The impact of forest thinning on microclimate in monarch butterfly ( Danaus pexippus L.) overwintering areas of Mexico

Abstract

The effect of forest density on minimum temperature was measured in several Abies religiosa forest quadrants located in and near colonies of the monarch butterfly ( Danaus pexippus ), in the Neovolcanic Belt of Mexico. Regression analysis with forest density, altitude and date of the studies show that the minimum temperature is an inverse function of forest density. The date of the studies and the altitude were also important components of the model. The lower temperatures in harvested (thinned) forests are explained by the absence of tree structures that prevent the escape of heat due to radiation during the night. Using the regression equation, we can predict that temperatures in forests that have been radically thinned will be more than one degree Celsius lower than in areas where butterflies have established their colonies. A subsequent drop in temperature of this magnitude, which is already at the death limit for butterflies during some winter storms, would surely increase colony mortality considerably.