In southwestern Oregon and northern California, Jeffrey pine (Pinus jeffreyi) savannas and Darlingtonia fens are part of a mosaic of serpentine soil-related plant communities separated by distinct ecotones. I characterized and quantified the biological and physical aspects of these transition zones. Changes in vegetation were distinguished across communities and across three sites, each a different time since fire, and water discontinuities appeared important for preserving these uniquely-adapted systems, while vegetation undergoes postfire succession from fen to shrub to savanna after approximately 100 years since fire. While many factors play a role in determining the composition and location of community boundaries, the role of water appears to be the most critical factor determining the structure and species composition of the fen and shrub communities in southwestern Oregon.