Variations in water use efficiency and carbon and nitrogen concentrations in red heart Chinese fir

Temperature can significantly ( P < 0.05) affect plant growth by modifying water use strategies, which are determined by intrinsic water use efficiency ( WUE i ). Red Heart Chinese Fir ( Cunninghamia lanceolata ) is one of the most important ecological and economic plantation species in China. However, the C. lanceolata water use strategy in response to increased temperatures and uneven temporal distribution of precipitation during the growing season is rarely reported. In a 7‐year‐old C. lanceolata plantation, differences in WUE i and C and N concentrations in different organs were analysed by anova , and the δ 13 C stable isotope, C, and N concentrations in stems determined at different tree heights. Stepwise regression and variance inflation factor were used to remove autocorrelated factors, and structural equation modelling was then used to explore relationships between WUE i and climate and biological factors. WUE i differed significantly between leaf and branch at different standardized precipitation evapotranspiration indices (SPEI). WUE i and N concentration decreased with age. The highest WUE i in branches and leaves were 92.7 and 88.4 μmol·mol −1 in 2020 (SPEI = 0.00), respectively. δ 13 C increased with relative tree height but N concentration and C/N ratio were not affected. Air temperatures has increased in between 2014 and 2020. WUE i and N concentration decreased with increasing branch and leaf age, but C concentration increased. SPEI significantly positively affected WUE i ( P < 0.05), and WUE i was significantly negatively related to C concentration, which is consistent with the trade‐off between C and water.