Water utilization of Pinus sylvestris var. mongolica in a sparse wood grassland in the semiarid sandy region of Northeast China

KEY MESSAGE : Mongolian pine in a sparse wood grassland stand maintains a relatively stable state under the current water conditions by varying its water use sources at a constant water use efficiency. In a semi-arid sandy ecosystem, water is one of the most important factors influencing survival and growth of Mongolian pine in a sparse wood grassland (MPSWG); however, the mechanism of water utilization of MPSWG has been yet poorly understood. Stable isotopes of hydrogen and oxygen of water in xylem, soil, precipitation and groundwater were analyzed to determine water sources used by MPSWG from April to October in two consecutive years (1.2 and 0.89 times of long-term annual precipitation in 2010 and 2011, respectively). Needles δ¹³C were measured simultaneously to assess water use efficiency. Results showed that MPSWG used soil water (sampling depth up to 60 cm) between April and July, and in October. However, water sources used by MPSWG exhibited significant differences in August and September between 2 years. In 2010, MPSWG only utilized soil water in August and September due to higher soil water content, but they used both soil water and groundwater in 2011 because of lower soil water availability, suggesting that MPSWG could utilize groundwater to replenish soil water deficiency. No significant differences in mean δ¹³C among months and between 2 years indicated that MPSWG did not suffer severely water stresses. In conclusion, MPSWG only used soil water during higher precipitation year, whereas they primarily utilized soil water and relied a little on groundwater during lower precipitation year. Water obtained by MPSWG (including water stored in trunk) basically satisfied its requirement for growth. Thus, MPSWG stand was relatively stable presently in the semi-arid sandy region.