Rapid recovery of the vegetation diversity and soil fertility after cropland abandonment in a semiarid oak ecosystem: An approach based on plant functional groups

In many parts of the world, including the semi-arid Mediterranean areas, land abandonment has led to significant changes in vegetation composition, plant diversity as well as soil fertility. In this study, new approaches were presented on how the understory plant functional group diversity and composition changed after land abandonment in Zagros oak forests (Quercus brantii Lindl.) in western Iran. In addition, the impact of land-use change on main soil properties were investigated. The diversity and nutrient status of the three stages of the secondary succession of abandoned agroforestry systems were compared: croplands after a short period of abandonment (similar to 5 years), croplands after a long period of abandonment (similar to 15 years) and oak forests representing the 'climax' stage. The plant species were classified in four functional groups (annual forb, annual grass, perennial forb, and perennial grass). The diversity indices were computed for each group and each stage. Results showed that the Shannon-Wiener diversity and Margalef richness of all functional groups significantly increased with time from the shortly abandoned croplands to the forest. Shortly after abandonment, annuals (especially annual forbs) were dominant whereas the abundance of the perennials increased after 15 years of abandonment and in the 'climax' forest. The soil content in lime decreased along the successional stages (from 52.7% to 26.4%) and was strongly negatively correlated with the diversity of the perennial forbs. In contrast, the soil total nitrogen and aggregate stability increased with succession (respectively from 0.04% to 0.19% and from 0.49 mm to 0.92 mm) and were strongly positively correlated with the diversity of this group. Recovery in soil fertility (and in particular, total nitrogen), provided suitable conditions for the establishment of a wide range of plant functional groups, which in turn increased the species diversity. The relatively fast recovery of soil fertility and plant diversity of abandoned agricultural lands indicated that the reestablishment of the forest vegetation could rely mostly on natural processes.