Abstract: A comparative study was conducted on the needles stable carbon isotope composition ( ¹³C), specific leaf area (SLA), and dry matter content (DMC) of 19-yearold Pinus sylvestris var.mongolica trees in a sparse wood grassland in the south edge of Keerqin Sandy Land under the conditions of extreme drought and extreme wetness, aimed to understand the water use of Pinus sylvestrisunder the conditions of extreme precipitation. The soil water content and groundwater level were also measured. In the dry year (2009), the soil water content in the grassland was significantly lower than that in the wet year (2010), but the  δ¹³C values of the current year-old needles had no significant difference between the two years and between the same months of the two years. The SLA of the current year-old needles was significantly lower in the dry year than in the wet year, but the DMC had no significant difference between the two years. Under the conditions of the two extreme precipitations, the water use efficiency of the trees did not vary remarkably, and the trees could change their needles SLA to adapt the variations of precipitation. For the test ecosystem with a groundwater level more than 3.0 m, extreme drought could have no serious impact on the growth and survival of the trees.

Key words: Pinus sylvestris var. mongolica, water use efficiency, specific leaf area, dry matter content.