Tree-ring δ²H records of lignin methoxy indicate spring temperature changes since 20th century in the Qinling Mountains, China

Stable hydrogen isotope ratios of wood lignin methoxy groups (δ²H_LM values) of mid-latitude forests are increasingly applied to reconstruct δ²H patterns in precipitation of regional to sub-continental scales. To date, the knowledge about the effect of topographic factors on δ²H_LM values is still scare, especially in the biogeographic boundary. We measured tree-ring δ²H_LM chronologies (1901–2018) from two typical conifer species at three sites with different altitudes of a north-ridge-south transect of the Qinling Mountains (central China), where covers the climate transition zone from warm temperate to subtropics. The δ²H_LM chronologies of the north and the south slopes were characterized by a decreasing trend between 1901 and 1955, which turned into a positive trend after 1955. In contrast, the mountain ridge site located at the highest altitude showed continuous increasing δ²H_LM values over the entire period from 1901 to 2018. The highest correlations between temperature and δ²H_LM chronologies were found for the northern slope and the mountain ridge region spring temperatures (March–May). The observed abrupt changes in the δ²H_LM values since the 1950s at lower elevation sites might reveal slope-related moisture effects which superimpose local temperature patterns. During the rapidly warming period 1955–2018, the higher correlations of the δ²H_LM chronologies with the spring and mean annual (previous November to current October) temperatures were detected on the warmer northern slope. In summary, although the variations of measured δ²H_LM of the three investigated sites reflect geographic-specific hydrological characteristics, the low-frequency temperature changes are documented in the δ²H_LM values.