Ecological Restoration of the Wairio Wetland, Lake Wairarapa: Water Table Relationships and Cost-Benefit Analysis of Restoration Strategies

摘要

The world’s wetlands are known for being highly productive environments and supportingsignificant numbers of fauna and flora species that rely on the wetland’s primary productionsfor survival. However, they were historically used by humans for hunting and fishing,wetlands were considered wastelands, best used when drained and filled for agricultural,industrial and residential development. Despite now having a greater understanding ofwetlands and their ecological importance, degradation of wetlands continues, mainly due toanthropogenic activities. Wetland restoration involves reconstructing natural sites that havebeen degraded or completely lost and re-establishing their functions and values as vitalecosystems. Important restoration components include control of invasive weeds, emphasison the presence of locally native species and restoration of the hydrological component.The Wairio wetland is part of the largest wetland complex in the southern North Island andsupports a number of native flora and fauna, of national and international importance. Wairiowetland has been destroyed by the effects of partial draining, unnatural hydrological control,clearing of native forest, construction of Parera Road separating once joined wetlands andthe establishment of invasive willow trees and agricultural grasses. The co-management bythe Department of Conservation and Ducks Unlimited, commenced in 2005, has begun apositive shift for the wetland. However, issues still remain due to the majority of the wetlandstill being used for farming, so there is no continuity between the three fenced restorationstages; artificial hydrological flow and water storage; and established willow trees along thewetland boundary.Most ephemeral wetland vegetation displays a strong pattern of zonation, through asequence from open water to dry land, which is correlated in some way with the duration andperiodicity of water inundation. This hydrosere reflects differences in the degree ofadaptation to aquatic life of different plant species. Two studies are reported here, conductedin two areas at the Wairio wetland over two desiccation periods. The first study conductedduring 2010/2011 at stage one, focused on determining the environmental conditions ofpeak abundance and limits to distribution of key native and exotic plant species along anenvironmental (hydrological) gradient. The second study, conducted in 2012 at stage three,further investigated the effect of topsoil removal on the plant community and was acomparison study with the initial study at stage one. Results indicated that the Wairiowetland plant communities display strong zonation patterns progressing from aquaticspecies, to turf communities, to exotic grass species. Over the two desiccation periods studied it was found that the introduced species most abundant in low soil moisture werecommon pasture grasses, especially yorkshire fog (Holcus lanatus), brown top (Agrostiscapillaris) and tall fescue (Schedonorus arundinaceus) as well as purple clover (Trifoliumpratense) and the high soil moisture invasive competitor was water plantain (Alisma spp.).We also found that topsoil excavation impacts the plant community; topsoil scraping in thehigh soil moisture areas leads to a more native dominated plant community, with thedominant species being water plantain (Alisma spp.) and Isolepis prolifera, but scraping inrelatively low soil moisture areas encourages the exotic grass weed species to grow.Wairio wetland on the Eastern shore of Lake Wairarapa has been adversely affected byanthropogenic activities since the 1960s. In 2005, Ducks Unlimited and the Department ofConservation signed a Land Management Agreement where Ducks Unlimited wouldcommence the restoration of the wetland. Survival of trees planted during the first few yearswas variable. Here, I report on the design and monitoring of a large scale field experimentinvolving the planting of around 2,500 trees of eight native wetland tree species Dacrycarpusdacrydioides, Podocarpus totara, Cordyline australis, Olearia virgata, Pittosporumtenuifolium, Coprosma robusta, Coprosma propinqua and Leptospermum scoparium. Thetrees were subjected to different methods of site preparation and aftercare to determine thebest combination of treatments for successful establishment of tree saplings. Treatmentsincluded the use or non-use of topsoil excavation, release spraying, weedmats, nurse trees(with two combinations of species) and different spacing between the nurse species.Survival and growth over the first six months was monitored. Preliminarily results showedsurvival of O. virgata and P. totara was influenced by surface water, but few immediateeffects of treatments upon growth rates. Olearia virgata, however, grew best in wet areasthat had been scraped free of topsoil or drier areas that had not been scraped. Monitoringover the next 18 months will give us a better understanding of which is the most costeffectivecombination of treatments. Early indications suggest high level survival under alltreatments.