Climate drives differences in the germination niche of a globally distributed invasive grass

Aims Exotic invasive species are often exposed to strong selection pressures in their new ranges that can often lead to substantial intraspecific variation. Population differentiation in the timing of life history events in response to climate gradients is thought to be an important mechanism facilitating the range expansion of many invasive species. For seed producing plants, the timing of seed germination determines the first environmental conditions experienced by newly emerged germinates, and can have important implications for the successful colonization, establishment and spread of invasive plants though the role of germination in the success of invasive plants remains poorly understood. Methods We assessed the variation in seed germination dynamics among 10 populations of the invasive plant Johnsongrass (Sorghum halepense) across its North American distribution, capturing both a temperature and precipitation gradient, and whether that variation is associated with home climate. Seeds were exposed to a wide range of temperatures (11-48 degrees C) and two water availability treatments. Important Findings We found that Johnsongrass seeds germinated across a wide range of temperatures, but there was substantial variation among populations in the proportion of seeds that germinated in response to both temperature and water availability. Evidence indicates that as Johnsongrass expanded its range from warmer climates into cooler climates, there was a concurrent shift in the germination temperature niche to cooler temperatures. Our results suggest that the germination of Johnsongrass seeds has adapted to home climate allowing this invader to maximize germination throughout its range, and that this may be an important contributing factor to its invasion into new environments.