Global impacts of transportation infrastructure on forest degradation and loss

Transportation networks threaten global forests, but prior assessments have been regional or limited to single metrics (e.g., forest cover). Here, we present a global analysis of multidimensional road effects on forests, using high-resolution remote sensing data and a Grid-wise Environmental Matching for Background Reference (GEM-BR) strategy. We detect 18.6% lower forest cover, 2.7 m shorter canopy height, 52.2 gC m^-2 yr^-1 reduced net primary productivity, and 23.0 patches per km² higher fragmentation within 1 km of roads compared to reference areas. Impacts extend up to 5 km with a clear distance decay effect, totaling 4.26 million km² of forest loss-equivalent to 10.7% of the 2020 global forest extent. The Global South (tropics accounting for 54.8%) faces severe, worsening degradation (2000-2020), while the Global North shows milder impacts, with partial recovery. Critically, 89% of grid cells exhibit conflicting long-term trends across metrics, highlighting the inadequacy of cover-only assessments. We further find that road-linked degradation is tightly coupled with local human activity, and that global protected areas have insufficient capacity to curb ongoing degradation. Differences in impacts among regions suggest that road-linked forest degradation is tied to governance choices-urging integrated transport-forest planning to balance development and conservation.