How Does the Forest “Shelter” the Butterflies?
Contributed by Dr. Bill Calvert

To answer this question we must know something about the impact of vegetation and forest structure on microclimate.

Outside of the Forest: Daily Temperatures Rise and Fall
In the open, where there is no vegetation higher than grass, the ground is directly influenced by the amount of solar radiation that strikes the area. In the tropics, during the day the intense heat (radiation) strikes the ground, heats up the ambient without any attenuating factors. During the night the ground radiates heat to the night sky in the same manner. The results are extreme heat during the day and extreme cold during the night!

Inside the Forest: Daily Temperatures Are More Stable
In a forest, the situation is very different. The incident radiation strikes the forest canopy during the day. Some is reflected away, some is absorbed. Depending on the thickness of the canopy, some fraction reaches the interior of the forest. Yet because much is reflected away, the forest remains cool and also moist in comparison to what is happening in nearby clearings.

So in the absence of intense tropical radiation outside, the forests remain cooler during the day, and at night warmer. Certain species of plants thrive in this microhabitat . These are adapted to supply nectar to monarch butterflies and in turn be pollinated by the masses of butterflies that inhabit the forests. The forest acts like a blanket retaining warmth during the cold nights and keeping out the heat during the day.

Temperature and also humidity variations are dampened; the extremes occurring in clearings are reduced within the forest, to the great benefit of the butterflies.

The high altitude forest of the transvolcanic belt of Mexico are extremely limited. (See map.) Overwintering monarch butterflies must have cool temperatures--but not freezing temperatures--and moist conditions--but not soggy ones. Thus, they have very limited habitat from which to choose.

This bus was not sheltered by the forest. It was parked outside on a cold winter night near the Sierra Chincua monarch sanctuary. Dr. Calvert wrote in the frost on the roof of the bus.