Research shows climate and competition with other plants stop the damaging invasive grass Johnsongrass from spreading further across the USA.

What causes an invasive species to go so far and no further? Originally from Asia, Johnsongrass (Sorghum halepense) invades cropland across the southeastern USA. Rebecca Fletcher and colleagues asked how far it can potentially spread as climate changes? Their study, in AoB PLANTS, examined whether climate, competition, or adaptation limit its range expansion. Their findings could help focus control efforts on a plant that causes millions of dollar’s worth of crop damage yearly.

Fletcher and colleagues grew Johnsongrass from across its US range in five common gardens spanning temperature and rainfall gradients. They found that northern and western edge plants grew smaller and flowered earlier, trading growth for reproduction. They also found that competition with other plants limited growth everywhere, especially in favourable climates.

Looking at the edges revealed two different tales. In the north, in New York, all Johnsongrass died over winter. It suggests that cold winters limit northward range expansion. In the west all Johnsongrass survived both the arid growing seasons and the winter. Yet Johnsongrass is rare in New Mexico, so it’s not clear what limits its expansion. Fletcher and colleagues suggest that it might be due to a critical period around germination, when Johnsongrass seedlings are most vulnerable.

The team also found that native weeds could help reduce the performance of Johnsongrass. While there was competition everywhere, Fletcher and colleagues found that plant-plant interactions had the biggest influence on growth in the regions where Johnsongrass grew best. This confirmed Fletcher and colleagues’ prediction that where the environment isn’t your biggest problem, it’s the neighbours that give you most trouble.

Johnsongrass is a bundle of trouble itself. It can reduce yields of sugarcane by half, as well as reducing yields in corn and soybean. It can also provide habitat for agricultural pests. Ironically, despite being introduced as a forage alternative for farmers, frost or parch damaged leaves can cause cyanide poisoning in cattle. Keeping it at bay, or even reducing its range would prevent a lot of harm in the future.

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Fletcher, R.A., Atwater, D.Z., Haak, D.C., Bagavathiannan, M.V., DiTommaso, A., Lehnhoff, E., Paterson, A.H., Auckland, S., Govindasamy, P., Lemke, C., Morris, E., Rainville, L. and Barney, J.N. (2023) “Adaptive constraints at the range edge of a widespread and expanding invasive plant,” AoB PLANTS, 15(6), p. lad070. Available at: https://doi.org/10.1093/aobpla/plad070


Cover: Johnsongrass. Image: Canva.

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