Silver fir tree-ring fluctuations decrease from north to south latitude—total solar irradiance and NAO are indicated as the main influencing factors

Silver fir (Abies alba Mill.) is a flexible European tree species, mainly vegetating within the mountainous regions of Europe, but its growth responses across its latitudinal and longitudinal range have not yet been satisfactorily verified under changing environmental conditions. This study describes the tree-ring increment of silver fir in research plots across a latitudinal gradient from the northern range in Czechia (CZ), through Croatia (HR) to the southernmost range in Italy (IT). The research aims to analyze in detail the dynamics and cyclicity of the ring-width index (RWI) and how it relates to climatic factors (temperature and precipitation), the North Atlantic Oscillation (NAO), and total solar irradiance (TSI), including the determination of latitude. The results show that the main drivers affecting fir growth are the seasonal NAO index and TSI. Monthly temperatures affect RWI early in the vegetation season, while lack of precipitation during the summer is a limiting factor for fir growth, especially in July. Seasonal temperatures and temperatures in June and July negatively impact, while seasonal precipitation totals in the same months positively influence the RWI in all research plots across meridian. The longest growth cycles in fir RWI were recorded in the northernmost studied plots in CZ. These cyclical fluctuations recede approaching the south. The cyclic increase in RWI is related to the TSI, which decreases its effect from north to south. The TSI's effects vary, positively impacting CZ but negatively influencing HR while remaining relatively neutral in IT. On the other hand, seasonal NAO tends to negatively affect silver fir growth in HR and CZ but has a mildly positive effect in IT. In conclusion, the TSI and the influence of the seasonal NAO index are prevalent in the fir RWI and are accompanied by a greater cyclicity of RWI in Central Europe (temperature optimum) than in the Italian Mediterranean region, where this tree species is limited by climatic conditions, especially lack of precipitation.