Background/Question/Methods The Maryland Conowingo Barrens (CB) forests and woodlands are located on a belt of variably deformed mafic-ultramafic to felsic rocks near the Susquehanna River with soils that vary in depth and degree of development. Ultramafic substrates are known for their profound effects on vegetation due to chemical (low essential nutrients, and toxic amounts of magnesium and nickel) and physical (thin, droughty, low organic matter content) stresses. On the other hand, soils formed on more compositionally evolved mafic substrates are likely to be different in texture and contain more exchangeable nutrients compared to soils that develop from felsic or ultramafic parent materials. Our study addresses the relationship between geodiversity and biodiversity in these woodlands, which serve as an excellent test area for geoedaphic/plant variation along a lithosequence. Results/Conclusions The “Serpentine Effect” associated with ultramafic rocks is well documented in areas with sharp contrasting zones, yet flora diversity studies in lithologic transitional areas are underrepresented. The CB provides such a gradational boundary to explore biogeodiversity changes. Edaphic factors contributing to forest tree community structure were investigated using 30 randomly stratified plots (25 x 50 m) with similar aspect and elevation oriented along the perceived environmental gradient to capture tree species diversity. In each plot we collected soils and rock parent material for physiochemical analysis. Soil pH ranged from 3.94 to 7.21. Ca: Mg ratio of all ultramafics is low (mean of 0.12 ± 0.05) but is greater than unity for more mafic parent materials. Soils on ultramafic sites were coarse textured (>40% sand and < 10% clay) while soils on mafic or felsic sites were more clay dominated (>30%). We focused on a 2.2 km sequence of plots across the geologically imposed environmental gradient to evaluate tree population and community response. Overall species richness or γ diversity = 48, whereas α diversity per each 1250 m^2 plot varied from 5 to 25. Twenty-seven species were amphitolerant while the others were more or less selective such as the sclerophyllous Quercus marilandica and Quercus stellata on ultramafics and Ulmus americana and Tilia americana on more evolved substrates. Pair wise turnover between adjacent plots yielded βw diversity (Whittaker) calculations ranging from .2 to a maximum βw diversity of .7. The largest turnover or βw diversity occurred at geologically abrupt areas with moderated βw in areas with gradational lithosequences. The data is consistent with physiognomy and species composition being a function of geoedaphic compositional traits.