Vegetation and soil development of restored freshwater wetlands in agricultural landscapes of northern New Zealand

Abstract

Wetland restoration has become more common in recent decades due to the increased recognition of the valuable services that wetlands provide, and to address historical wetland losses. In the north Auckland region of New Zealand, much of the wetland restoration is occurring in agricultural landscapes but there is a lack of understanding of the development of these restored wetlands following initial restoration actions. In this research I used a chronosequence approach to compare the soil and vegetation characteristics of 13 restored swamp and marsh wetlands (ranging from 3 – 13 years since restoration) to degraded and undisturbed wetlands. To characterise the vegetation at each site I measured standing plant biomass and plant species richness while vegetation community composition was analysed using ordination. Soil samples were retrieved from each site to evaluate soil bulk density, soil pH, soil organic carbon (SOC) stock, organic layer biomass, and soil texture. At the sites I assessed, planting of native species during restoration caused an immediate change in vegetation community composition, increasing similarity of restored wetlands to undisturbed wetlands in composition. However, after the immediate benefit of restoration ongoing change stalls, with the restored vegetation communities failing to converge on that of the undisturbed wetlands. The trajectory of the vegetation communities in the restored wetlands may be impacted by seed dispersal limitation slowing a continued increase in compositional similarity with undisturbed wetlands after restoration. Rapid changes also occurred in standing plant biomass and soil bulk density after restoration, but, conversely, organic layer biomass, SOC stock, soil pH, and soil texture were similar across degraded, restored and undisturbed wetland sites. The poor accumulation of organic matter and SOC in the restored and undisturbed wetlands may be due to high rates of decomposition caused by nutrient enrichment and alterations to the hydrological regime arising from historic land-use change in the landscapes surrounding these wetlands. Due to the importance of improving soil condition in wetlands for processes such as carbon storage, legal processes that require the protection and restoration of wetlands need to incorporate and emphasise the restoration of soil. Further assessments of older restored wetlands would help to determine the trajectory of vegetation communities older than 13 years, providing more insight into the potential impact that seed dispersal limitation is having. It is also important that wetland restoration practitioners consider the potential of shifting baseline syndrome when selecting a reference ecosystem for restoration projects.