Woody colonization of road embankments: A large spatial scale survey in central Spain

Planting of woody species is a commonly used method to restore road embankments. Given the importance of road verges as potential corridors and refuge for biodiversity, natural plant regeneration processes may also play an important role in establishing vegetation into these novel landscape elements. Most studies on woody colonization of roadsides have considered only a few sites covering a very limited environmental range. Therefore, it is unclear whether or not there are general patterns that may explain the development of woody vegetation. We analyzed woody vegetation colonization in embankments over a large and heterogeneous territory, using aerial photographs, available repositories of environmental and land management data sets and some embankment features. We addressed the following questions: (1) To what extent does the presence of planted woody plants influence patterns of natural recruitment in road embankments? and (2) What are the key factors underlying natural/passive plant colonization in road embankments? We used Multi-Model Inference (MMI) analysis to model woody vegetation cover. According to our results, woody-planted vegetation does not have a facilitating effect on natural colonization, questioning the efficiency of reforestation measures in the ecological integration of areas affected by road construction. Passive natural plant colonization occurs spontaneously in road verges and shows a highly site-dependent pattern, driven mainly by the age of embankments and the immediate surrounding vegetation. Therefore, we suggest that natural succession may be sufficiently effective as a passive restoration measure on embankments in the long term.