Can the restoration of natural vegetation be accelerated on the Chinese Loess Plateau? A study of the response of the leaf carbon isotope ratio of dominant species to changing soil carbon and nitrogen levels

For the heavily degraded ecosystem on the Chinese Loess Plateau, it would be of great significance if vegetation restoration could be accelerated anthropogenically. However, one major concern is that if the late successional species were planted or sown in degraded habitats, would they still be competitive in terms of some critical plant traits associated with specific habitats? Water use efficiency (WUE) is a major plant trait shaping the pattern of species turnover in vegetation secondary succession on the Loess Plateau. We hypothesized that if late successional stage plants could still hold a competitive advantage in terms of WUE, the prospects for an acceleration of succession by sowing these species in newly abandoned fields would be good. We tested this hypothesis by comparing the leaf C isotope ratio (delta C-13) value (a surrogate of WUE) of dominant species from different successional stages at given soil C and N levels. Results indicated that leaf delta C-13 of the two dominant species that co-dominated in the second and third stages were significantly more positive than that of the dominant species from the first stage regardless of changing soil C and N. Yet the dominant species from the climax stage is a C-4 grass assumed to have the highest WUE. In addition, increasing soil nutrition had no effects on leaf delta C-13 of two dominant species in the late successional stage, indicating that dominant species from the late successional stages could still have a competitive advantage in terms of WUE in soil C- and N-poor habitats. Therefore, from the perspective of plant WUE, there are great opportunities for ecosystem restoration by sowing both dominant species and other species that co-occur in late successional stages in newly abandoned fields, for the purpose of enhancing species diversity and optimising species composition.