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

Abstract

© 2015 John Wiley & Sons Ltd. 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.We thank the referees for improving a previous version of the manuscript. We are particularly indebted to all the support provided by the people from the “Laboratorio de Sanidad Forestal” (Mora de Rubielos, Aragón Government), particularly Rodolfo Hernández, Juan Manuel Gil, Miguel Ros, Víctor Pérez Fortea and Araceli Órtiz. We also thank Melissa Hoffer and Justin Waito for their help with laboratory work. Antonio Gazol is supported by a Postdoctoral grant from MINECO (Contrato Formacion Postdoctoral MINECO – FPDI 2013–16600). This study was supported by projects CGL2011–26654 (Spanish Ministry of Economy and Competitiveness), 387/2011 and 1032S/2013 (Spanish Ministry of Agriculture and Environment, OAPN, Spain). The research was also partly funded by the Canada Research Chair Program and a NSERC discovery grant to J.C. Tardif. We are also indebted to all people who helped us sampling in the field. We thank AEMET, CRU and ESRL-NOAA for providing climatic and CO2 data.Peer Reviewe