Nature-Based Solutions Increase Sowing Success for Mediterranean Grassland Restoration: A First Short-Term In Situ and Ex Situ Comparison

Abstract

Active ecological restoration of Mediterranean semi-natural grasslands faces challenges such as limited seed dispersal and poor establishment of dominant perennial species. The aim of our study was therefore to test different seedling type along a gradient of nature-based solutions including a commercial seed mixture with seeds of regional provenance, hay transfer, and seeds from ant refuse piles both harvested in neighboring non-degraded grassland. The primary objective was to compare these three mixtures ex situ, in a greenhouse, in order to distinguish the effects of each sowing treatment under controlled common conditions with the same experiment conducted in situ across four sites in the Crau Plain (Southern France). Using a set of soil rehabilitation treatments, we compare alluvial quarries backfilled with former grassland or orchard soils, abandoned orchards treated with normal or deep plowing, and the reference grassland. In the short term (6 months), results already differed significantly between in situ and ex situ experiments. Ex situ, the commercial mixture showed significantly higher density than the other treatments and a high species richness. On the other hand, in situ, sowing from hay transfer and ant refuse piles displayed higher species richness, with a composition more similar to the reference grassland, while the commercial mixture remained similar to controls. These results underline the predominant impact of habitat conditions in constrained ecosystems, highlighting the importance of increasing the use of more nature-based solutions such as hay transfer or ant refuse piles spreading to maximize grassland restoration success.