Long tree-ring chronologies reveal 20th century increases in water-use efficiency but no enhancement of tree growth at five Iberian pine forests

We investigated the tree growth and physiological response of five pine forest stands in relation to changes in atmospheric CO2 concentration (c(a)) and climate in the Iberian Peninsula using annually resolved width and delta 13C tree-ring chronologies since ad 1600. 13C discrimination (delta approximate to c(i)/c(a)), leaf intercellular CO2 concentration (c(i)) and intrinsic water-use efficiency (iWUE) were inferred from delta 13C values. The most pronounced changes were observed during the second half of the 20th century, and differed between stands. Three sites kept a constant c(i)/c(a) ratio, leading to significant c(i) and iWUE increases (active response to c(a)); whereas a significant increase in c(i)/c(a) resulted in the lowest iWUE increase of all stands at a relict Pinus uncinata forest site (passive response to c(a)). A significant decrease in c(i)/c(a) led to the greatest iWUE improvement at the northwestern site. We tested the climatic signal strength registered in the delta 13C series after removing the low-frequency trends due to the physiological responses to increasing c(a). We found stronger correlations with temperature during the growing season, demonstrating that the physiological response to c(a) changes modulated delta 13C and masked the climate signal. Since 1970 higher delta 13C values revealed iWUE improvements at all the sites exceeding values expected by an active response to the c(a) increase alone. These patterns were related to upward trends in temperatures, indicating that other factors are reinforcing stomatal closure in these forests. Narrower rings during the second half of the 20th century than in previous centuries were observed at four sites and after 1970 at all sites, providing no evidence for a possible CO2 'fertilization' effect on growth. The iWUE improvements found for all the forests, reflecting both a c(a) rise and warmer conditions, seem to be insufficient to compensate for the negative effects of the increasing water limitation on growth.