Aim Forest responses to global‐change drivers such as rising atmospheric CO2 concentrations (Ca), warming temperatures and increased aridification will depend on tree species and site characteristics. We aim to determine if rising Ca enhances growth of coexisting pine species along broad ecological gradients in a drought‐prone area. Location Iberian Range, Spain. Methods We sampled 557 trees of five pine species encompassing a wide climatic gradient and measured their radial growth. We used nonlinear flexible statistics (generalized additive mixed models) to characterize growth trends and relate them to Ca, temperature and water balance. Results The sites most responsive to the growing‐season water balance were dominated by Pinus pinaster and Pinus nigra at low elevations, whereas those most responsive to temperatures were high‐elevation Pinus sylvestris and Pinus uncinata stands. From 1950 onwards, most sites and species showed decreasing radial growth trends. Growth trends were coherent with a CO2‐related fertilization effect only in one P. sylvestris site. Main conclusions We found little evidence of growth stimulation of Iberian pine forests due to rising Ca. The results indicated that any positive effect of a Ca‐induced growth increase was unlikely to reverse or cancel out the drought‐driven trends of reduced growth in most Mediterranean pine forests. Further assessments of CO2‐fertilization effects on forest growth should be carried out in sites where climatic stressors such as drought do not override the effects of rising Ca on forest growth.

Little is known about environmental controls on vessel features in ring-porous tree species. Our objectives were to assess (i) the association between tree-ring descriptors (vessels and width) and climate in two oak species, white oak, Quercus alba L., and red oak, Quercus rubra L., and (ii) the utility of vessel series in climate reconstruction. The study was conducted in southern Quebec and 10 trees of each species were analyzed. For each species, 11 chronologies (vessel and ring width) were developed and compared. Few differences were observed between the oak species. All vessel chronologies were associated with those of ring dimension and none revealed a unique climate signal. Current growing season conditions were mainly associated with latewood features, whereas those of the year prior to ring formation were mainly associated with earlywood features. The best climate variable to reconstruct was the July Canadian Drought Code and the best reconstruction model was derived from earlywood, latewood, and ring-width chronologies. We conclude that vessel chronologies for Q. alba and Q. rubra have limited use in dendroclimatology. Vessel features are best used to identify event years recorded during the life of a tree. Vessel series could prove useful, however, in calibrating physiologically based models of tree growth.

[1] The CO 2 fertilization hypothesis stipulates that rising atmospheric CO 2 has a direct positive effect on net primary productivity (NPP), with experimental evidence suggesting a 23% growth enhancement with a doubling of CO 2 . Here, we test this hypothesis by comparing a bioclimatic model simulation of NPP over the twentieth century against tree growth increment (TGI) data of 192 Pinus banksiana trees from the Duck Mountain Provincial Forest in Manitoba, Canada. We postulate that, if a CO 2 fertilization effect has occurred, climatically driven simulations of NPP and TGI will diverge with increasing CO 2 . We use a two-level scaling approach to simulate NPP. A leaf-level model is first used to simulate high-frequency responses to climate variability. A canopy-level model of NPP is then adjusted to the aggregated leaf-level results and used to simulate yearly plot-level NPP. Neither model accounts for CO 2 fertilization. The climatically driven simulations of NPP for 1912-2000 are effective for tracking the measured year-to-year variations in TGI, with 47.2% of the variance in TGI reproduced by the simulation. In addition, the simulation reproduces without divergence the positive linear trend detected in TGI over the same period. Our results therefore do not support the attribution of a portion of the historical linear trend in TGI to CO 2 fertilization at the level suggested by current experimental evidence. A sensitivity analysis done by adding an expected CO 2 fertilization effect to simulations suggests that the detection limit of the study is for a 14% growth increment with a doubling of atmospheric CO 2 concentration.

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