Interactions of climate, topography, and soil factors can enhance the effect of a single factor on spring phenology in the arid/semi-arid grasslands of China

Understanding the interactions among climate, topography, and soil factors on vegetation spring phenology can help reveal the feedback mechanisms of vegetation ecosystems in response to environmental conditions and climate change. Nonetheless, the coupling effects of these environmental elements on vegetation start of season (SOS) are still poorly understood. We took Xinjiang Province, a typical arid/semi-arid region, as the research area and quantitatively analyzed the explanatory power and interactions of climate, topography, and soil factors on grassland SOS through the geographical detector model based on the data extracted by Timesat 3.3 software. The results showed that: (1) From 2001 to 2020, early SOS predominantly occurred from 75 to 105 days in Northern Xinjiang. While later SOS was widespread in Southern Xinjiang from 105 to 135 days. The area of the SOS advanced region was larger than that of the delayed region; (2) There was obvious spatial hierarchical heterogeneity between SOS and climate, topography, and soil factors. Altitude, maximum temperature, and soil type were the primary drivers of SOS changes in the study area (P<0.001); (3) The interactions among environmental factors strengthened the explanatory power of individual factors, either through mutual reinforcement or non-linear effects. The interactions between elevation and climate factors were particularly significant, with an explanatory power above 0.3. This study highlighted the importance of the coupling effects of environmental variables on SOS and suggested that drives should be considered more comprehensively in future analyses.