A dendroecological approach to growth dynamics of Mediterranean forests in Southwestern Spainclimate change impacts, vulnerability and adaptive capacity

Resumen

Tree-ring chronologies were studied using samples of dead holm oaks (Q. ilex ssp. ballota [Desf.] Samp.) in declining open-woodlands in southwestern Spain and samples of stone pine (Pinus pinea L.) from forests along a latitudinal gradient in Spain, Portugal and Italy. Growth trends and climate change-related chronology signal in Q. ilex were analyzed to assess the role of climate change in the tree mortality process. Growth responses of P. pinea to spatiotemporal variability of climate were examined to gain knowledge about the species’ adaptive capacity. Different techniques to extract the climatic signal from the tree-ring series were applied according to standard dendroclimatological approaches. A cubic spline with a period of 30 years was the best approach to remove the non-climatic variance related to silviculture-induced growth releases. The climate variability was obtained from meteorological station records and gridded climate datasets from international databases. There are common Mediterranean macroclimatic features, but the distribution of precipitation over the year and the duration and intensity of meteorological droughts vary among sites. Temperatures and aridity increased since the 1970s in all sites. The most significant growth-climate correlations were found with precipitation and drought indices in all sites, indicating that water availability is the main limiting factor for growth. Growth reductions in Q. ilex occurred during the intense and prolonged droughts of the recent decades, and temporal changes of growth-climate correlations indicates trees’ sensitivity to the increasing aridity. Climate change played an important role as stress factor determining the inception of mortality processes, and probably aggravated the impacts of other mortality factors. This is the first evidence of the direct effects of climate change on the current widespread stand decline and increase of mortality in oak open-woodlands in the southwestern Iberian Peninsula. Growth-climate correlations patterns in P. pinea varied over time and space, indicating the species’ growth adaptation to site-specific climatic conditions and suggesting physiological adjustments in response to climatic variations. The plasticity of this species suggests that different populations have different capacities for acclimation to climate change, and this will probably influence future vegetation dynamics. This is the first assessment of tree species’ plasticity in Iberian Mediterranean forests from a dendrochronological perspective. The vulnerability to drought of southern Iberian forests should be considered when making decisions for adaptive management. This thesis presents the first tree-ring dataset in southwestern Spain and a novel dendroecological approach for studying climate-related forest growth dynamics in this region.