Use of atmospheric CO2-sensitive trees may influence dendroclimatic reconstructions

We examined recent radial growth increases in western juniper trees using an 11‐site chronology dating from AD 1000–2006. By various measures, radial growth during the late 20th/early 21st centuries was exceptional, with increases occurring absent of regional climatic change. We found that 54% of annual radial growth variability was explained by June Palmer Drought Severity Index (PDSI) values, but the inclusion of atmospheric CO2 values accounted for a 14% increase in explanatory power. We reconstructed June PDSI both including and excluding CO2, and found that PDSI values were overestimated at the end of the record with CO2 omitted from the model. We conclude that: 1) western juniper radial growth was associated with rising CO2 during the late 20th/early 21st centuries; and, 2) the use of CO2‐sensitive trees such as western juniper for dendroclimatic reconstructions may influence the results if the impacts of CO2 fertilization are omitted.