Contribution of winter precipitation to tree growth persists until the late growing season in the Karakoram of northern Pakistan

Identifying the influence of seasonal source water on tree growth is essential to understand the effect of climate change on forest dynamics. However, due to the lack of available long-term monitoring studies, little is known on how long moisture originating from winter precipitation influences tree growth in the following growing season. Herein, we developed a tree-ring latewood oxygen isotope (delta O-18(LW)) chronology from the Karakoram (1955-2013) in northern Pakistan. We investigated whether winter climate signals are incorporated in the delta O-18(LW) and how long winter precipitation through soil moisture contributed to tree growth during the growing season. Significant correlations were found between delta O-18(LW) with both, winter to spring (January-May) temperature (r = 0.43, p < 0.05) and summer (July-August) precipitation (Local: r =-0.38, p < 0.05; Moisture source region: r =-0.44, p < 0.05). As shown by the cross-spectrum and commonality analysis, the climate signals in the delta O-18(LW) are frequency-dependent. Winter temperature signals stored in the delta O-18(LW) variations dominate at low frequencies more than 7 years (59% variance explained), while the summer precipitation signals dominate variations with higher frequencies shorter than 7 years (47% variance explained). Further analysis using reanalysis data shows that the growing season soil moisture at root depth (10-40 cm) is more strongly related to winter precipitation than to summer rainfall. Winter precipitation formed as ground snow provides root-zone soil moisture and tree growth via snowmelt during the growing season. Our study highlights that winter precipitation is the dominant water source contributing to Karakoram tree growth until the late growing season. Given the possibility of a decrease in winter precipitation under climate change in the Karakoram region, local forest is potentially threatened by the declining water supply during the growing season.