Accelerated development in johnsongrass seedlings (Sorghum halepense) suppresses the growth of native grasses through size-asymmetric competition

Authors: SCHWINNING, SUSANNE - Texas State University; MECKEL, HEATHER - Texas State University; REICHMANN, LARA - University Of Texas; Polley, Herbert; Fay, Philip

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2017
Publication Date: 5/3/2017
Publication URL: https://handle.nal.usda.gov/10113/5852151
Citation: Schwinning, S., Meckel, H., Reichmann, L.G., Polley, H.W., Fay, P.A. 2017. Accelerated development in johnsongrass seedlings (Sorghum halepense) suppresses the growth of native grasses through size-asymmetric competition. PLoS One. 12(5):e0176042. https://doi.org/10.137/journal.pone.0176042.

Interpretive Summary: Biological invasions of exotic plant species are caused in part by globalization and human activities. Plant invasions can cause losses of native species and economic losses in agricultural ecosystems. Ecologists have sought for decades to identify the plant characteristics conferring invasiveness in exotic species. This question is especially perplexing in the case of invasive warm-season grasses that invade North American grasslands dominated by warm season native grasses such as big bluestem, switchgrass, and little bluestem. Understanding how these widespread and abundant native warm season grasses differ from invasive warm season grasses such as Johnsongrass (Sorghum halepense) is crucial to designing effective management strategies to minimize their harmful impacts on natural and agricultural ecosystems. We previously showed that Johnsongrass gains a growth advantage over these native grasses in the first two weeks of growth from seed. Here, we examine consequences of this initial growth advantage for subsequent competitive interactions between Johnsongrass and native species grown together in pots in a greenhouse. Over a full growing season, Johnsongrass increased its size advantage over big bluestem, little bluestem and switchgrass by up to 70:1. Johnsongrass also suppressed the nitrogen content of its pot mates but itself had low tissue nitrogen concentration but high total nitrogen uptake. The initial early size advantage of Johnsongrass is thus maintained over a full growing season, and appears to be founded in its ability to establish priority within days of germination and to disproportionately acquire nitrogen. Native grasses could be favored against Johnsongrass by management that stimulates earlier spring growth.

Technical Abstract: Superior competitive ability is an expected characteristic of grassland invaders, but not all invaders exhibit traits that convey a persistent growth advantage. Here we examine priority, expressed as a brief seedling growth burst, as the driving mechanism through which the exotic weed Johnsongrass (Sorghum halepense) gains dominance during establishment. Previously, we determined that Johnsongrass had four times more biomass than three native grasses, big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparius) and switchgrass (Panicum virgatum) 17 days after germination. Here, we examine consequences of this initial growth advantage for subsequent competitive interactions between Johnsongrass and native species. In a greenhouse, the same four species were grown from seed either alone or with one Johnsongrass plant in 100 cm deep x 11.3 cm wide containers at two fertilizer levels. Plants were periodically harvested for biomass analysis. Johnsongrass increased its size advantage over native pot mates to 40:1, 45:1 and 70:1 on average for big bluestem, little bluestem and switchgrass, respectively. Fertilization increased plant biomass and enhanced competitive suppression of switchgrass by Johnsongrass, but did not influence other competitive interactions. Johnsongrass suppressed the nitrogen content of its pot mates but itself had low tissue nitrogen concentration regardless of competition or fertilizer level. The competitive superiority of Johnsongrass appears to be founded in its ability to establish priority within days of germination. Thereafter, the size advantage is magnified, presumably by asymmetric resource competition. We conclude that some invasive traits are expressed only in the earliest stages of life and could be missed in trait screenings.