With advances in wood product development and building code acceptance, mass timber structural systems have become viable alternatives to steel and concrete structural systems (Post 2015). These mass timber systems have environmental benefits, such as carbon sequestration ability and lower greenhouse gas emissions than steel and concrete systems. How can mass timber materials such as cross-laminated timber (CLT) reduce the environmental impacts of buildings, and how certain is this reduction? In order to truly answer this question, environmental impact assessments of CLT and other wood materials must first address variation and uncertainty in forest management and biogenic carbon accounting.