Mechanisms of competitive suppression of native grasses by Johnsongrass

Background/Question/Methods

Native perennial grasses are widely considered alternatives to corn for biofuel production. However, there is concern that nitrogen fertilization of perennial grasses makes them just as vulnerable to noxious weeds. One of the most problematic weeds in grasslands is the C4 perennial grass Johnsongrass (Sorghum halepense). We conducted a greenhouse study to investigate the mechanistic basis by which Johnsongrass may outcompete three native C4 grasses, big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparium), and the biofuel crop, switchgrass (Panicum virgatum). We hypothesized that Johnsongrass suppresses native grass species due to differences in the allocation of biomass between roots, stems and leaves. Grasses were planted as seeds in 1 m tall, 10 cm wide pots (15 liters) and grown for six months. The four species were grown at two levels of nitrogen (N) (+0.14 g and +0.42 g slow-release N fertilizer per pot in 70/30 sand to topsoil), either alone or in combination with Johnsongrass in a full factorial randomized complete block design. Each month, leaf gas exchange was measured and a subset of plants was harvested to measure biomass allocation to leaves, stems, reproductive tissues, and roots. Data were analyzed using ANOVA. 

Results/Conclusions

In the absence of competition, Johnsongrass accumulated total biomass much faster than the other species. For example, after 6 weeks, across N levels, it had 12x, 28x and 22x more total biomass than big bluestem, little bluestem and switchgrass, respectively. Among the four species, it allocated the most biomass to stem (40% vs. 30%, across native species) and least to leaves (30% vs. 40%). Nevertheless it had 9x, 17x and 12x more leaf area than big bluestem, little bluestem and switchgrass, respectively, due to lower specific leaf area. Johnsongrass had the lowest rate of net photosynthesis during early growth, while switchgrass had the highest. Competition with Johnsongrass led to almost complete growth suppression in little and big bluestem, independent of N-level. However, switchgrass grown with Johnsongrass at high N achieved 50% of the above-ground biomass of plants grown without Johnsongrass, compared to only 3% at low N. This suggests that the competitive ability of switchgrass was enhanced at high N. We conclude that Johnsongrass achieves its competitive dominance within the first weeks of growth by outgrowing and overtopping native species, trading off leaf biomass for stem biomass, and high photosynthetic rates for more leaf area.