Background/Question/Methods The U.S. National Vegetation Classification (USNVC) has recently been completely revised through the Federal Geographic Data Committee (FGDC) Vegetation Subcommittee. That committee, representing  a partnership between federal agencies, the Ecological Society of America’s Vegetation Panel, NatureServe, and other organizations worked together with vegetation scientists from Canada, Mexico and South America in the development of this current standard. The new USNVC is based on a substantial revision of the 1997 hierarchy, which relied heavily on physiognomy at all levels above the alliance.  The revised USNVC treats cultural and  natural vegetation separately, allowing for an independent set of levels for cultural vegetation. This separation allows the USNVC to better address unique needs of both the natural and the cultural categories.
The new 8-level natural vegetation hierarchy emphasizes physiognomy in an ecological context at three upper levels (Formation Class, Formation Subclass, and Formation), and increasingly integrates biogeography and floristics at three middle levels (Division, Macrogroup, and Group). The new middle levels bridge the large conceptual gap between alliance and formation, an important improvement over the 1997 hierarchy. The upper levels of the USNVC hierarchy are based on dominant and diagnostic growth forms that reflect environment at global to continental scales.  The mid-levels are based on dominant and diagnostic growth forms and compositional similarity reflecting biogeography and continental to regional environmental factors. The lower levels (alliance and association) are based on diagnostic and/or dominant species and compositional similarity reflecting local to regional environmental factors.

Results/Conclusions

The revised USNVC breaks new ground on how to approach vegetation classification of existing vegetation. Floristic and physiognomic criteria are still the primary properties of natural vegetation used to define all units of the classification. However, how these criteria are applied in classification depends on the ecological and biogeographic context. The new hierarchy integrates the physiognomic and floristic levels based on ecologic vegetation patterns, rather than developing the physiognomic and floristic levels independently and then forcing them into a hierarchy. Implementation of the revised USNVC will facilitate information transfer between federal agencies and nonfederal partners, inform new approaches to regional and national vegetation mapping, and better inform the application of research findings to natural resource management. A working draft of the revised Hierarchy has been released for the entire U.S. and is available for review.