Recovery of species richness lags behind functional recovery in restored grasslands

Restoration of grasslands on abandoned croplands is a major opportunity for land development in degraded landscapes. In this study, we compared the success of three restoration measures, spontaneous recovery, perennial-crop-mediated restoration, and sowing native grasses, by using a unique combination of species-based and trait-based approaches. We sampled 373 4-m(2) plots in nearly 1,000 ha of 20-year-old-restored grasslands and 72 4-m(2) plots in three neighbouring ancient grasslands. Species richness and target species cover were higher in spontaneously recovered sites than in other restored sites, but reference sites had higher scores than all restored sites. Spontaneous sites had higher functional diversity than the other restored sites. Spontaneous and reference sites showed little difference in their functional diversity and in the community-weighted means of relevant traits. Our findings indicate that the establishment limitation effect of the initial application of perennial crops or sowing competitive grasses can last for decades, hindering the recolonization of target species and the recovery of functional diversity; thus, they should be applied in landscapes with high cover of natural habitats only when inhibiting undesirable species is of paramount importance. The similar levels of functional niche saturation in spontaneous and reference sites may also indicate a self-developed establishment-limited situation after 20 years of succession. In combination with this, the lagging recovery of species richness entails lower functional redundancy in spontaneous sites. Our results underline the necessity of postrestorative management methods that reduce establishment limitation and contribute to the recolonization and subsequent coexistence of functionally similar species.