Soil water availability effects on seed germination account for species segregation in semiarid roadslopes

Previous studies report that the low colonisation success on eroded roadslopes of semiarid environments is controlled by microsite limitations. We predicted that soil water availability, through its effect on seed germination, is a determinant factor in the colonisation process of roadslopes in semiarid environments. Moreover, we predicted that the success of species establishment on the harshest roadslope conditions (i.e., south-facing roadcuts) is either due to the ability of seeds to germinate fast at low water potentials (colonising species) or to the ability of plants to sprout (resistant species). Specifically we present evidence for: (1) soil drying occurs faster on roadcuts than on roadfills after a rainfall event; (2) germination is a filtering process that influences the success of species establishment on roadslopes; (3) species able to colonise successfully south-facing roadcuts have higher germination rates and a shorter time to germination under water-stress conditions than species able to colonise successfully but exclusively the most favourable roadslopes (i.e., roadfills); (4) species that live on south-facing roadcuts and have the ability to sprout do not necessarily germinate with germinating rates and speeds as high as species that colonise successfully these slopes but are unable to sprout. To test these hypotheses we compared water dynamics in the soil among roadslope types and aspects as well as the seed ability to germinate at low water potentials among species showing different regeneration strategies and establishment success on roadslopes. Soil water availability after rainfalls occurring during the germination period played a major role in the germination of seeds. The patterns of seed germination under water-stress conditions were consistent with the success of colonising species on roadslopes and with the distribution of adult plants in the roadslopes 8 years after these latter were built. We discuss the usefulness of these results for the improvement of revegetation projects in semiarid areas by means of an appropriate selection of species adapted to the local environmental conditions. We suggest that the ability of species to germinate under water stress could be an indication of a species' potential for success under semiarid conditions.