Background/Question/Methods The relative abundance of carbon isotopes in plant leaves is a function of discrimination against 13C during photosynthesis and is related to plant water use efficiency due to the effects of stomata on both CO2 supply and water loss. Stomatal function and photosynthetic 13C discrimination can be influenced by a variety of factors including leaf habit, phylogeny, life history, and water availability. However, it is unclear to what extent these factors, which are well studied at local and regional spatial scales, have consistent effects across different ecosystem types or contribute to patterns at the global scale. To evaluate large-scale patterns in leaf C isotope variability, we assembled a database of woody plant leaf C isotope values, representing nearly 300 species from more than 80 field sites and 7 biomes. Results/Conclusions When environmental factors are constrained by limiting comparisons between woody plant functional groups to sites where each group is represented, consistent differences among functional groups emerge: 1) angiosperms have higher leaf 13C discrimination than gymnosperms, and 2) deciduous species have higher leaf 13C discrimination than evergreen species. Our results suggest that these differences are largely consistent across a variety of ecosystem types with unique community composition and provide support for adding leaf 13C discrimination and water use efficiency to a growing list of traits that differ among woody plant functional groups. Our data also show significant differences in average leaf 13C discrimination among biomes, with greatest discrimination occurring in tropical rainforests and lowest discrimination in xeric woodlands and shrublands of the temperate zone. These differences hold even when controlling for phylogeny and leaf habit, and may be driven by variation in water availability across biomes. Positive linear relationships between leaf 13C discrimination and mean annual precipitation were found across all biomes, as well as within distinct latitudinal zones (e.g. temperate vs. tropical) and within each woody plant functional group.