Combined restoration techniques underperform in alien grass-dominated sandy soils: Lessons from southern Brazil

The study aimed to define techniques that favor the ecological restoration of a degraded area with abundance of invasive alien species (IAS) and sandy soil poor in organic matter, as well as to determine factors that can interfere with ecological restoration. The study area is located on the left bank of the Forqueta River in the municipality of Travesseiro, Rio Grande do Sul, Brazil. The area exhibits low levels or organic matter and high levels of macro and micronutrients, and is mostly dominated by invasive alien grasses. Four ecological restoration techniques were applied, constituting five treatments: (1) Planting of seedlings in small clusters - Anderson groups (NA); (2) NA with soil transposition (NA + TS); (3) NA with artificial perches (NA + PA); (4) NA + TS + PA; and (5) natural succession (TC), each with three replications in a completely randomized experimental design. Mechanical mowing and herbicide application were conducted before the application of the techniques, followed by two mechanical mowings at five-month intervals. The evaluation of seedling survival and the monitoring of germination of seeds from transposed soil and around the perches, along with phytosociological sampling at three time (T1 = 3 months; T2 = 9 months; T3 = 13 months), revealed that neither treatment was really successful, and this had several causes, primarily alien species. IAS influenced the reduction of richness and percentage cover of native plant species, regardless of treatment. Soil conditions, low precipitation, and high temperatures also hindered the growth and establishment of native plants. The techniques used did not favor species diversification and the formation of vegetation cover of native plants, thus failing to contribute to local ecological restoration, even when two or three techniques were combined. ∗ Invasive alien grasses reduced native species cover in all tested restoration methods. ∗ Low rainfall and high temperatures hindered native plant growth and establishment. ∗ Restoration techniques were ineffective in sandy soils with low organic matter. ∗ Combined restoration techniques did not overcome barriers to native vegetation cover.