir boughs. 2. Thermal measurements taken at similar elevations with a weather station on the Sierra Chincua and within a Cerro Pelon and a Sierra Chincua overwintering area indicated that clustering on the fir trunks provides dual microclimatic benefits for the butterflies. a. At night, the minimum surface temperatures of all firs combined averaged 1.4 °C warmer than ambient forest temperatures, thereby enhancing protection against freezing for monarchs that are either wet or dry. We term this the ‘hot water bottle effect.’ b. During the day, the maximum surface temperatures of all firs combined averaged 1.2 °C cooler than ambient, a difference sufficient to lower the loss of the butterflies’ lipid stores over the 154‐day wintering season. 3. Larger diameter trees increase both microclimate benefits. 4. The results add a new dimension to improving the conservation management guidelines for the Monarch Butterfly Biosphere Reserve. Strict enforcement against culling of larger trees and in favour of promoting old‐growth oyamel forests will enhance two microclimatic benefits: butterfly mortality during severe winter weather will be reduced, and the butterflies’ lipid savings over the winter will be enhanced.