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Cellulosic biofuels offer environmental benefits not available from grain-based biofuels and are a cornerstone of efforts to meet transportation fuel needs in a future low-carbon economy, even with electrified vehicles and other advances. Bioenergy with carbon capture and storage (BECCS) is also key to almost all Intergovernmental Panel on Climate Change mitigation scenarios that constrain end-of-century atmospheric CO₂ to 450 parts per million. Some cellulosic feedstocks can come from industrial and agricultural by-products or from winter cover crops, but a substantial fraction must come from cellulosic biomass crops—perennial grasses and short-rotation trees planted for this purpose. Land requirements, however, are substantial and raise crucial questions about the environmental sustainability of a future bioenergy economy. First, if planted on existing croplands, will biofuel crops increase …