Variability increased underneath growth enhancement of Mongolian pine natural forests in northeastern China in response to climate change

Clarifying tree growth variability in response to long-term climate change and short-term climate extremes is crucial for predicting future trends of forest performances. With climate change accompanied by more frequent climate extremes, tree mortality has been recently observed in the natural distribution areas of Mongolian pine (Pinus sylvestris var. mongolica), an important species used for afforestation in water-limited environments of northern China. However, the growth dynamics of the naturally grown pine trees and their associations with climate change are not well understood. We combined tree-ring and remote sensing approaches to investigate long-term growth performances and their responses to climate change from tree individual to landscape scales of Mongolian pine natural forests in a water-limited region in northeastern China. We found that the tree-ring width index (RWI) was highly correlated (r = 0.927, P < 0.0001) with satellite-derived normalized difference vegetation index (NDVI) during growing season (May-September) and both metrics consistently showed an obvious increasing trend in recent two decades, indicating enhancements of growth and their strong associations at both individual tree and landscape scales. However, long RWI time series also revealed obviously greater variations in radial growth (reflected by increased coefficient of variation) and lower resilience to potential disturbances (higher first-order antocorrelation) underneath such enhanced growth during the past two decades than earlier periods. RWI was found to be positively influenced by water-related climate factors (i.e., precipitation, relative humidity and drought index-SPEI) and negatively affected by temperature during growing season, which were further corroborated by similar relationship patterns between these climate variables and NDVI. Such impacts seemed to have strengthened over time, particularly at short time scales, which likely contribute to the increased growth variability with increasing climate extremes. Comprehensive examinations on growth performances of Mongolian pine natural forests through integrating multiple approaches imply that such forests would suffer from increasing growth instability and risks of die-off under climate change.