Season and plant life history stage in revegetation influence competition of foundation species, subordinate species, and weeds in a reclaimed grassland.
River embankments offer potential semi-natural habitats for grassland species, but their large-scale reconstruction leads to vast vegetation loss. To restore both ecosystem services and biodiversity on reconstructed river embankments, we tried to achieve rapid cover by native grasses and to enhance the richness of species typical of semi-natural grasslands. We conducted a 4-year experiment to clarify whether season and plant life history stage in revegetation determine competitive interactions among foundation species (Imperata cylindrica), subordinate species (eight species typical of semi-natural grasslands), and weeds (any other species). Square frames measuring 1 m × 1 m × 30 cm depth were filled with soil used for the construction of river embankments. Two I. cylindrica life history stage modalities (seeds, seedlings) and a control were established at three timings (March, May, July) with three replications. Fifty seeds of each subordinate species were sown in each plot. In the March and May timings, annual weeds were dominant in year 1. In the July timing, I. cylindrica became dominant because high temperatures inhibited the germination of weeds and several subordinate species. Planting seedlings in any season was successful in achieving dominance of I. cylindrica, but only in the absence of rhizomes of invasive Solidago altissima. Many individuals of subordinate species were recorded in the May timing. Subordinate species with higher competitive ability or with less niche overlap with I. cylindrica performed better. Even so, overgrowth by I. cylindrica or weeds generally emerged, leading to low survival rates of subordinate species in all treatments. Season or life history stage had no effect on richness. Since rapid coverage of river embankments by native grasses is essential, planting in July is desirable for restoration. A shortage of dominant weeds in year 1 and relatively slow dominance by sown I. cylindrica led to less severe competitive exclusion of subordinate species. Thus, sowing in July would enhance the dominance of I. cylindrica and the establishment of subordinate species. Since the germination rate in July sowing was the smallest, further studies of successful introduction methods in the warm season are needed.