Fig. 7. Correlation between seasonal growth factor and ring width residual.

Growth factors were aggregated site-wisely and seasonally and calculated for (‘act’) the target year (year of tree ring formation), (‘-1’) the previous year, and (‘-2’) the year before that, as past conditions might also influence current growth. Seasonal periods were defined as follows: January to March, April to May, June to July, August to September, and October to December. Given that tree ring development is presumed to be complete by the end of September, the analysis timeframe for the target year also concludes with September. Subsequently, Pearson correlation coefficients were computed between the ring residuals and the growth factors. Blue fields indicate a positive correlation between the growth factor and the residual, while red fields indicate a negative correlation. The more intense the colour, the stronger the correlation. Across all tree species, it becomes evident that the growth conditions in the current year, especially during the growing season of June to July, exert the strongest influence on ring width. As expected, the correlations are significantly more pronounced during the summer months than outside the growing season. Soil moisture consistently correlates positively with growth, underscoring the importance of water availability for forest productivity. In contrast, temperature exhibits an opposite effect: warmer conditions are associated with smaller ring widths in the target year. Quercus cerris is the only exception here: Increased temperatures, especially during the period outside the growing season, lead to wider growth rings. This indicates a possible suitability concerning a warming climate.