Coupling Tree-Ring Blue Intensity and Carbon Isotope (δ¹³C) Reveals Shifting Climate Change Drivers on Tree Growth in Kurdistan Forests, Northern Iraq

Abstract. This study investigates the relationships between climate changes and growth in semi-arid forests located in the Kurdistan Region of northern Iraq by utilizing tree-ring Blue Intensity (BI) parameters alongside stable carbon isotopes (δ¹³C). The sensitivity of detrended tree-ring width (TRW), earlywood blue intensity (EWBI), latewood blue intensity (LWBI), and delta blue intensity ΔBI to temperature, precipitation, and physiological responses are measured on three species of tree Quercus aegilops, Juglans regia and Fraxinus. Tree-ring Blue Intensity chronologies for 100 years, are used to look at climate-growth interactions over time for all three species. Results show a dominant temperature control on wood formation. EWBI is correlates with temperature (r ≈ 0.37, p < 0.001) and with precipitation in Quercus aegilops, whereas ΔBI is a positive temperature proxy (r ≈ 0.37, p < 0.001), this shows that when years are warmer, the density contrast of earlywood and latewood is increased. In Juglans regia, the strongest temperature sensitivity in TRW is found in LWBI (r ≈ -0.45, p < 0.001). The positive correlation of ΔBI with δ¹³C (r ≈ 0.30, p < 0.01) indicates a positive structural and physiological response to drought stress. The TRW sensitivities of Fraxinus are weaker (r = 0.128 in Erbil, r = 0.122 in Duhok), and the temperature BI(EW-LW) relationships are negative and significant (r ≈ -0.340, p < 0.01) (r ≈ -0.280, p < 0.01). Wavelet coherence analysis indicates a shift in the growth trend from precipitation-driven growth prior to ~1980 to the temperature-related growth thereafter, with increasing non-stationarity and episodic drought signatures. The integrated BI-δ¹³C approach captures anatomical and physiological adjustment, revealing that recent warming led to a decrease in radial growth, less earlywood development and changed wood density structure across species. The results show the potential of BI as an effective indicator of detecting climate-driven ecosystem shifts and temperature variability.