LB 24-261 - Prairie species’ vulnerability to biological invasions: velvet bean (Mucuna pruriens) suppresses big bluestem (Andropogon gerardii) growth and development.

Background/Question/Methods:

As a consequence of land-use change, agricultural repurposing, overgrazing, and biological invasions, the historical North American prairie ecosystem coverage has substantially declined in the past few decades. While agricultural practices substantially deplete soils of nutrients and enhance erosion, the native prairie vegetation features species with long and dense root systems, which enhance species’ water use efficiency and prevent soil erosion. Their interactions with the soil biome enhance soil texture, quality and contribute to efficient nutrient cycling, thus providing particularly valuable ecosystem “services”. Big bluestem (Andropogon gerardii) is a warm season, perennial species, one of the “Big Four” (with Indian grass, switchgrass, and little bluestem) native grass species to dominate the tallgrass prairies of Central North America. The current work focused on better understanding the vulnerability of native prairie species to invasions and native-invasive species competition. In a controlled environment, greenhouse study we potted 1 g big bluestem seeds, alone, or together with seeds of the invasive velvet bean (Mucuna pruriens), in pots, ranging from 250mL to 2.5L. Plants’ emergence, growth, development, and health, as reflected by pigment concentrations and antioxidant levels were assessed weekly, for 8 weeks. Finally, plants were harvested and their above- and belowground biomass was determined.

Results/Conclusions:

We found that big bluestem emergence, and growth decreased by 46% and 40%, respectively when co-planted with velvet bean. Surprisingly, velvet beans planted alone emerged over a week earlier, compared to the co-planted beans, indicating a mutually suppressive effect of the two species in their initial developmental stage. Nevertheless, after 8 weeks, no differences were detected in the velvet bean’s development, height, and overall biomass, when planted alone, or in combination. On the other hand, big bluestem biomass decreased by 75%, even though, the shoot/root biomass ratio favored a higher root mass in the plants exposed to competitive pressure. Leaves of big bluestem growing alone showed a two-fold higher total chlorophyll (7 mgg^-1DW) and a three-fold lower total ascorbate content (2.3 µmolg^-1FW) than the competing grass, indicating that the competing grass is experiencing “stress-like” conditions. We attribute this effect to a potential belowground chemical interaction between the two competing species. Soil volume (and differences in the pot size) did not affect the observed effects, and we excluded the potential big bluestem shading including a set of controls in which the velvet bean was trimmed back. The exact nature of these potentially chemical interactions will be further investigated in the future.