Relative importance of plant traits and ecological filters in road embankment revegetation under semiarid Mediterranean conditions

Revegetation in semiarid road embankments is not always successful because most of the sown species disappear and only a few survive. To improve hydroseeding success, there is a need to understand the underlying ecological processes that determine the outcome of sown species in restoration works.The objective of this work is to determine the relative importance of different factors in determining the final species composition after sowing in road embankments and with this aim, we conducted three different experiments: (1) experimental sowing in road embankments to determine species performance in field conditions; (2) greenhouse sowings, with the same species than the road embankment experiment, to study the effect of ecological filters (water stress and plant coexistence) on the performance of the species; and (3) analysis of relations between plant traits of the sown species (specific seed mass and specific plant biomass) and sowing success and competitive abilities in the greenhouse experiment. Relative success of the species in the embankments was compared with the relative success of the same species in greenhouse experiments and with the seed density sown in the road embankments.Plant coexistence, water stress and plant traits affected aboveground plant biomass production per species in the greenhouse experiment. However, the effects of plant traits on aboveground plant biomass were lower than the effect of plant coexistence but higher than the effect of water stress. The performance of the species in the water stress monoculture treatment at the greenhouse correlated positively with the performance of the species in the field 2 years after they were hydroseeded, thus indicating that water stress was the most influencing factor on species performance in road embankments. At the same time, plant traits as specific seed mass and plant biomass indirectly affected plant performance in the field since they affected aboveground plant biomass in the greenhouse experiment. On the contrary, species coexistence and seed density at sowing had influence on species performance in the road embankments neither 1 nor 2 years after hydroseeding.