Effects of competition, shade and soil conditions on the recolonization of three forest herbs in tree‐planted riparian zones

Abstract

AbstractQuestionsIn mesic forests, ecological filters due to past agricultural land use reduce forest herb recolonization. Is the recruitment of such species also limited in tree‐planted riparian zones by local filters such as competition, shade level and soil conditions?LocationTwo agricultural watersheds, southeastern Québec, Canada.MethodsThree herbs characteristic of natural riparian forests were selected for this study: one graminoid, Glyceria striata, and two ferns, Matteuccia struthiopteris and Onoclea sensibilis. Effects of shade level (75% vs 50%) and soil type (forest vs agricultural soil) on seedling emergence were evaluated in a seed‐sowing greenhouse experiment. In a 2‐yr transplant field experiment, seedling and sporophyte establishment was monitored in five natural riparian forests and five tree‐planted post‐agricultural riparian zones on microsites with understorey vegetation kept intact or cleared and on forest or agricultural soils. Using a priori contrasts, we assessed the influence of habitat type (natural riparian forests or tree‐planted riparian zones), competition and post‐agricultural soil type on transplant survival and growth.ResultsSeedling emergence tended to be higher on forest soils for G. striata while sporophyte emergence increased under 75% shade for M. struthiopteris. Transplanted seedlings and sporophytes of the three species survived and grew as well in tree‐planted riparian zones as in natural riparian forests. In tree‐planted riparian zones however, competing understorey vegetation reduced the survival and growth of G. striata and agricultural soil reduced the growth of M. struthiopteris. For O. sensibilis, only sporophyte survival was reduced by competition in tree‐planted riparian zones.ConclusionsPlanting trees in post‐agricultural riparian zones fosters establishment of forest herbs similar than those observed in natural riparian forests. Additional environmental filters specific to tree‐planted riparian zones, however, offset the positive influence of trees and limit recolonization of the three studied species. Considering the partial restoration success of establishment niches by tree planting, controlling spontaneous vegetation after tree planting is advised when conceivable and cost‐effective to promote the recolonization of environmentally‐limited forest herbs. Long‐term transplant experiments should be more often conducted to identify the ecological filters that reduce plant recolonization, and thereby design the most effective restoration strategies.