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Increases in mean air temperature are thought to improve productivity in northern forests but site-specific observations have shown that high temperatures can have deleterious effects. This study reconstructs leaf area index (LAI) time series for the Great Lakes–St. Lawrence forest region of Ontario using SPOT vegetation optical satellite data and the University of Toronto v2 LAI algorithm to study the impact of a spring heatwave in 2010. The results indicate a decline in 2010 LAI values as compared to prior normal years (2006–2009), coincident with the onset of high spring temperatures, which lasts throughout the growing season. Fitting a linear mixed model shows a significant link between high temperatures and LAI declines throughout the study area. These results provide the first evidence of sustained declines in LAI, and by proxy productivity, following high spring temperatures at the landscape scale.