Sources of water used by Pinus sylvestris var. mongolica trees based on stable isotope measurements in a semiarid sandy region of Northeast China

Dieback of Mongolian pine (Pinus sylvestris var. mongolica) forests in the semiarid sandy region of northern China occurs when the trees are approximately 30–35 years old. Water deficit was the most important reason for this dieback, but the water sources used by pine trees are poorly understood. We aimed to identify the water sources utilized by pine trees and to explore the potential mechanisms inducing Mongolian pine dieback. The δD and δ18O of water in twig xylem, soil, rain and groundwater were analyzed in a 29-year-old pine stand during two consecutive years 2010 and 2011. Soil water content (SWC), tree transpiration and groundwater level were measured concurrently. The results demonstrated that trees utilized soil water in spring (April–May) and autumn (September–October). In contrast, trees used both soil water and groundwater in summer (June–August) during the two observed years. Additionally, 70.1% of water used by trees came from soil water when the SWC was high (6.6%), but 55.8% of water used by trees came from groundwater when the SWC was low (3.5%). The groundwater contribution to tree transpiration increased from 10.0% to 14.4% with the rainfall decreasing from 580mm to 460mm, with a mean value of 12.2%. However, in the year of 60% of the long-term annual rainfall, the mean SWC only was 2.7%. These findings indicate that groundwater was increasingly critical with decreasing rainfall. Groundwater should be thus the primary water source for the growth and survival of pine trees in the years of extreme low rainfall. The dieback of Mongolian pine forests occurred when the groundwater level declined suddenly in the years of extreme low rainfall.