Early Dynamics of Plant Communities on Revegetated Motorway Slopes from Southern Spain: Is Hydroseeding Always Needed?

The increasing global rate of road construction is leading to a parallel increase of areas of degraded soil conditions and steep slopes that need revegetation. Hydroseeding with commercial seeds of fast‐growing grasses and legumes is a common practice in revegetation of motorway slopes. We carried out 3 years of monitoring of vegetation dynamics on hydroseeded and nonhydroseeded motorway slopes (48 slopes) in a maritime Mediterranean zone in Málaga (southern Spain). Our main objectives were to test whether hydroseeding significantly increases species richness and plant cover and whether hydroseeded species act as starters, facilitating the establishment of the vegetation and quickly disappearing once the communities are established. A hydroseeding success index (HSI, ranging from 0 to 1) was used to assess the relative abundance over time of the 14 species from the hydroseeding mixture. Species richness and cover was significantly higher on embankments (50–70 species per embankment, 80–90% cover) than on roadcuts (6–10 species per roadcut, 18–30% cover). Performance of hydroseeded species was poor from the very beginning (HSI, 0.2–0.3). On embankments, either presence or abundance of hydroseeded species did not significantly vary throughout the study. Both hydroseeded and nonhydroseeded communities exhibited a significant decrease in species richness, a significant increase in plant cover, and a highly dynamic species composition over time, with Sorensen index of 0.3–0.5 between years. There were no significant differences in plant cover, species richness, and aboveground biomass between hydroseeded and nonhydroseeded plots on embankments throughout the study. Our results demonstrate that there are situations in which the use of hydroseeding for revegetation is not needed. Further research should focus on understanding the establishment of autochthonous species and identifying environmental conditions under which the addition of commercial seeds may not be needed, or indeed situations where it may be harmful in suppressing autochthonous species.