Spatio-temporal variability in Scots pine radial growth responses to annual climate fluctuations in hemiboreal forests of Estonia

In this study, we used a comprehensive tree-ring network from Estonia and investigated Scots pine (Pinus sylvestris L.) radial growth responses to changing climate conditions, considering differences in site conditions and local climates. To assess whether climate influences Scots pine radial growth consistently across the country, we developed thirteen tree-ring width chronologies for pine populations growing in four forest site types - Cladonia, Calluna, Myrtillus, and Rhodococcum - in four sub-regions of Estonia and compared these to climate data. A correlation analysis between ring-width indexes and monthly resolved temperature and precipitation applied over the period of 1955-2006 revealed significant positive correlations with winter/early spring temperatures and the total precipitation of late summer in the year prior to growth. High mean temperatures in August of the year prior to growth were negatively related to pine growth, particularly on islands and in the Northeast of Estonia. Scots pine growth on mesic and medium fertile Myrtillus and Rhodococcum sites in the Southeast exhibited greater sensitivity to mean February−April temperatures, while high temperatures and low precipitation at the end of the summer of the previous growing season limited radial growth of pine on the islands and in the North-eastern sub-region. A principal component analysis conducted on mean index chronologies and a hierarchical cluster analysis performed on bootstrapped correlation coefficients showed that local climate is the main driver of common growth, followed by ecological site conditions. A moving correlation analysis, performed over the period of 1955-2006, using 30-year windows shifted by one year showed that climate-growth relationships are not stable in changing climatic conditions. Associations between Scots pine tree-ring width and winter temperatures are getting weaker.