Tracing signatures of dual moisture sources in tree-rings: insights from xylem and leaf water isotopic study

 

           Seasonal variations in moisture sources are found in many regions, and tree ring isotopic records from such regions are known to respond to variations in these moisture sources. However, how the relative strengths of the distinct moisture sources influence tree ring isotopic records is not well understood.

The Western Himalayas (WH), a hydro-climatically sensitive region, has two distinct moisture sources: westerlies, which provide snow during winter, and the South-west monsoon, bringing rain during summer. To understand how these moisture sources leave their imprint in the tree ring, isotopic characterization of precipitation, xylem and leaf water was carried out for two years (2023-2024). Forty trees, of species commonly employed in dendroclimatic research, from two climatologically distinct locations in the WH, Manali and Keylong, were studied. Precipitation samples were collected throughout the sampling interval, while xylem and leaf samples were collected immediately before (June) and after (October) the monsoon. The isotopic composition of xylem and leaf water collected during June and October is expected to reflect the isotopic signature of snow and rain, respectively. Cryogenic vacuum extraction was employed to extract water from xylem and leaf samples. Stable isotopic analysis (δ¹⁸O, δ²H) of all samples was performed using an IRMS (Delta V Plus, Thermo Scientific).

The results indicated winter precipitation was enriched in ²H and ¹⁸O compared to monsoon rain. The d-excess of winter precipitation was higher than that of the monsoon, suggesting source of moisture was from a comparatively drier region. Precipitation at Keylong showed a signal depleted in ²H and ¹⁸O for all seasons compared to that at Manali. The isotopic composition of xylem water mimicked seasonal isotopic variability in precipitation, suggesting that trees in the WH indeed sample water from snow and rain sequentially during the growing season. The leaf water exhibited higher enrichment in ¹⁸O than in ²H (higher than that predicted by equilibrium fractionation) over the xylem isotopic composition. This suggested δ²H of leaf water was better at reflecting the isotopic composition of precipitation than δ¹⁸O, especially when the relative humidity is lower. Our results suggested intra-annual isotopic characterization of tree rings from the WH has the potential to reveal past variations in the strengths of westerlies and monsoon.