FOREST CANOPY STRUCTURE AT OVERWINTERING MONARCH BUTTERFLY SITES - MEASUREMENTS WITH HEMISPHERICAL PHOTOGRAPHY

Using hemispherical photography and digital image analysis, we have quantified forest canopy structure and light conditions at monarch butterfly (Danaus plexippus) overwintering sites near Santa Barbara, California, Hemispherical photographs were taken from 1.75 m height in more than 30 forest groves, including permanently occupied, transient, formerly occupied, and unoccupied sites. Analysis of the photographs permitted us to calculate site factors that quantify the proporation of indirect and direct radiation received relative to completely open conditions. The permanently occupied sites exhibited a narrow range of indirect radiation and a slightly wider but still narrow range of annual direct radiation. Transient sites exhibited a wider range of indirect and annual direct radiation than the permanent sites. Within the largest aggregation site, horizontal and vertical variation in site factors was considerable. Changes in monthly direct radiation with height may be unpredictable and reduce confidence in extrapolations from near ground level to the height of butterfly clusters. Sites that formerly supported aggregations and since have had trees removed exhibited high indirect and direct radiation and apparently are now too open to support butterflies. We simulated proposed tree removals at two sites by editing digitized canopy photographs, which enabled us to predict resultant changes in indirect and direct radiation. These results provide quantitative guidelines for silvicultural management to maintain and enhance remaining aggregation sites along the California coast.