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The carbon isotopic compositions of leaves and stems of woody legumes growing in coastal mediterranean and inland desert sites in California were compared. The overall goal was to determine what factors were most associated with the carbon isotope composition of photosynthetic stems in these habitats. The carbon isotope signature (delta(13)C) of photosynthetic stems was less negative than that of leaves on the same plants by an average of 1.51 +/- 0.42 parts per thousand. The delta(13)C of bark (cortical chlorenchyma and epidermis) was more negative than that of wood (vascular tissue and pith) from the same plant for all species studied on all dates. Desert woody legumes had a higher delta(13)C (less negative) and a lower intercellular CO2 concentration (C-i) (for both photosynthetic tissues) than that of woody legumes from mediterranean climate sites. Differences in the delta(13)C of stems among sites could be entirely accounted for by differences among site air temperatures. Thus, the delta(13)C composition of stems did not indicate a difference in whole-plant integrated water use efficiency (WUE) among sites. Zn contrast, stems on all plants had a lower stem C-i and a higher delta(13)C than leaves on the same plant, indicating that photosynthetic stems improve long-term, whole-plant water use efficiency in a diversity of species.