The area was hard hit by a severe ice storm in January of 2009. The largest overstory trees came crashing down, and the remainder were stripped of all their branches. You can see a large canopy gap in the photo below.
The pondberry was thriving in ankle-deep water at the base of this behemouth blown-down tree. See the person in the background for scale.
The pondberry appears to do best with specific hydrology: it tolerates being innundated with water, but apparently doesn't want water that is too deep (you can see here that it is growing only on the edges of this deeper pond. The water levels are most likely lower than usual this year: southern MO has had a relatively dry winter/spring). In other areas of the preserve, it is growing in somewhat drier areas. Although the pondberry has responded well to the canopy opening created by the ice storm, it is being outcompeted in the drier locations by greenbriar, poison ivy and other shrubs and vines which have also benefitted from the greater light levels. Pondberry seems to thrive only where it is just wet enough to keep the competition down.