Climatic controls of Pinus pumila radial growth along an altitude gradient

Recent temperature rise caused by human activities has led to profound changes in the forest ecosystem. Shrubs play an important role in the boreal forest. However, it is not clear how shrub growth responds to climate change, especially whether there is a difference in the response of shrub at different altitudes to climate change. Here, we developed three ring-width chronologies of Pinus pumila along an altitudinal gradient in the Aokelidui Mountains, northeast China, then analyzed the relationship between the growth of P. pumila and climate at different altitudes. The results showed that the radial growth of P. pumila at low altitude had more prominent climate signals than those at medium and high altitude and was more sensitive to climate change. The chronological similarity was the highest in low and middle altitudes (72.5%, p < 0.01). Summer temperature had a significant negative effect on the growth of shrubs, and summer precipitation was conducive to the formation of wider rings at medium altitude. The warmer winter of the previous year favored P. pumila growth at low and medium altitudes but just the opposite at high altitudes. After rapid warming, the negative correlation between temperature and P. pumila growth in the growing season changed to a positive correlation at high altitude. On the contrary, the relationship between precipitation and P. pumila growth in the growing season changed from a positive to a negative correlation. We also found that the sensitivity of the growth of P. pumila to the Standardized Precipitation Evapotranspiration Index (SPEI) has gradually increased at medium-altitude since the 1980s. Therefore, in recent decades, the effect of moisture on the radial growth of shrub has become more and more important at medium altitudes. At the same time, the temperature rise was beneficial to the growth of P. pumila at low altitude. Our results suggest that the treeline advancement in this area is unlikely to occur in the near future since the shrub at high altitudes is not sensitive to climate warming.