Soil Moisture Is the Key Factor Facilitating Giant Ragweed Invasions in Grasslands of the Yili Vally, China

Giant ragweed (GR; <i>Ambrosia trifida</i> L.), an invasive alien species, causes significant harm to grassland ecosystems and farmlands in some areas but is challenging to control. GR has invaded the hilly grasslands of Yili Valley, China, since 2013, and preliminary observations have shown that GR populations on the lower slopes of hills are more successful than those on the middle or upper slopes. To clarify the factors determining GR's invasion success, we compared GR population distributions among slope positions and the relationship between non-biotic factors and the invasion of GR. Of the soil physicochemical properties, only soil moisture differed significantly among slope positions, with the wettest soils found on the lower slopes. GR biomass increased with the soil water content, irrespective of native plant diversity. In our experiment, when the annual average soil volume moisture content exceeded 20.3% and 25.3%, GR could reduce the biomass of native herbs by more than 50% and 80%. Therefore, water is the determining factor of a successful GR invasion in the grasslands of the Yili Valley. On a global scale, it was discovered for the first time that GR can invade temperate grasslands, but also has risks of invading other grasslands that share similar conditions. So, GR invasions of temperate grasslands must be closely monitored, particularly in low-lying areas or those with increasing precipitation.