Geographically structured and temporally unstable growth responses of Juniperus thurifera to recent climate variability in the Iberian Peninsula

Geographically structured tree-ring networks are needed to fully understand the spatiotemporal variability in climatic sensitiveness of trees and to study their future responses to global warming. We aim to identify the spatially constrained structure of radial-growth patterns of the Spanish juniper (Juniperus thurifera L.) and to assess whether their climate–growth responses were unstable during the late twentieth century. Tree–ring width chronologies were built for 13?J. thurifera stands in Spain using dendrochronological methods and related to monthly climatic data. Sites were grouped according to their growth patterns using hierarchical cluster analysis. The relationships among geographical, climatic and stand features and their influence on radial growth were evaluated using redundancy analysis. The climate–growth relationships and their temporal stability were assessed using Pearson’s and moving bootstrapped correlations, respectively. Stands formed three geographical groups according to their high-frequency growth variation: North West and Centre, North East and South East. We found that J. thurifera radial-growth patterns depended on geographical and climatic factors, but not on the stand structure, and responded to a northwest–southeast gradient of decreasing rainfall and influence of Atlantic Westerlies and Mediterranean cyclonic activity. The positive response to June precipitation was unstable during the late twentieth century and started earlier in populations from western mesic sites than in eastern xeric sites. This pattern may be related to either decreasing water availability in western than in eastern sites or the resilience of J. thurifera growth from xeric sites in response to the increasing summer aridity.