Trees present a vast array of performance in density, growth speed, height, fibre strength, and rates of carbon absorption. This paper, presented at SBE Berlin in 2022, explores how biodiverse mass timber is a way to support forest health and improve the health conditions of the most vulnerable—those who have experienced the pollution of mineral-based materials for generations. This symbiotic relationship between forests and urban environments incentivizes better care for both spaces, the transfer of materials should be reciprocal. Fringe Timber looks at the forest surrounding a city to inform its scale of growth and reduce its carbon footprint. Using three case studies in New York City, Denver, and Tulsa, this paper posits city-specific, species-driven supply chains beyond softwoods for mass timber, including hardwoods and hybrids, to support the appropriate scale and speed for urban development.