Multi-Species Tree-Ring Networks reveals seasonal shifts in Vapour Pressure Deficit trends and evolving Ocean-atmospheric Teleconnections in the Baspa basin, Northwestern Himalayas

Climate warming is increasing atmospheric moisture demand globally, intensifying hydroclimatic variability and ecosystem stress, particularly in climate-sensitive mountain regions. The Himalayan climate system, particularly Northewestern Himalayas (NWH), is shaped by moisture driven through Indian Summer Monsoon and Western Disturbances interacting with complex orography, resulting in highly dynamic climatic conditions. Recent increases in global temperatures have altered this circulation system, leading to enhanced climatic variability. However, long-term, region-specific climate records remain sparse across the NWH, limiting our understanding of these changes. Tree rings serve as high-resolution natural archives of past climate variability, and offer critical insights into the region's climatic history and its driving forces. The present study develops tree-ring chronologies from a dense network of five sites in the Baspa Basin, NWH, using 275 increment cores (89 from Cedrus deodara and 186 from Pinus wallichiana). Individual ring-width series were detrended using age-dependent splines, and chronologies were developed employing ‘Signal-free’ method. Composite regional chronologies were generated for both species through averaging same species ring widths as having high inter-site correlation and species-specific growth–climate relationships were assessed. The analyses identified vapour pressure deficit (VPD) as the dominant limiting factor of radial growth with spring VPD (February–April; FA-VPD) strongly constraining Cedrus deodara growth (r = −0.77) and summer VPD (June–July; JJ-VPD) limiting Pinus wallichiana growth (r = −0.63). VPD integrates the combined effects of temperature and humidity, influencing stomatal conductance and carbon assimilation, and thus exerting primary control on tree growth. While temperature shows a negative relationship and precipitation a comparatively weaker positive influence. Based on these relationships, we developed basin-scale, multi-season tree-ring reconstructions of FA-VPD (1771–2023 CE) and JJ-VPD (1834–2023 CE) using Cedrus deodara and Pinus wallichiana, respectively. These reconstructions explain approximately ~59% and ~40% of the variance in FA-VPD and JJ-VPD, respectively, during the calibration period. The FA-VPD reconstruction reveals a long-term increasing trend, characterized by two phases (1771–1917 and 1918–2023), with 1917 identified as a significant change-point year. In contrast, JJ-VPD shows a decreasing trend since the early twentieth century, consistent with enhanced monsoonal moisture availability in the basin. These divergent seasonal moisture trends imply future shifts in forest composition, with increasingly favourable conditions for Pinus wallichiana and heightened vulnerability of Cedrus deodara. Phase-wise teleconnection analyses indicate a weakening influence of El Nino Southern Oscillation and Interdecadal Pacific Oscillation, alongside an increasing role of the Indian Ocean Dipole, a pattern further supported by sea surface temperature spatial correlation analyses. Our findings highlight the critical role of large-scale climate drivers in shaping local hydroclimatic stress in the NWH. The seasonally resolved VPD reconstructions offer actionable baseline information for climate adaptation strategies, including forest management, species selection, drought preparedness, and risk reduction planning for climate-sensitive Himalayan communities.