The protective role of roadside forest belts: Linking canopy health to carbon assimilation

Roadside vegetation strips are critical ecotones that provide essential ecosystem services, including carbon sequestration, while being constantly exposed to anthropogenic stress from traffic emissions. This study assesses the ecological state and carbon assimilation capacity of roadside plant communities along three classes of motorways (Class 1–3, total length 684.4 km) in the Penza region, Russia, based on field surveys and remote sensing data from 2024. We found significant non-compliance with standard roadside widths, with the actual width of Class 1 highway strips reaching only 57.5% of the required value. This spatial deficit is linked to visible biotic stress responses: canopy closure in forest belts follows a second-order parabolic function of belt width, reaching levels typical of regional forests (70–80%) only when the belt width exceeds 30 meters. Carbon assimilation by forest vegetation is best described by a logistic growth curve, reflecting biological productivity limits under varying width and density conditions. Herbaceous communities exhibited an average carbon sequestration rate of 2.81 t/ha, with biomass dynamics perpendicular to the road described by a third-order parabola, indicating a gradient of stress and recovery. The compensatory capacity of existing strips for traffic carbon emissions ranged from only 11.9% on heavily used Class 1 highways to 57.3% on Class 3 roads. Our integrated mathematical model, based on biological response curves, reveals an optimal forest belt width of 35 meters, beyond which marginal gains in assimilation plateau. Using this model, we derived a practical parabolic function linking the required grassy strip width to traffic intensity (up to 5000 vehicles/day) to achieve carbon neutrality. This study provides a quantitative framework for understanding how anthropogenic pressure shapes roadside plant communities and offers biologically sound guidelines for designing resilient green infrastructure