Succession in plant communities and ecological filters for three forest species in post-agricultural riparian zones after tree planting
The recolonization of plant communities in restored ecosystems corresponds to a bottle-neck process structured around three main successive steps, i.e. the dispersal of species to restored sites, their germination and the establishment of reproductive populations. Seed sowing and transplant experiments can help to determine the ecological filters conditioning these steps. We assessed the effect of light availability, competition and soil on the germination, survival and growth of three forests species, Glyceria striata (Lam.) Hitchc., Matteuccia struthiopteris (L.) Tod. and Onoclea sensibilis L., in tree-planted riparian zones within agricultural landscapes. Seedlings (360 per species) were transplanted in five riparian zones planted with trees since 15 years and five natural riparian forests within two watersheds of southeastern Québec (Canada). We manipulated soil (forest or agricultural soil) and competition (elimination of vegetation or not) in 90 subplots. Their survival and growth were monitored after two years and analyzed using a priori contrasts. Their germination on forest or agricultural soils under two light regimes (representing natural forests and tree-planted sites) was assessed in greenhouse and analyzed using ANOVA. These results were compared to the plant succession followed by riparian communities along a chronosequence from 3 to 17 years after tree planting obtained by PRC.
M. struthiopteris had higher germination under low light availability (p=0.0507) and higher survival (p=0.0506) and growth (p=0.0681) on forest soil, pointing out an ecological filtering by light availability at the germination step and by soil at the seedling step. G. striata which germinated better on forest soil (p=0.0568), but survived (p=0.0235) and grew (p=0.0112) better in plots without competition, seemed influenced mainly by soils at the germination step and by competition at the seedling step. The germination, the survival and the growth of O. sensibilis was not affected by soil, competition or light availability suggesting dispersal limitation for the recolonization of this species. No difference in survival and growth were found for these three species between natural riparian forests and tree-planted riparian zones showing that tree planting re-establish suitable light availability for forest species. This result was consistent with the sigmoidal pattern of plant succession followed by understory riparian communities after tree planting. While species composition remained stable during 14 years after planting, they then abruptly shifted to become similar to natural riparian forests after 17 years. Identifying the critical life-cycle steps and the ecological filters that lower species recolonization can provide meaningful information to design ecosystems restoration.