## 4 Discussion
These findings highlight SIF as a sensitive indicator of the physiological impacts of drought on vegetation, complementing structural indices like NDVI to provide a more comprehensive understanding of vegetation responses to climate change. In arid and semiarid ecosystems, water stress is the primary factor influencing vegetation growth. However, studies have suggested that vegetation in drought areas may adopt various strategies to adapt to environmental pressures, which may not always result in clear stress signals in remote sensing indices [43; 44]. Therefore, the overall upward trends in NDVI and SIF can be interpreted as evidence that vegetation adapts to climate change through physiological mechanisms. Conversely, the decreasing trends observed in certain densely vegetated areas of northern Xinjiang probably reflect the ecological impact of increased water stress in these regions. Significant differences exist in the responses of vegetation to climate change across the different ecological zones. The vegetation in northern Xinjiang showed a slightly higher sensitivity to climate change than that in southern Xinjiang, with physiological responses (e.g., photosynthetic activity as reflected by SIF) being more pronounced than structural responses (e.g., vegetation cover changes indicated by NDVI). Natural grasslands respond more strongly to climate change than forest areas. The relatively warmer and more humid climate in northern Xinjiang likely contributed to its heightened sensitivity to climate variability. In regions with water deficit, the sensitivity of vegetation to drought tended to decrease [45]. The response of vegetation to climate change inherently involves a degree of lag; physiological responses, such as adjustments in photosynthetic rate and water-use efficiency, can react quickly to environmental changes, whereas structural changes often require a longer timescale. Thus, SIF, which is closely related to photosynthetic activity, may respond more rapidly to environmental stresses, such as water or heat stress [46; 47; 48]. Grassland plants typically have shorter growth cycles and higher growth rates, enabling them to rapidly adjust their physiological processes in response to climate change. This rapid adaptability allows grasslands to sustain their growth under fluctuations in precipitation and temperature. Contrarily, forest vegetation with longer growth cycles generally exhibits structural changes that lag behind physiological responses. In terms of response times, certain northern ecological zones (R1, R2, and R3) in Xinjiang were primarily influenced by long-term climate variability (SPEI-12), whereas the southwestern areas of the southern ecological zones (R4 and R5) were mainly affected by short-term climate change (SPEI-3). Natural forests are more responsive to long-term climate changes, whereas natural grasslands are more sensitive to short-term climatic fluctuations, reflecting their distinct adaptation strategies at different timescales. Studies indicate that plants adapt to water availability at different timescales, with short-term adjustments relying on stomatal regulation to maintain water balance and medium-term adaptations involving reductions in photosynthetic activity [49; 40], which is particularly crucial in water-limited conditions. In the long term, plants respond to sustained water stress through deeper root development and growth strategy adjustments, such as growth dormancy. Vegetation response mechanisms to climate change vary by region owing to differing climate characteristics and vegetation types across geographic zones. Natural forests are mainly distributed in northern Xinjiang, with natural grasslands being more prevalent in the north than in the south. In water-limited environments, extensive root systems may reach deeper soil layers to access additional moisture, with deep-soil moisture responding more slowly to precipitation changes, thereby alleviating the effects of water stress on plant growth.