Long-term management is required for the recovery of pollination networks and function in restored grasslands

Abstract Grasslands face rapid decline worldwide. Among the main threats to these ecosystems are changes in land use, such as abandonment and forestation, which promote forest vegetation to the detriment of grassland plant diversity. To support the conservation and restoration of grasslands, it is key to understand what ecological processes limit the recovery of their biodiversity and ecosystem functions after perturbations. However, we still know little about the legacy effects of past forestation on the ecological mechanisms involved in grassland recovery, especially regarding long-lasting impacts on plant?pollinator interaction networks and plant reproduction. Here, our objective is to fill this knowledge gap by comparing plant and pollinator diversity, the degree of network specialization, the pollination success and pollen limitation of native plant species across 28 plant?pollinator networks of old and restored grasslands with different ages since restoration (from 2 to 75?years). We hypothesized that past forestation would have long-lasting negative legacy effects on plant richness and plant?pollinator networks, increasing pollen limitation for native plants, thereby delaying the recovery of grassland communities in restored grasslands. The results showed that restored grasslands exhibited significantly lower plant richness, less specialized (more generalized) interaction networks, lower pollination success and pollen-limited seed reproduction of native plants compared with the old grasslands. Lower network specialization was associated with reduced pollination function and decreased plant reproductive success, which likely led to slower recovery of plant diversity in restored meadows. We showed that such negative legacy effect was long-lasting and maintained in the grasslands even 75?years after recovery. Synthesis and applications: Our findings suggest that the recovery of specialized plant?pollinator networks by enhancing plant diversity is essential for restoring pollination function. To restore such pollination networks and function, grassland management, including mowing, should be maintained for a certain period that exceeds at least 75?years. Encouraging the abundance of bees and butterflies is a key to facilitating plant recovery. Moreover, it is crucial to maintain neighbouring plant- and pollinator-rich old grasslands, which support the recovery by serving as sources of native seeds and pollinators.