Abstract Trees adjust their architecture to acclimate to various external stressors, which regulates ecological functions that are needed for growth, reproduction, and survival. Human activities, however, are fragmenting natural habitats apace and could affect tree architecture and allometry, but quantitative assessments remain lacking. Here, we leverage ground surveys of terrestrial LiDAR in Central Amazonia to comprehensively assess forest edge effects on tree architecture and allometry, and their associated impacts on the forest biomass 40 years after fragmentation. We found that young trees colonising the forest fragments have thicker branches and architectural traits that maximise light capture, and can produce 50% more wood than their counterparts of similar stem size and height in interior forests. Large trees that have survived disturbances arising from forest fragmentation are able to acclimate and maintain their wood production, but damages that reduce tree height near the edges can lead to a 30% decline of their woody volume. Despite the large wood production of colonising trees, changes in tree architecture lead to a net loss of 6.6 Mg ha-1 of the forest aboveground biomass, which account for 20% of all edge-related aboveground biomass losses of fragmented Amazonian forests (34.3 Mg ha-1). Our findings show a strong influence of edge effects on tree architecture and allometry, and reveal an additional unaccounted factor that exacerbates carbon losses in fragmented forests.