Warming and drying increased the interspecies consistency of tree-ring lignin methoxy δ2H variability at the alpine treeline

Climate warming-induced changes in the hydrological habitat for the alpine treeline can significantly impact tree growth and community dynamics. However, composite indicators that capture typical alpine species' responses to temperature and precipitation are lacking. This study investigates the use of the δ2H values of lignin methoxy groups (δ2HLM) from the dominant deciduous conifer tree (Larch chinensis) and evergreen broadleaf shrub (Rhododendron purdomii) in the upper treeline of the Qinling Mountains from 1960 to 2018 as potential hydroclimatic indicators. The δ2HLM values of the R. purdomii shrubs were enriched by 23.3‰ in 2H compared with those of the L. chinensis trees. However, the multispecies δ2HLM composite significantly reflected the temperature and precipitation during the growing season (May to September). Rapid warming and drying after the 1990s increased the interspecies consistency of δ2HLM variability, accompanied by a decreasing enrichment trend. The weighted self-calibrated Palmer Drought Severity Index showed a similar variation tendency. Similar isotopic patterns were detected for the δ2H values of precipitation (δ2HPre) and the composite δ2HLM values. The latest empirical relationship mode can effectively verify the reconstructed δ2HPre values using the δ2HLM composite from the alpine treeline. These results indicate that the cross-species composite tree-ring δ2HLM can accurately indicate the alpine hydroclimatic conditions during the growing season, including the warming and drying periods.