Synergistic Effects of Microenvironmental Heterogeneity and Propagule Pressure Drive the Adaptive Expansion of a Wetland Invasive Plant in an Arid Region

Understanding how invasive species establish in environments outside their typical climatic range is crucial for predicting and managing biological invasions. This study experimentally assessed the survival mechanisms of giant ragweed (<i>Ambrosia trifida</i> L.), a wetland plant, in the arid Yili Valley of Northwest China-a region with climatic conditions markedly different from its known distribution. Using comprehensive niche analysis methods (PCA and kernel density estimation), we confirmed that the Yili Valley represents a novel climatic space for this species, with mean annual precipitation (200 mm) far below its typical requirements (> 800 mm). Field experiments demonstrated that topographically driven moisture accumulation creates critical microhabitat refugia, with lower slope positions maintaining significantly higher soil moisture than middle and upper slopes throughout the growing season. Linear mixed-effects models revealed that slope position (<i>F</i> = 109.77, <i>p</i> < 0.001) and propagule pressure (<i>F</i> = 225.55, <i>p</i> < 0.001) were the primary drivers of seedling establishment, with their significant interaction (<i>F</i> = 5.18, <i>p</i> < 0.001) indicating that high propagule input can partially compensate for suboptimal moisture conditions. Seeds from different collection years (2010-2022) showed variable performance, with the collection year significantly affecting long-term survival (<i>F</i> = 20.24, <i>p</i> < 0.001) but not initial establishment, suggesting potential population-level changes during the invasion period that warrant further investigation with common garden experiments. Our findings demonstrate that microenvironmental heterogeneity provides crucial stepping stones for invasive species in climatically unsuitable regions, highlighting the importance of considering fine-scale habitat variation in invasion risk assessments.