Abstract: Pinus sylvestris var. mongolica (MP) is the most important evergreen conifer tree species for afforestation in the Three North Shelter Forest Program. Since the beginning of 1990s, however, the MP plantations in the earliest introduced area (more than 30-35 years old) have declined, which are characterized by withered top, low growth rate, dead stems and no natural regeneration. A series of experiments in comparison with the sparse MPgrassland in Keerqin sandy land and natural MP forests in Hulunbeier sandy land have been conducted by using thermal dissipation, stable isotope and remote sensing technologies. As a result, the mechanisms of the decline of the MP plantations have been almost determined. In this paper, we systematically summarized the previous results about the decline mechanisms of the MP plantations. The major conclusions are as follows: the soil moisture threshold of severe water stress for the MP plantation was determined (20% of field capacity); the water use strategy (9-21 years old trees only used soil water, but 31-41 years old trees used both soil water and groundwater) and the water balance of MP plantations (water imbalance, 39 mm of groundwater was used to maintain water balance) were tested; as well as the contribution of transpiration of MP plantations to consumption of water resources at a regional scale was quantified (consuming 12.3% of total water resources). Finally, the decline processes of the MP plantations were revealed. Based on these findings, countermeasures solving the decline of MP plantations and afforestation programs have been proposed, which provides reference for preventing decline and rational management for existing MP plantations, and spatial layout for this tree species in the future.

Key words: elevation, leaf ecological stoichometry, Leontopodium leontopodioides, the Qinghai-Tibetan Plateau.