Urban wilderness areas, including remnant and derelict or vacant lands, offer important yet understudied opportunities for biodiversity conservation and carbon sequestration in cold-temperate cities. Understanding how plant diversity and structural attributes relate to aboveground carbon stock in these contrasting habitats is critical for evidence-based urban rewilding and climate mitigation. We compared plant community traits, species diversity, and aboveground carbon stock between Remnant and Protected Wilderness (RPW) and Derelict and Vacant Lot Wilderness (DVW) in Harbin, China, and examined biodiversity-carbon coupling patterns. Field surveys of woody and herbaceous vegetation were conducted in 21 sites, and plot-level aboveground carbon stocks were estimated using regionally derived species-specific allometric equations with sensitivity analyses. RPW plots showed significantly higher tree-layer diversity and aboveground carbon stock, averaging 63.3 Mg C ha(-1) compared to 39.0 Mg C ha(-1) in DVW plots, whereas DVW plots had greater aboveground carbon in the herbaceous layer. In both types, species richness in woody and herbaceous layers correlated positively with carbon density, indicating an overall synergy between biodiversity and aboveground carbon stock, but negative associations between woody diversity and herbaceous biomass revealed vertical trade-offs. These results show how biodiversity-carbon relationships shift with succession stage and land-use legacy, underscoring the need for differentiated management. Conservation of mature RPW sites should maintain ecological integrity, while DVW areas offer scope for passive rewilding and structural enhancement to improve carbon function and biodiversity outcomes. Management implications are discussed within a broader multi-service perspective (microclimate, stormwater, and social value), alongside potential trade-offs.