A relict tallgrass prairie area at University of Colorado (39°55′N, 105°12′W), USA, was mined for gravel over 50 years, topsoil replaced and a mix of native grasses sown. A study was undertaken to assess plant response (species cover and richness) and restoration success to subsequent nutrient enrichment and reduction techniques, and partial grass removal (to reduce grass competition) in experimental plots.
In 1998, the topsoil (stockpiled for 40 years) was replaced, forming a layer 12-18 cm deep, and drill-seeded with nine native grass species. Initial establishment of most was good, but productivity lower than surrounding grasslands. In 2001, 140 plots (7 randomised blocks of 20, 2 x 2 m plots) were used to evaluate five soil nutrient amendments (each of 5 levels replicated 4 times in each block) on seedling establishment:
1) untreated (control); 2) carbon (sucrose) + phosphorus (P) additions to reduce nitrogen (N) availability; 3) gypsum to reduce P availability; 4) carbon and gypsum to reduce N and P availability; and 5) P additions.
In spring 2002, half of the plots were subjected to partial grass removal (grass-specific herbicide, Ortho Grass-B-Gone, applied to a 2 x 0.67 m strip per plot). This greatly reduced grass growth and killed some species in the treated strip.
During 2004, species richness and percent cover estimates (point-intercept sampling, hence relative cover values can exceed 100%) were made.
Precipitation over 5-years (2000-2004) was 79, 90, 69, 109 and 134% of the annual average. A species-poor plant community developed (total 25 species; 7 seeded). The two dominant species prior to gravel extraction (both in seed mix) were virtually absent: big bluestem Andropogon gerardii was not recorded in any plot; switchgrass Panicum virgatum averaged <1% relative vegetation cover.
Several common plants of saline soils uncharacteristic of the original flora prevailed. In control plots, alkali sacaton Sporobolus airoides comprised almost 50% of relative cover. The only treatment that reduced its abundance (sugar + P) achieved only a marginal reduction. Seeded species composed about 92% relative cover and weeds 8%. Three years amendments which lowered plant-available N and P, increased cover of seeded species to 97.5%.Two non-native annual Bromus (cheatgrass B. tectorum and Japanese brome B. japonicus) had highest cover on P addition plots.
Control plots averaged 4.7 seeded plus 3.9 non-planted species, herbicide plots 5.4 non-planted species. Thus seeding was beneficial in reducing undesired weeds. Restoration of the former tallgrass community was not achieved, but what developed was less vulnerable to invasives than surrounding (more fertile) grasslands.
If using or referring to this published study, please read and quote the original paper, this can be viewed at: http://onlinelibrary.wiley.com/doi/10.1111/j.1526-100X.2008.00418.x/full