Since the 21st century, global forest ecosystems have experienced systemic decline driven by multifaceted stressors, including escalating climate change, increasing drought frequency, and severe anthropogenic disturbances. To elucidate the characteristics and underlying mechanisms of forest decline, this study conducted a systematic literature review and synthesized multidisciplinary evidence to comprehensively assess its current manifestations and causative factors. The key findings were as follows: (1) Quantitative evaluations of forest decline exhibited multiscale patterns, with core indicators-such as biodiversity, soil moisture content, carbon sequestration capacity, thermal response, evapotranspiration efficiency, and nutrient concentrations (e.g., phosphorus, potassium, iron, and copper)-demonstrating significant negative correlations with decline severity, whereas land surface temperature showed a pronounced positive response. (2) The mechanistic drivers of decline could be attributed to the synergistic interplay of endogenous physiological processes (e.g., drought-induced hydraulic dysfunction, carbon starvation, and genetic adaptation constraints) and exogenous stressors (including pest and disease outbreaks, geohazards, anthropogenic pressures, and interspecific competition). Notably, the coupled effects of hydraulic failure and carbon metabolism disruption have been identified as the predominant pathway driving forest decline in arid environments. This review further highlighted critical limitations in current research: (1) Integration of multiscale modeling frameworks was inadequate; (2) The assumptions in hydraulic failure mechanisms were oversimplified, which often neglected interspecific variability in embolism resistance among plant functional types; and (3) Persistent challenges were still existed in quantitatively disentangling anthropogenic impacts from natural stressors. Therefore, future research should prioritize defining critical thresholds for forest decline under global change scenarios, developing early-warning systems for drought-induced decline, and exploring forest resilience after drought. By consolidating existing knowledge, this study provides a theoretical foundation for understanding the cascading effects of forest decline and informs adaptive management strategies to mitigate ecosystem decline.