Abstract: Mongolian pine (Pinus sylvestris var. mongolica) has been widely planted on sandy soil in northern region of China, but a decline in forest growth and even death has occurred in the earliest Mongolian pine plantations on Keerqin sandy soil located at Zhanggutai of Liaoning Province. Aiming to study the decline problem of Mongolian pine, a water gradient experiment was conducted to assess the effects of soil water stress on the membrane lipid peroxidation, osmotic substances, and protective enzyme system of twoyear old P. sylvestris var. mongolica seedling’s needles. The water gradient experiment was designed with reference to the annual mean precipitation of main sites on Keerqin over 50 years. The results showed that with the aggravation of drought stress, the SOD activity of the seedling’s needles increased after an initial slight decrease, while POD activity had gradually increased. The osmotic substances including soluble sugar (SS), soluble protein (SP) and proline (Pro) presented distinct ascending trend with drought stress, and the ascending range was the largest for SS and SP, e.g., the contents of SS and SP at -16.18 bars were 1.8 and 2.4 times than those at -4.50 bars, respectively. The MDA content, plasma membrane permeability (RC), and rate of O₂^-_· increased gradually with decreasing soil water potential, and the increasing range of the rate of O₂^-_· was the largest. The above results suggested that P. sylvestris var. mongolica could endure and adapt to drought stress in some extents through physiological and biochemical mechanisms, especially through increasing osmotic substances. However, the cellular tissues at the soil water potential of -16.18 bars were un-reversibly damaged due to the accumulation of O₂^-_· and the aggravation of membrane lipid peroxidation.

Key words: Peach orchards, Arthropod communities, Species diversity, Community evenness, Seasonal dynamics