Radial growth responses of <i>Pinus sylvestris</i> var. <i>mongolica</i> with different stand ages and diameter classes to drought events.

We explored the differences in the impacts of drought events on <i>Pinus sylvestris</i> var. <i>mongolica</i> of different ages (30 and 40 years) and different diameter classes (large 20-24.9 cm, medium 15-19.9 cm, small 10-14.9 cm) in the Saihanba Nature Reserve. Based on the tree ring width index (RWI), we analyzed the correlation between radial growth and climatic factors and their ecological resilience to drought events. The results showed that the RWI of 30-year-old small-diameter trees was significantly positively correlated with standardized precipitation evapotranspiration index (SPEI) from September to December of the previous year and February of the current year. RWI of 30-year-old large-diameter and medium-diameter trees was correlated with SPEI from September of the previous year to June of the current year, but the correlation was statistically non-significant. The RWI of 40-year-old large-diameter trees was significantly negatively correlated with the maximum mean temperature in October of the previous year and June of the current year, as well as the mean temperature in June of the current year. The RWI of 40-year-old medium-diameter trees was significantly negatively correlated with the maximum mean temperature and mean temperature in October of the previous year and significantly positively correlated with SPEI in July of the current year. The RWI of 40-year-old small-diameter trees was significantly positively correlated with SPEI from September of the previous year to June of the current year. The resistance of radial growth of trees with different ages to four drought events (40 years old significantly higher than 30 years old) and the resilience exhibited a significant downward trend, while the recovery showed a significant upward trend (40 years old significantly lower than 30 years old). Within the same age group, the responses of <i>P. sylvestris</i> var. <i>mongolica</i> with different diameter classes to drought events were different. The resistance and resilience of large and medium diameter classes of 40-year-old trees were significantly higher than those of small diameter class trees, but their recovery showed no significant difference. For 30-year-old trees, there were no significant differences in resistance, recovery, or resilience among different diameter classes. <i>P. sylvestris</i> var. <i>mongolica</i> of different ages and diameter classes experienced varying degrees of drought stress, resulting in a significant decrease in resilience. The 40-year-old trees exhibited high resistance, while the 30-year-old trees showed high recovery capability. Small diameter class trees were most severely affected by drought stress.