Wood anatomical traits responses to climate in Scots pine at the leading edge of its natural distribution

Global warming is occurring at an accelerated pace in many high-latitude regions around the world due to the phenomenon of Arctic amplification, a positive feedback effect that stimulates further climate warming. Forests will be directly affected by the changes in climatic conditions and indirectly by changes in disturbance regimes. Therefore, tree species will likely shift their range to follow climate change, moving northward; while at lower latitudes there is high chance of loss of actual species and the possibility of immigration for warmth-requiring ones. Climate sensitivity of populations at the edge of their range is of key importance for understanding species responses to future warming conditions. Therefore, the present study examines which factors influence the growth of Pinus sylvestris L. (Scots pine) in Alta (Norway), at the northern edge of its natural distribution. By investigating the xylem anatomical structure of Scots pine, I aimed to retrospectively infer how climate influences the processes of cell enlargement and wall thickening, combining the methodological approach of dendrochronology and quantitative wood anatomy. Chronologies of mean ring width (MRW), cell number (CNo), mean lumen area (MLA), theoretical hydraulic conductivity (Kh), mean thickness of radial (CWTrad), tangential (CWTtan), and total (CWTall) cell walls were correlated with an 85-year long (1935-2020) climate series to identify the influence of climate on xylem anatomical traits. In addition to the conventional yearly chronologies, lumen area and total cell thickness series were split into ten tangential intra-annual sectors of equal width. Our analysis showed that changes in ring width are strongly associated with cell number and theoretical hydraulic conductivity: the greater the number of tracheids, the greater are the ring width and the conductance of the entire stem. July’s temperature resulted the main limiting factor for growth in the study area, confirming previous literature. Lumen area and cell wall thickness show correlations with temperature increasing from early and mid-June until early August, respectively, and then decreasing until mid-August. The different impacts of climate on cell enlargement and wall thickening indicate that different morphogenetic mechanisms are responsible for the different traits of tracheids, also shown by the opposite correlations between the chronologies of the first and last sectors. This study increases our understanding of the climatic influence of Scots pine growth at the northernmost area of its range to highlight how climate change will impact the xilogenesis of the species.