Response of Pinus sylvestris var. mongholica tree-ring density to climatic factors in Northeast China under climate warming background

Under the background of increasing global climate change, understanding the impact of climate variables on forest ecosystems has become an important topic in ecology and climatology. To explore the connection between tree-ring density and climatic variables, Pinus sylvestris var. mongholica (Ps) trees growing at the northwestern foot of the Greater Khingan Mountains (GKM) were selected as research subjects. Correlations between their density index and climatic factors were analyzed using the basic theory and methods of dendrochronology. The impacts of either climatic variable (temperature and precipitation) on the tree-ring density and growth of Ps were also analyzed under warming conditions. Results showed that the tree ring width index (TRW) was positively correlated with the earlywood width index (EW), latewood width index (LW), minimum density (MND), density of earlywood (EWD), maximum density (MXD), and density of latewood (LWD) (all at p < 0.01). These results imply that the inter-annual changes of the three groups of indicators exhibit a high degree of synchronization. The analysis of the density index to climatic variables showed that the density index of different growth rings of Ps was considerably affected by temperature and precipitation. Both TRD and EWD showed a positive correlation with maximum temperature in May, while LWD and MXD showed a positive correlation with precipitation in March. However, EWD as well as MND showed a considerably negative correlation with temperature in the previous autumn and likewise with precipitation in spring and summer. The results obtained from a follow-up redundancy analysis further validated those above from the response function analysis. The sliding correlation analysis indicated that the dynamic stability of the ring density index became stronger or weaker over time. This study reveals the response differences of the ring density indices of Ps to climate factors and their temporal stability, which is helpful to understand the response relationship between conifer species growth and climate in northern GKM.