Schwinning Lab

Yellow bluestem forms nearly monocultures on Texas rangelands

A Johnsongrass seedling grows faster than the seedlings of many native grasses our research showed

The invasion of American grasslands by exotic grasses is a challenge to grassland management, but also raises fascinating questions in plant community ecology: Why is it that some grass species introduced from Europe, Asia or Africa have been so exceptionally successful in North America? At the surface, there seems to be nothing special about the newcomers that would explain their apparent competitive advantage over native species. Over the years, students in the Ecology Lab have examined both mechanims of invasive success and methods for the control of established invasive species in central Texas grasslands.

In collaboration with Dr. Philip Fay at USDA/ARS (Temple, TX), and through funding from USDA, we have conducted greenhouse and field experiments to examine when and how the invasive species Johnsongrass (Sorghum halepense) begins to establish competitive dominance over native species and which traits might be involed. Former graduate student Heather Meckel and current graduate student Alexandra Salinas found that, above all, Johnsongrass tillers develop much faster than those of native species, whether establishing from seed or rhizome buds. Johnsongrass tillers can overtop almost any native grass species within a week or two of emergence. Adding to its success is the species' nitrogen use efficiency, although it is not higher than that of some native species.

The spread of yellow (KR) bluestem (Bothriochloa ischaemum) in Texas is just one example of an invasive grass all but replacing a diverse native grass flora after introduction. We have conducted experiments to quantify the potential allelochemical effect of KR bluestem on native and recentlly arrived invasive species such as silky bluestem (Dicanthium sericeum). In contrast to earlier reports, we found no evidence of allelochemical suppression of competitors by yellow bluestem.

On the issue of combating invasive grasses in the field, we have collaborated with Dr. Kelly Lyons at Trinity University in San Antonio. In this work, we focussed on determining prescribed fire regimes more detrimental to invasive species than to native species. Former graduate students Erin Ruckman and Scott Havill determined that prescibed fire in a drought year or in summer is potentially more damaging to KR bluestem than to native species, which have likely adapted to the combination of drought and fire in semi-arid American grasslands.

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, 20176042 pdf

Reichmann, L.G., Schwinning, S., Polley, H.W,, Fay, P.A. 2015. Traits of an invasive grass conferring an early growth advantage over native grasses. Journal of Plant Ecology DOI:10.1093/jpe/rtw014 pdf

Havill, S. Schwinning, S., Lyons, K.G. 2015. Fire effects on invasive and native warm-season grass species in a North American grassland at a time of extreme drought. Applied Vegetation Science 18:637-649 pdf

Ruckman, E., Robinson, T., Lyons, K.G., Schwinning, S. 2012. Comparative seed heat tolerances among native and a non-indigenous, invasive species: implications for selective management of grassland using fire. Ecological Restoration 30: 136 – 142. pdf

Ruckman, E., Schwinning, S., Lyons, K. 2011. Rainfall - burn time interactions in the recovery of an invasive grass after prescribed burn. Restoration Ecology 20: 756 - 763.