Terrain-Driven Water Accumulation Amplifies Propagule Pressure to Facilitate Ambrosia trifidaInvasion in Arid Grasslands: A Non-Climatic Similarity Perspective

Invasive alien plants pose significant threats to global ecosystems and economies. Understanding the drivers of their successful spread and colonization is critical for predicting and managing invasion events. Ambrosia trifida, native to humid regions, has rapidly invaded the arid hilly grasslands of the Yili Valley in China in recent years, challenging traditional climate niche theories. This study first use global-scale data to compare the climatic niches of the native range, global invaded regions, and the Yili Valley. It was found that the Yili Valley is characterized by a typical arid climate (aridity index < 0.3), differing from other distribution areas. Subsequently, field experiments were conducted on Terrain-Driven Water Accumulation and Propagule Pressure at different hillslope positions. Findings indicated that water accumulation on the lower hillside significantly increased soil moisture compared to the middle and upper hillsides, providing a suitable microhabitat for colonization. Under the same Propagule Pressure, the colonization rate at the lower hillside was significantly higher. Additionally, seeds from plants that had experienced drought stress exhibited higher colonization rates after germination than those from plants with less or no drought stress. These results suggest that the synergistic effect of Terrain-Driven Water Accumulation and Propagule Pressure is crucial for the successful invasion of giant ragweed in arid grasslands. Traditional climate niche models may underestimate its invasion risk in arid regions. We recommend that monitoring and management efforts in arid areas focus on terrain-formed water accumulation zones and consider Propagule Pressure to develop effective control strategies.

外来入侵植物对全球生态系统与经济均构成严重威胁。明晰其成功扩散与定殖的驱动机制，对预测和管控入侵事件至关重要。三裂叶豚草（Ambrosia trifida）原产于湿润区域，近年来已快速入侵中国伊犁河谷的干旱丘陵草原，对传统气候生态位（climatic niche）理论构成挑战。本研究首先借助全球尺度数据集，对比分析该物种原生分布区、全球入侵区与伊犁河谷的气候生态位。研究发现，伊犁河谷属于典型干旱气候，其干旱指数（aridity index）＜0.3，与该物种其他分布区域存在显著差异。随后，本研究针对不同坡位开展了地形驱动水分富集（Terrain-Driven Water Accumulation）与繁殖体压力（Propagule Pressure）的野外控制实验。结果显示，相较于坡中与坡上区域，坡下区域的水分富集可显著提升土壤含水量，为豚草定殖提供了适宜的微生境。在同等繁殖体压力条件下，坡下区域的定殖率显著更高。此外，经历干旱胁迫的母株所结种子，在萌发后展现出比轻度或未受干旱胁迫母株种子更高的定殖率。上述结果表明，地形驱动水分富集与繁殖体压力的协同效应，是三裂叶豚草成功入侵干旱草原的关键机制。传统气候生态位模型或会低估该物种在干旱区域的入侵风险。我们建议，干旱区域的监测与管控工作应聚焦于地形形成的水分富集区，并结合繁殖体压力因素制定高效防控策略。