Restauración ecológica de taludes de carretera en ambiente mediterráneocomunidades de suelo y efectos del manejo

Abstract

This Doctoral Thesis studies the development of biological communities in roadslopes from different highways in Madrid, as well as the effects of varying management techniques on the composition, diversity and ecological processes that are in turn responsible for ecosystem functionality. The goal is to provide new insight and knowhow to improve the effectiveness of efforts to assist the recovery and reassembly of soilborne biota, pedological processes, and resilient plant communities in these highly perturbed settings. Following an introductory chapter, chapters II and III of the thesis analyze microarthropod and bacterial communities in heavily disturbed, anthropogenic soils, or Anthrosols in roadside embankments. These topics were given priority in the thesis in view of the importance of these soilborne communities in basic soil processes that underlie long term development and resilience of terrestrial ecosystems. The next two chapters of the thesis describe the effects of different commonly used techniques employed to restore and maintain roadside ecosystems. Chapter IV evaluates the effect of mowing, fertilizing, and also experimental biomass addition, on the composition and diversity of soil bacterial communities, and on ecological processes such as rate of litter decomposition and soil nutrient availability, especially nitrates and ammonium. This experiment was carried out in the same four road embankments studied in Chapter III, in each of which experimental plots were set up. Results revealed that roadside ecosystems are sensitive in their response to treatments with both bacterial community structure and processes being rapidly and significantly affected. In particular, biomass addition and fertilization affected diversity and community composition, while mowing affected soil properties and litter decomposition. The fact that neither structure nor function was affected under the effect of a given treatment may be related to bacterial community attributes such as functional plasticity or redundancy. Finally, Chapter V analyzes primary colonization and early community assembly in a south-oriented roadcut constructed in 2008 with slope greater than 40%, and where no management techniques were applied prior to our investigations. Abiotic filters such as water and nutrient availability, as well as the abundance of safe-sites for incoming seeds and propagules are the main constraints for plant community development in this kind of relatively harsh environment. Thus, in this chapter we describe an experiment in which we manipulated abiotic filters through the application of two different treatments, namely topsoil spreading and shallow tillage. We also set up control plots. Specifically, we analyzed the effect of treatments on seed availability (both soil seed bank and seed rain), seedling emergence, and seedling survival, as well as their effects on plant community attributes such as species richness and plant cover from 2010 to 2011. Results showed that topsoil spreading facilitated early community assembly by improving soil properties in term of nutrients, pH, soil stability, and texture. Additionally, it favored the emergence of higher species and improved seedling survival, with further consequences on plant community richness and cover. This treatment also increased species recruitment over two years after road construction. Although there was no effect of tillage on seed availability, emergence, and survival, by the end of the experiment in 2011, it appeared that this treatment had increased plant species richness on the upper-slope zone. In conclusion, our results showed that local soil conditions mainly related with organic carbon, pH, soil texture, and structure, were in large part responsible for the organization of soil communities and secondary succession processes on roadside sites undergoing restoration in our study area. Moreover, the application of conventional techniques oriented to overcome abiotic filters through the improvement of soil conditions did facilitate plant community reassembly. However, other management techniques that have been traditionally used in roadslope engineering and restoration, such as fertilization and mowing, significantly changed soil community structure, by increasing evenness and the abundance of certain groups related to low resource availability; they also affected functionality, by accelerating biodegradation of leaf litter. Given that changes in belowground communities may also induce changes in aboveground communities and vice versa, our results should be taken into account when selecting management techniques in order to ensure the maintenance of the ecosystem services that these ‘young’ or emerging ecosystems are expected to provide. Based on our results, conventional management techniques applied in roadside systems should be oriented to improve soil conditions after roadslope building, and aim to preserve and promote soil stability in the subsequent stages in order to ensure secondary succession processes and ecosystem development.