Abstract
Global change in the Anthropocene contributed to rising atmospheric CO2 concentrations (Ca), warming temperatures, and changing precipitation patterns. Related to this, the frequency and intensity of drought events increased in the last decades. This impacts on species and ecosystems, including carbon allocations in trees and forests. More studies are still needed to enhance our understanding of how species- and site-specific tree-climate relations are influenced by the structural composition of the forest along eco-climatic gradients. The combined analysis of tree ring width and stable carbon isotopes (δ 13C) has shown to be a viable approach for studying forest responses to longterm climatic and environmental stress. However, the labor- and time-intensive cellulose extraction procedure limits large-scale tree ring research using δ 13C measurements. Studies of this thesis are composed of three research article manuscripts (Chapters III, IV and V) which are aimed at investigating Pinus sylvestris L. response to drought and inter-annual and local climatic variations in pure and mixed forest stands. Chapter II examines the substitute use of bulk wood instead of cellulose extraction to measure δ 13C in tree rings. Chapter III and Chapter IV aimed at understanding the intrinsic functional traits and radial growth responses of Pinus sylvestris L. to long-term climate change, respectively. The studies were conducted on pure and mixed-species P. sylvestris forest stands at dry Mediterranean, sub-Boreal, and Temperate climate sites in Spain, Italy and Poland, respectively. The occurrence and severity trends of long-term droughts were examined using the standard precipitation evaporation index. Combined analysis of tree-ring width (RW) and δ 13C measurements was employed to reconstruct annually resolved radial growth and tree intrinsic physiological response variables, such us of δ 13C discrimination (Δ13C) and intrinsic water-use efficiency (iWUE). Trees in pure and mixed forests were compared to assess how forest structural composition (species mixing) influences P. sylvestris' response to climate. Trees in pure forests were used to assess how trees' response to climate varies across study sites along a water availability gradient. The results showed that in P. sylvestris, δ13C proxies measured in bulk wood and cellulose are strongly correlated (R2=91- 98%) with comparable sensitivity to climate variables and water availability. iWUE increased markedly over time in all the sites. The dry Mediterranean site exhibited the lowest Δ13C and highest iWUE compared to the sub-Boreal and Temperate sites. Summer precipitation was the principal controlling factor for P. sylvestris growth at the Mediterranean and sub-Boreal climate sites. Radial growth was aided by warmer early-season temperatures in the Mediterranean and Temperate climate sites. This shows that the tree radial growth has a divergent response pattern to the raised iWUE as a function of 7 species composition of forest stand and water availability across ecological gradients. The negative association between RWI and iWUE in a dryer climate suggests a risk of tree growth decline under water stress conditions. Overall, this research significantly contributes to our insightful understanding of how tree radial growth reacts to elevated CO2 levels and severe drought, under contrasting forest species composition and across a water availability gradient. Growing P. sylvestris in admixture with Quercus might help forest managers reduce radial growth loss under drought stress in water-stressed environments like the Mediterranean region.