Glossy buckthorn (Rhamnus frangula L. or Frangula alnus Mill.) is a tall shrub to small tree, native to Eurasia and North Africa, which can invade both wetland and upland sites in northeastern and midwestern North America. Invasion by R. frangula is often patchy in riparian savannas of the Allegheny High Plateau of northwestern Pennsylvania: Some sites are heavily invaded, whereas others nearby are sparsely invaded or not invaded at all.
Because microtopography in riparian savannas can be diverse, with slightly elevated dry hummocks and wet depressions providing habitats for a range of upland and wetland plant species, we tested the hypothesis that the patchy nature of R. frangula invasion of riparian savannas is determined in part by soil drainage, which may ultimately affect shrub establishment and growth. We used depth to mottling by iron oxides as a primary, indirect measure of soil drainage and inundation and also measured the depth of leaf litter, humus, and A and B horizons in belt transects that were either heavily or sparsely to noninvaded by R. frangula at three riparian savanna study sites in the Allegheny National Forest. Sites with relatively well-drained soils, indicated by greater mottling and B horizon depths, supported significantly greater densities of R. frangula seedlings and shrubs than did sites with apparently wetter soils having shallower mottling and B horizon depths. We suggest that physiological stress associated with wet soils may restrict establishment of R. frangula, explaining in part its patchy invasion patterns in Allegheny High Plateau riparian savannas.