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Urban natural area forests provide cities with crucial ecosystem services, including carbon storage and sequestration. Although previous work from a single city has suggested that urban natural area forests may be carbon‐rich and dominated by native species, it is unclear if that pattern is widespread. Indeed, little is known about the species composition and carbon storage in these green spaces, or how urban natural area forests compared with similar rural forests. Here, we use data collected by the Forests in Cities network to quantify carbon stored in urban natural area forests across 1852 plots in eight of the most populous US cities and examine patterns in woody vegetation carbon storage by tree species composition and size class. We also compare the carbon storage in these urban natural area forests to similar rural forests using USDA FS Forest Inventory and Analysis data. We found that urban natural area forests store between 214 and 267 Mg C ha−1, with the majority (on average 55%) in the above‐ground portion of live trees and shrubs. This carbon was primarily stored by native trees and increased with the proportion of native species, as native trees tended to be larger. The proportion of non‐native species was higher in smaller size classes. Overall, the woody vegetation carbon density in urban natural area forests was higher than similar forest types in rural settings, a phenomenon driven by the presence of more large trees. Synthesis and applications: Together these results demonstrate the importance of urban natural area forests as carbon‐dense ecosystems and havens for native tree species. However, because of the greater number of non‐native species in smaller size classes, these forests may also be at risk of transitioning away from native‐dominated systems. Therefore, greater monitoring efforts and increased management activities such as non‐native species removal will be crucial to maintaining the high carbon storage and health of these ecosystems. [ABSTRACT FROM AUTHOR]