image: Monarch butterflies rely on fall-blooming flowers during their long migration. view more
Credit: Photo by Jerry A. Payne, USDA Agricultural Research Service, from Forestry Images
US Forest Service (FS) research in the Ouachita Mountains of Arkansas suggests that decades of fire suppression have reduced the area's food supply for migrating monarch butterflies—and that restoration efforts that include prescribed burning can reverse this trend. Craig Rudolph and Ron Thill, research ecologists with the FS Southern Research Station (SRS), along with SRS ecologists Charles Ely, Richard Schaefer and J. Howard Williamson, report their findings in the latest issue of the Journal of the Lepidopterists' Society.
Every fall, masses of monarch butterflies migrate across eastern North America to remote sites in central Mexico—a long flight fuelled only by nectar from flowers still blooming on the way. In recent years, there have been concerns about the continued health of monarch populations, and of the migration itself, as land use change has altered both breeding habitat and migratory pathways.
Studies have focused on food availability for larvae, pesticides, and loss of overwintering sites in Mexico as possible threats. Less attention has been paid to how landscape-level changes affect the availability of the butterfly's preferred nectar sources along their migratory routes. In September and October, large numbers of monarchs pass through the Ouachita Mountains, a largely forested area in Arkansas and Oklahoma where fire suppression, logging, and pine production practices have altered forest structure, leading to a drastic reduction in the quality and abundance of the flowering plants the butterflies rely on for nectar.
"The area was dominated by fire-maintained shortleaf pine forests until the early 20th century," says Rudolph. "The forest was open, with a high mostly pine canopy, sparse midstory, and a diversity of herbaceous plants in an understory dominated by bluestem grass—the kind of habitat enjoyed by the endangered red-cockaded woodpecker."
Typical forests in the area now have a younger and thicker canopy, a dense woody midstory, and a very sparse herbaceous understory. In 1979, to provided habitat for the endangered red-cockaded woodpecker, the Forest Service began restoring parts of the Ouachita National Forest to the shortleaf pine-bluestem grass ecosystems that existed before European settlement.
SRS ecologists have been involved in a range of studies on the effects of restoration, including ongoing research on butterflies and their nectar resources. For the published study, researchers observed butterflies feeding on flowers during the migration period over four years, finding that they fed most frequently on tickseed sunflower (70 percent), with goldenrod, late boneset, and other asters also used as nectar resources.
Data from monarch counts and plant inventories on both restored and control sites showed that the restored sites supported a higher abundance of monarch butterflies during migration than unrestored, fire-suppressed controls. This pattern was most noticeable during the first October after a prescribed burn. The same pattern was observed in the abundance and diversity of the nectar resources. Within restored sites, nectar resources were more abundant the first year after prescribed burns, and least abundant the third year post-burn. In unrestored areas, nectar resources were found in abundance only in disturbed sites such as fence rows, utility rights-of-way, and road verges.
"Without frequent fire, nectar resources in the forests of the Ouachita Mountains are generally low," says Rudolph. "The large numbers of monarchs passing through the area each fall get most of their nectar from flowers in the small patches represented by disturbed sites. Though we can't really know how nectar resources were dispersed across the region's forests before pre-European settlement, our findings from restored sites suggest that fire suppression has limited the availability of these resources—and that using prescribed fire to restore these forests may be important to future monarch migrations."
The full text of the article is available at http://www.srs.fs.usda.gov/pubs/24947.