Causes and research prospects of the decline of Pinus sylvestris var. mongolica plantation

Planted forests account for 70% of the world’s forest area. Over the past 30 years, the area of planted forests has been increasing worldwide and as a natural climate solution for carbon sequestration and climate change mitigation. The impact of climate change on forest sustainability is complex and the question of how to maintain the long-term stable growth of plantations is a global technical problem. Since the 1950s, a large area of Pinus sylvestris var. mongolica plantation has been built for wind prevention and sand fixation, and soil and water conservation in northern China, which has become an important part of the green ecological barrier in North China. At present, part of the Three-North Shelterbelt Forest Program has declined, especially P. sylvestris plantation. Although a lot of research has been carried out on the decline of P. sylvestris plantation on sandy land, at present, the understanding of the decline mechanism is still unclear. Most of the previous studies focused on the water factor, and considered that the water deficit in the introduction site was the main factor leading to the decline of P. sylvestris plantation, focusing on the soil water change, water balance and water use strategy. However, the physiological mechanism of the decline and even death of P. sylvestris caused by water deficit is still unclear, and there is a lack of research on the response and regulation mechanism of the non-structural carbon to water deficit, especially the interaction between diseases and pests, hydraulic failure and carbon starvation during the decline process of P. sylvestris. In this paper, the main factors leading to the decline of P. sylvestris plantation are summarized and a conceptual model of the decline process is proposed. Hydraulic failure and carbon starvation are the two main physiological mechanisms leading to the decline of P. sylvestris plantation. Hydraulic failure and carbon starvation reduced the ability of P. sylvestris to resist pests and diseases, while pests further promoted the development of hydraulic failure and carbon starvation until P. sylvestris seriously declined or even died. Future research should focus on the physiological process of P. sylvestris decline and its interaction with pests and diseases. The following aspects should be strengthened: (1) The synergistic mechanism of multiple factors on declined P. sylvestris plantations, such as the interaction between diseases and insect pests and hydraulic failure and carbon starvation, the impact of the interaction between diseases and insect pests and drought on the hydraulic characteristics and NSC, the impact of NSC on the synthesis of metabolites resistant to diseases and insect pests; (2) the decline mechanism of P. sylvestris plantation on stand or landscape scale based on the study of individual scale and the ecological response mechanism to environmental factors at stand or landscape scale; (3) the P. sylvestris plantation response to environmental stress and its regulation mechanism, such as how to balance the hydraulic efficiency and safety, and the strategic model of xylem water transport and NSC regulation in response to drought and other emergencies; (4) the suitable range of ecological suitability evaluation of P. sylvestris plantation based on long-term positioning observation data analysis.