Adaptive constraints at the range edge of a widespread and expanding invasive plant

Identifying the factors that facilitate and limit invasive species' range expansion has both practical and theoretical importance, especially at the range edges. Here, we used reciprocal common garden experiments spanning the North/South and East/West range that include the North American core, intermediate and range edges of the globally invasive plant, Johnsongrass (Sorghum halepense) to investigate the interplay of climate, biotic interactions (i.e. competition) and patterns of adaptation. Our results suggest that the rapid range expansion of Johnsongrass into diverse environments across wide geographies occurred largely without local adaptation, but that further range expansion may be restricted by a fitness trade-off that limits population growth at the range edge. Interestingly, plant competition strongly dampened Johnsongrass growth but did not change the rank order performance of populations within a garden, though this varied among gardens (climates). Our findings highlight the importance of including the range edge when studying the range dynamics of invasive species, especially as we try to understand how invasive species will respond to accelerating global changes. Invasive species often face novel abiotic and biotic interactions at their range edge that may limit further expansion into new geographies or elicit fitness trade-offs that do not occur in the range core. Using one of the world's worst weeds, Johnsongrass, that is found across the Southern US grown in a large experiment across five US states, we found that expansion into new diverse environments was likely not the result of local adaptation. Contrary to the weedy nature of Johnsongrass, competition in the first year strongly limits performance of this invasive plant, especially at the range edge where populations are currently rare. Our study highlights the importance of understanding the factors limiting geographic expansion, in particular at the range edge, especially as we try to understand how invasive species will respond to climate change.