Radial growth rate increases in naturally occurring ponderosa pine trees: a late-20th century CO

• The primary objective of this study was to determine if gradually increasing levels of atmospheric CO 2 , as opposed to ‘step’ increases commonly employed in controlled studies, have a positive impact on radial growth rates of ponderosa pine ( Pinus ponderosa ) in natural environments, and to determine the spatial extent and variability of this growth enhancement. • We developed a series of tree-ring chronologies from minimally disturbed sites across a spectrum of environmental conditions. A series of difference of means tests were used to compare radial growth post-1950, when the impacts of rising atmospheric CO 2 are best expressed, with that pre-1950. Spearman’s correlation was used to relate site stress to growth-rate changes. • Significant increases in radial growth rates occurred post-1950, especially during drought years, with the greatest increases generally found at the most water-limited sites. Site harshness is positively related to enhanced radial growth rates. • Atmospheric CO 2 fertilization is probably operative, having a positive effect on radial growth rates of ponderosa pine through increasing water-use efficiency. A CO 2 -driven growth enhancement may affect ponderosa pine growing under both natural and controlled conditions.