Attributing forest responses to global-change drivers: limited evidence of a CO2-fertilization effect in Iberian pine growth

AimForest responses to global-change drivers such as rising atmospheric CO2 concentrations (C-a), warming temperatures and increased aridification will depend on tree species and site characteristics. We aim to determine if rising C-a enhances growth of coexisting pine species along broad ecological gradients in a drought-prone area. LocationIberian Range, Spain. MethodsWe 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 C-a, temperature and water balance. ResultsThe 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 conclusionsWe found little evidence of growth stimulation of Iberian pine forests due to rising C-a. The results indicated that any positive effect of a C-a-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 C-a on forest growth.