Active wetland restoration kickstarts vegetation establishment, but natural development promotes greater plant diversity

Samenvatting

Rapid vegetation establishment is an important goal for many freshwater wetland, coastal and terrestrial restoration projects. To accelerate the restoration process, managers often introduce target plant species, potentially generating priority effects that can lead to either divergence or convergence of community compositions over time, depending on the local biotic and abiotic conditions. These alternative trajectories pose the question of whether active restoration by initial steering of plant community composition results in a different long-term community assembly when compared to restoration via natural development. Here, we tested vegetation development subject to active restoration or natural development strategies with plant introduction and protection against herbivory by geese (Anser anser). This was facilitated by a 1300 ha restoration project in which new, bare wetland habitat was created from soft sediments (“Marker Wadden”, the Netherlands). We studied vegetation cover, plant community composition and soil elevation in a full-factorial experiment under grazed, ungrazed, planted and unplanted conditions for 6 years. Without herbivory by geese, the planting of reed (Phragmites australis) accelerated vegetation establishment and resulted in greater vegetation cover. However, cover was dominated by reed, and plant diversity remained low. With herbivory, grazing on reed increased plant diversity. With herbivory and without reed planting, vegetation established slower. However, after 6 years, plant cover became similar to the treatment with planted reed rhizomes in the absence of herbivory—albeit with higher plant community diversity. Across all treatments, locations with higher vegetation cover decelerated soil subsidence, which maintained important abiotic conditions for the developing food web. Synthesis and applications: Active steering of wetland restoration by introducing target species and preventing herbivory accelerated vegetation development, while natural development took longer. However, natural development resulted in more diverse plant community compositions over time. These results imply that restoration projects face a trade-off between actively stimulating vegetation cover to facilitate food web development and prevent soil subsidence, or waiting for more diverse plant communities to develop naturally.