Date Published: January 25, 2019
Publisher: Public Library of Science
Author(s): Jennifer M. Fill, Eleanor Pearson, Tiffany M. Knight, Raelene M. Crandall, Mai-He Li.
The presence of native grasses in communities can suppress native forbs through competition and indirectly benefit these forbs by suppressing the invasion of highly competitive exotic species. We conducted a greenhouse experiment to examine the potential of direct and indirect interactions to influence the aboveground biomass of four native forb species in the presence of the native perennial grass Schizachyrium scoparium and exotic invasive Lespedeza cuneata. We examined patterns of growth for the invasive legume, the perennial grass, and four native species in four scenarios: 1) native species grown with the grass, 2) native species grown with the legume, 3) native species grown with both the grass and legume together, and 4) native species grown alone. Schizachyrium scoparium significantly decreased biomass of all forb species (p<0.05). In contrast, L. cuneata alone only significantly affected biomass of Asclepias tuberosa; L. cuneata increased the biomass of A. tuberosa only when the grass was present. When S. scoparium and L. cuneata were grown together, L. cuneata had significantly lower biomass (p = 0.007) and S. scoparium had significantly greater biomass (p = 0.002) than when each grew alone. These reciprocal effects suggest a potential pathway by which L. cuneata could alter forb diversity in grassland communities In this scenario, L. cuneata facilitates grass growth and competition with other natives. Our results emphasize the importance of monitoring interactions between exotic invasive plant species and dominant native species in grassland communities to understand pathways of plant community change.
Native perennial grasses are a keystone functional group of grassland and savanna communities. They influence fundamental ecosystem processes such as fire regimes and nutrient cycling [1–3] but they also have been shown to increase ecosystem resilience and community resistance to invasion [4–6]. As long-lived individuals that achieve high biomass, native perennial grasses may be particularly influential in competing with other plant species and in determining community invasion dynamics over the long term .
Native perennial grass and invasive legume presence had different effects on the growth of native grassland forbs. The perennial grass alone significantly decreased the aboveground biomass of all native forbs (Table 1; Fig 1). In contrast, the invasive legume alone only significantly affected biomass of A. tuberosa (Table 1). When the grass was absent, A. tuberosa biomass was higher when growing with the invasive (Fig 1). When the grass was present, A. tuberosa biomass was not affected by the presence or absence of the invasive (Fig 1). None of the forb species were significantly affected by plant density (Table 1).
When grown separately, the native perennial grass, but not the invasive legume, suppressed native species’ aboveground biomass. Native forbs grown with S. scoparium had lower biomass than those grown alone, but counter to our hypothesis, native forbs were generally not affected by the invasive legume. The exception to this result was Asclepias tuberosa, which had more biomass where L. cuneata was present and the grass was absent. Although light limitation is a mechanism by which both L. cuneata and perennial grasses can negatively affect co-occurring native species [29, 34], this type of competition was unlikely in our study. The four native forbs are relatively tall-statured species that quickly exceeded the height of S. scoparium. It is possible that belowground interactions have a greater effect on growth. Lespedeza cuneata is a nitrogen-fixer and increased nitrogen in pots with L. cuneata might have ameliorated any negative direct effects of competition between L. cuneata and natives, and even facilitated growth (e.g., A. tuberosa). Lespedeza cuneata is also known to alter soil bacterial and fungal community composition [35, 36] which may have differentially influenced forb growth.