Abstract: Dryland forests account for 27% of the world's forests and play a key role in global ecological balance and environmental protection. With the intensification of climate change, dryland forests are experiencing extensive decline, which seriously restricts or even threatens local ecological security and human social development. However, there is a lack of comprehensive assessment and analysis of the progress of research on the status and mechanisms of dryland forest decline on a global scale. Based on the extensive collection of relevant literature at home and abroad, we have systematically assessed and analyzed the concept, characteristics, current situation, influencing factors, and formation mechanisms of dryland forest decline. In terms of concepts and characteristics, we have summarized the concept, and the general characteristics of dryland forest decline. In terms of the current situation of forest decline, we have summarized the current situation of forest decline in global and Chinese dryland forests, and described the differences in dryland forest decline in regions, forest categories, and tree species. In terms of influencing factors, we have summed up the process of natural factors (climate change, biological invasions, site conditions, natural disasters) and anthropogenic factors (anthropogenic disturbances, anthropogenic management measures) influencing the decline of dryland forests. In terms of decline mechanisms, we have discussed the role of mechanisms at different scales (hydrodynamic failure hypothesis, carbon starvation hypothesis, biological attack hypothesis at individual scale; population-scale, community-scale like nutrient cycling, competitive effects; and ecosystem-scale) on dryland forest decline according to the research scales, and talked about the relationship between dryland forest decline mechanisms and the difficulties in the research. Based on these results, future research directions should focus on: (1) strengthening research on the ecological mechanisms of dryland forest decline at large scales; (2) strengthening the collection of long-term high-quality research data and systematically analyzing the integrated effects of multiple factors on dryland forest decline; and (3) strengthening integrated research on multiple mechanisms of dryland forest decline. We systematically reviews the current status and mechanisms of dryland forest decline, aiming to enhance the understanding of degradation mechanisms and provide theoretical support for ecological restoration practices in dryland.

Key words: dryland forest, forest decline, influential factors, formation mechanisms