Effect of Drought and Topographic Position on Depth of Soil Water Extraction of Pinus sylvestris L. var. mongolica Litv. Trees in a Semiarid Sandy Region, Northeast China

Drought and topographic position are the most important factors influencing tree growth and survival in semiarid sandy regions of Northeast China. However, little is known about how trees respond to drought in combination with topographic position by modifying the depth of soil water extraction. Therefore, we identified water sources for 33-year-old Mongolian pine (Pinus sylvestris L. var. mongolica Litv.) trees growing at the top and bottom of sand dunes by comparing stable isotopes delta H-2 and delta O-18 in twig xylem water, soil water at various depths and groundwater during dry and wet periods. Needle carbon isotope composition (delta C-13) was simultaneously measured to assess water use efficiency. Results showed that when soil moisture was low during the dry period, trees at the top used 40-300 cm soil water while trees at the bottom utilized both 40-300 cm soil water and possibly groundwater. Nevertheless, when soil moisture at 0-100 cm depth was higher during the wet period, it was the dominant water sources for trees at both the top and bottom. Moreover, needle delta C-13 in the dry period were significantly higher than those in the wet period. These findings suggested that trees at both the top and bottom adjust water uptake towards deeper water sources and improve their water use efficiency under drought condition. Additionally, during the dry period, trees at the top used shallower water sources compared with trees at the bottom, in combination with significantly higher needle delta C-13, indicating that trees at the bottom applied a relatively more prodigal use of water by taking up deeper water (possibly groundwater) during drought conditions. Therefore, Mongolian pine trees at the top were more susceptible to suffer dieback under extreme dry years because of shallower soil water uptake and increased water restrictions. Nevertheless, a sharp decline in the groundwater level under extreme dry years had a strong negative impact on the growth and survival of Mongolian pine trees at the bottom due to their utilization of deeper water sources (possibly groundwater).