Effects of soil erosion on the floristic composition of plant communities on marl in northeast Spain

This study explored the validity of three responses of vegetation to increa sed soil erosion: reduction of vegetation cover, number of species and redu ced substitution of species. 201 releves, including edaphic and geomorphological data, were surveyed in the intensely eroded Eocene marls of the Pre-pyrenees (NE Spain). Changes i n plant species' presence in releves from different degradation stages were compared. The level of vegetation degradation was defined as the total pha nerogam cover which, in the studied area, was correlated to the degree of s oil erosion. The considered trends were validated. Reduction of phanerogam cover and species number were gradual from low to high-eroded areas. Vegeta tion degradation explained 48% of the species number variance. In the later stages of degradation a significant substitution of species was not observ ed, only a lower frequency of occurrence of several species that appeared i n the whole set of releves. Through the process of degradation, 47% of spec ies displayed significantly reduced frequencies as degradation increased, n one showed a significant increase in frequency. It is concluded that there are no characteristic species in these plant com munities that survive in the severely eroded marls. Among the few species t hat had increased in frequency, most only colonised favourable micro-enviro nments.