Increasing water‐use efficiency and age‐specific growth responses of old‐growth ponderosa pine trees in the Northern Rockies

We examined radial growth responses of ponderosa pine ( Pinus ponderosa var. ponderosa ) between 1905–1954 and 1955–2004 to determine if the effects of increased intrinsic water‐use efficiencies (iWUE) caused by elevated atmospheric CO 2 concentrations were age‐specific. We collected 209 cores from five sites in the Northern Rockies and calculated iWUE using carbon isotope data from 1850 to 2004. Standardized radial growth responses were age dependent, with older trees exhibiting significantly higher values than younger trees during the later period at four sites and all sites combined. No significant differences in radial growth existed either for the individual sites or combined site during the earlier period. Increases in iWUE during 1955–2004 were 11% greater than during 1905–1954, and pentadal fluctuations in iWUE were significantly correlated with the radial growth of older trees from 1850 to 2004. Radial growth of younger trees and iWUE were not significantly correlated. Our results suggest that: (1) responses to elevated atmospheric CO 2 in old‐growth ponderosa forests are age‐specific; (2) radial growth increases in older trees coincided with increased iWUE; (3) ponderosa had increased growth rates in their third, fourth, and fifth centuries of life; and (4) age‐specific growth responses during 1955–2004 are unique since at least the mid‐16th century.