Post - drought growth responses of western juniper ( Juniperus occidentalis var . occidentalis ) in central Oregon Paul

Increased atmospheric CO2 may affect the physiological response of natural trees to drought. We examined radial growth rates developed from five western juniper chronologies to determine if post-drought growth responses have changed. Using prior year October to current year June precipitation from 1896-1998, we identified drought recovery years as having standardized scores (z-scores) > 0 and preceded by a year with a z-score <-0.6. We defined our analysis by an early period, 1896-1930, when atmospheric CO2 concentrations were close to preindustrial evels, and a late period, 1964-1998, when concentrations were substantially higher. Mean growth index values of recovery years between early and late periods were significantly greater (p<0.05) for four of the five sites, and for all sites combined. These results are consistent with the drought-ameliorating effects of CO2 shown by controlled laboratory studies and suggest hat rising levels of atmospheric CO2 may impact western juniper growth rates.