Trees outside forests and their carbon dynamics over Maryland, U.S.A

Carbon monitoring is needed to track the exchange of carbon between the land and atmosphere. However, comprehensive monitoring of terrestrial ecosystems is challenging, partially because of the fine-scale spatial heterogeneity of land cover and vegetation structure. While forest ecosystems are relatively well studied, knowledge of trees outside forests (TOFs) is limited, even though TOF contribute substantially to many ecosystem services. In this study, we leveraged advances in remote sensing and modeling to quantify TOF and their contribution to the 2011–2023 Maryland state carbon budget. Specifically, the Ecosystem Demography model was initialized with 1 m airborne optical and LiDAR data. These datasets were combined with 30 m National Land Cover Database maps to identify and monitor TOF statewide. In the base year, TOF contributed to 22.86% (2795.59 km ^2 ) of the total tree cover, 14.70% (15.98 Tg C) of the total aboveground live tree carbon stocks, and 22.56% (0.39 Tg C yr ^−1 ) of the corresponding carbon fluxes in the state, with variation by land cover class. TOF on developed and planted/cultivated lands contributed approximately 90% of the total TOF tree cover and carbon stocks, as well as over 96% of the total TOF carbon fluxes. Most TOF occurred as polygons smaller than 1 km ^2 and were within 600 m distance from forests. From 2011–2023, persistent TOF carbon stocks were estimated to increase by 5.15 Tg C, representing 23.71% of the state’s total increase in tree carbon stocks over the interval, with substantial interannual variability in carbon fluxes. These results suggest TOF are an important contribution to statewide tree cover and produce carbon stocks and fluxes that should be highlighted in carbon budgets for effective monitoring and assessment. Moving forward, efforts are needed to expand the availability and application of high-resolution remote sensing data and models needed for TOF quantification in carbon budgets.