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 δ¹³C 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. δ¹³C 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.