Direct monitoring of air quality does not cover more than the last three decades in most industrialized countries. For that reason studies using growth-ring carbon isotopes (δ13C) of several species of trees have recently investigated isotopic responses in the contexts of stationary and diffuse pollution in humid continental conditions. Here, the growth-ring δ13C series (1880–2009) of spruce trees living in sub-humid subarctic conditions were measured to assess if they represent indicators for air quality changes near oil sands (OS) developments initiated in northeastern Alberta in 1967. The measured δ13C pre-operation rings at two forest sites were analyzed along local climatic conditions to develop response-to-climate statistical models and predict the natural isotopic behaviour for the most recent part of the ring series. The measured trends and climate-modelled (natural) δ13C values strongly depart during the operation period, depicting anomalies which can be nicely reproduced by multiple regression models combining climate and a proxy for OS airborne emissions. This research allows envisioning the use of carbon dendroisotopic indicators to compensate for the lack of long-term air quality measurement, and monitor environmental conditions in the sub-humid terrestrial ecosystem exposed to emissions from oil sands operations which are predicted to increase in the future.