Revegetation success at the roadside: What matters when all fails?

Roads create a singular edge environment. Geomorphologically, road cuts and fills differ from the equilibrium profile of natural slopes; ecologically, vegetation grows on substrates deprived of their original soils; succession at the roadside is either not observed or limited to the replacement of seeded commercial grasses by local recruitment, differentially enriched in non-native species. Besides, revegetation is hampered by the calendar requirements fixed by the civil construction. In this study, we search for the determinants of revegetation success at the medium term in a continental Mediterranean environment. We have compared the standard hydroseeding technique with an improved solution using native species, and we have monitored the edaphic and microclimatic environment, and the quality and quantity of the seed bank and rain. After three years, our main conclusions are: (1) plant recruitment at the roadsides is not limited by seed availability, (2) topsoiling did not enhance erosion in slopes of 34 degrees, (3) initial (before hydroseeding ) topsoil contents in organic matter and nitrogen predict plant cover for the following two years with and increasing correlation coefficient, (4) both commercial (starters) and native species failed when hydroseeded in an unfavourable season. Our results suggest that the selective pressures imposed by the roadside environment determine plant recruitment, which suggests that hydroseeding, particularly out of the optimal season, is an inadequate technique for road slope revegetation.