Seeding is sometimes used in attempts to increase grass forage production in invaded rangelands, but insufficient long-term data prevent determining if seeded grasses are likely to become and remain productive enough to justify this expensive practice. We quantified long-term seeding outcomes in a widespread Rocky Mountain foothill habitat invaded by leafy spurge (Euphorbia esula L.) and several exotic grasses. Fourteen yr after seeding, the most productive grass (bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) Á. Löve]) produced 900 (100, 12 000) kg ha–1 [mean (95% CI)], which was about 70% of total plant community biomass. This result was not greatly altered by grazing according to an unreplicated, grazed experiment adjacent to our replicated ungrazed experiment. Regardless of treatment, E. esula gradually became less productive and seeded and unseeded plots produced similar E. esula biomass 14 yr after seeding. P. spicata reduced exotic grasses about 85%. Our results resemble those of another foothills study of another invasive forb (Centaurea stoebe L. ssp. micranthos [Gugler] Hayek) and a Great Plains study of E. esula, so foothills seeding outcomes seem somewhat insensitive to invader composition, and seeding can increase forage across much of E. esula's range. While there is always some risk seeded grasses will fail to establish, our study combined with past studies identifies invaded habitats where seeded grasses have a good possibility of forming persistent, productive stands.
Invasive plants with poor forage characteristics have replaced forage grasses on millions of hectares of western US rangelands (Duncan et al. 2004). Examples include leafy spurge (Euphorbia esula L.), an invasive forb not typically heavily consumed by cattle, elk, and deer (Trammell and Butler 1995), and downy brome (Bromus tectorum L.), an invasive annual grass lacking palatability and protein content beyond early growth stages (Cook and Harris 1952).
Attempts to increase forage quantity and quality sometimes involve seeding. Seeding is considered failure prone, though seeded populations are rarely tracked long enough to conclusively determine their fates. When grasses remain sparse a few years after seeding (e.g., Piper and Pimm 2002; Fansler and Mangold 2011), it is tempting to assume they will remain sparse indefinitely and this assumption is sometimes correct according to a few longer-term studies (Ferrell et al. 1998; see fig. 8 of Rinella et al. 2012; Ott et al. 2019). However, in areas infested with sulfur cinquefoil (Potentilla recta L.), seeded grass cover jumped from 8% to 50% between 3 and 6 yr after seeding (Endress et al. 2007; Endress et al. 2012), and in areas invaded by spotted knapweed (Centaurea stoebe L. ssp. micranthos [Gugler] Hayek), seeded grasses went from sparse to dominant between 2 and 15 yr after seeding (Rinella et al. 2012). Ott et al. (2019), Prodgers (2013), and Copeland et al. (2019) provide further examples of seeded grasses gradually increasing. In short, sparse grass stands are not a good sign a few years after seeding, but it is not assured stands will remain sparse indefinitely.
On the other hand, productive stands are a good sign early after seeding, but it is not assured quickly proliferating stands will remain productive over the long term. After quickly proliferating, seeded grasses sometimes gradually decline and other times remain productive for prolonged monitoring periods (5–12 yr) (Ferrell et al. 1998; Thacker et al. 2009; Robins et al. 2013; Davies and Boyd 2018; Stonecipher et al. 2019). When grasses gradually decline, it is likely a consequence of invaders recovering from herbicides and/or tillage combined with seeding (e.g., Benz et al. 1999; Metier et al. 2018).
In summary, short-term results are unreliable indicators of long-term seeding outcomes, so more long-term data are needed to determine which seeded grasses can flourish in which invaded environments. The rough fescue (Festuca altaica Trin.) habitat we studied is prevalent in northern Rocky Mountain foothills (Taylor 1994). Long-term research indicates seeding can return forage production to foothills invaded by the short-lived, tap-rooted perennial forb spotted knapweed (C. stoebe L. ssp. micranthos [Gugler] Hayek) (Rinella et al. 2012), and if this is also true for the long-lived, rhizomatous weed of this study (E. esula), it will suggest seeding outcomes are somewhat consistent across invader species. Finally, long-term research shows seeded grasses can maintain robust stands in E. esula–invaded northern Great Plains habitats (Ferrell et al. 1998), and if this is also true for foothill habitats, it will suggest seeding can reverse forage losses throughout E. esula's invasive range.
We seeded five grasses, and previous studies led us to hypothesize one or more of these grasses would be productive 14 yr after seeding (Ferrell et al. 1998; Rinella et al. 2012). We hypothesized the most productive grasses would suppress E. esula and exotic grasses. Grasses were sown with and without herbicide, and based on Rinella et al. (2012), we hypothesized seeded biomass would remain greater where herbicide was applied 14 yr earlier.