Previous work indicates that an evergreen shrub layer dominated by Rhododendron maximum L. and/or Kalmia latifolia L. is a critical component of Appalachian forests, potentially exerting control over forest structure and succession. Moreover, communities characterized by thickets of evergreen understory shrubs exhibit distinct patterning on the landscape associated with topographic gradients and disturbance. We compare distributions of R. maximum and K. latifolia in the warmer and drier Ridge and Valley physiographic province and on the cooler and wetter Allegheny Plateau of western Maryland and adjacent Pennsylvania and find that the relationships between community composition and the environment differ between these two physiographic provinces. Kalmia latifolia dominance is strongly associated with topographically dry locations and a history of gypsy moth defoliation, while R. maximum is associated with topographically wet positions in the Allegheny Plateau, but less so in the Ridge and Valley. ANOVA identified significant differences in midstory volume and basal area and leaf area index of canopy tree species where evergreen understory shrub species were prominent. Specifically, basal area and leaf area were lower above K. latifolia understory communities, and midstory volume was found to be lower in areas where K. latifolia and R. maximum were abundant. However, the results show that cover of K. latifolia must be sufficiently continuous (i.e., not patchy) for the midstory volume to be affected. In plots with a history of gypsy moth infestation, basal area was found to be significantly lower in plots where K. latifolia thickets were present than where it was absent, suggesting that K. latifolia impedes structural recovery from this disturbance.