Dominant plant species modulate responses to hydroseeding, irrigation and fertilization during the restoration of semiarid motorway slopes

Restoring roadside slopes in semiarid regions of the Mediterranean Basin is often constrained by the difficulties arising when developing restoration projects (absence of nearby natural ecosystems serving as reference sites and slow natural colonization) and by the contradictions found between short-term (reduce soil erosion) and long-term (increase plant diversity) restoration goals. Restoration techniques developed in temperate climates are commonly applied in these regions without taking into account their specific characteristics; as a consequence, they often fail. We evaluated the effectiveness of three treatments widely used by practitioners (hydroseeding, fertilization and irrigation) to foster community composition changes that control soil erosion and increase species diversity (restoration goals) during the restoration of motorway embankments. The study was carried out during an 18-month period in five embankments from semiarid central Spain. The most outstanding result was that responses of the plant community to the treatments evaluated were site-specific. Several fast-growing dominant species, some hydroseeded and some already present in the study sites, were responsible for this idiosyncratic variation between sites. On embankments, where plant cover can easily reach values high enough to prevent erosion, the use of non-native herbs that can potentially dominate the community should be avoided. These fast-growing species, although effective as starters the first years following motorway building, can constrain vegetation dynamics in the long term. Our results indicate that these species should be controlled in the field, and their presence avoided in the commercial seed mixtures when the target is to enhance biodiversity and ecosystem stability and resilience.