An innovative technique utilizes a tree-ring marker to investigate long-term changes in the frequency of cool summers in Interior North America (INA), a region that currently suffers important gaps in knowledge concerning annual to secular temperature changes. Using multivariate adaptive regression splines (MARS), we established a threshold for the formation of climatically-induced light rings recorded in Pinus banksiana trees from INA. Then, we used the MARS model to reconstruct negative departures in summer maximum temperatures (April-September) from 1717 to 2007. The estimates explain 45 of the variance in instrumental temperature data. The reconstruction indicates the presence of significant multidecadal changes in the frequency of cool summers, with maximums in 1780, 1900 and 1960 and minimums in 1740, 1860, 1920 and 2000. No evidence of secular changes was found.
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