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.