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

Abstract

The world’s wetlands are known for being highly productive environments and supporting significant numbers of fauna and flora species that rely on the wetland’s primary productions for 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 of wetlands and their ecological importance, degradation of wetlands continues, mainly due to anthropogenic activities. Wetland restoration involves reconstructing natural sites that have been degraded or completely lost and re-establishing their functions and values as vital ecosystems. Important restoration components include control of invasive weeds, emphasis on 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 and supports a number of native flora and fauna, of national and international importance. Wairio wetland has been destroyed by the effects of partial draining, unnatural hydrological control, clearing of native forest, construction of Parera Road separating once joined wetlands and the establishment of invasive willow trees and agricultural grasses. The co-management by the Department of Conservation and Ducks Unlimited, commenced in 2005, has begun a positive shift for the wetland. However, issues still remain due to the majority of the wetland still being used for farming, so there is no continuity between the three fenced restoration stages; artificial hydrological flow and water storage; and established willow trees along the wetland boundary. Most ephemeral wetland vegetation displays a strong pattern of zonation, through a sequence from open water to dry land, which is correlated in some way with the duration and periodicity of water inundation. This hydrosere reflects differences in the degree of adaptation to aquatic life of different plant species. Two studies are reported here, conducted in two areas at the Wairio wetland over two desiccation periods. The first study conducted during 2010/2011 at stage one, focused on determining the environmental conditions of peak abundance and limits to distribution of key native and exotic plant species along an environmental (hydrological) gradient. The second study, conducted in 2012 at stage three, further investigated the effect of topsoil removal on the plant community and was a comparison study with the initial study at stage one. Results indicated that the Wairio wetland plant communities display strong zonation patterns progressing from aquatic species, 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 were common pasture grasses, especially yorkshire fog (Holcus lanatus), brown top (Agrostis capillaris) and tall fescue (Schedonorus arundinaceus) as well as purple clover (Trifolium pratense) 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 the high soil moisture areas leads to a more native dominated plant community, with the dominant species being water plantain (Alisma spp.) and Isolepis prolifera, but scraping in relatively 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 by anthropogenic activities since the 1960s. In 2005, Ducks Unlimited and the Department of Conservation signed a Land Management Agreement where Ducks Unlimited would commence the restoration of the wetland. Survival of trees planted during the first few years was variable. Here, I report on the design and monitoring of a large scale field experiment involving the planting of around 2,500 trees of eight native wetland tree species Dacrycarpus dacrydioides, Podocarpus totara, Cordyline australis, Olearia virgata, Pittosporum tenuifolium, Coprosma robusta, Coprosma propinqua and Leptospermum scoparium. The trees were subjected to different methods of site preparation and aftercare to determine the best combination of treatments for successful establishment of tree saplings. Treatments included 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 showed survival of O. virgata and P. totara was influenced by surface water, but few immediate effects of treatments upon growth rates. Olearia virgata, however, grew best in wet areas that had been scraped free of topsoil or drier areas that had not been scraped. Monitoring over the next 18 months will give us a better understanding of which is the most costeffective combination of treatments. Early indications suggest high level survival under all treatments.