Low weed and cool‐season grass abundances likely necessary for warm‐season grass, forb, and shrub establishment

Abstract

Plants seeded in degraded grasslands often fail to establish. In the Northern Great Plains, perennial cool‐season grasses are easiest to establish, and they sometimes competitively suppress warm‐season grasses, shrubs, and forbs. Seeding cool‐season grasses at low rates sometimes benefits other seeded plants but risks greater weed abundances. To identify grass seed rates low enough to allow other plants to establish but high enough to constrain weeds, we varied cool‐season grass seed rates while holding warm‐season grass, shrub, and forb seed rates fixed. The first couple growing seasons after seeding, we hypothesized cover of other seeded plants and weeds would decrease with increasing cool‐season grass seed rate. During later growing seasons, we hypothesized weed cover would become independent of grass seed rates due to seeded plants increasing in plots seeded at low rates. Neither hypothesis was supported. Because weed abundances were high, warm‐season grasses, shrubs, and forbs apparently experienced similarly intense competition regardless of grass rate, so low rates did not increase seeded plant establishment. Regardless of seed rate, cool‐season grass cover did not increase between the second and final (i.e. fourth) growing season, perhaps because of low precipitation. Increasing warm‐season grass, shrub, and forb abundances will require controlling weeds in addition to lowering cool‐season grass seed rates. Even these steps will not always increase establishment because native plants sometimes died before controlling weeds with herbicides became feasible and grass competition became important. Lowering grass rates without implementing weed control risks sites becoming weedy for prolonged periods.