Ecological restoration of Wairio Wetland, Lake Wairarapa: The response of native wetland vegetation to eutrophication and revegetation management strategies

Abstract

Wetlands are highly productive ecosystems that support abundant native fauna and flora and provide many essential functions and services, for example water purification, erosion stabilisation, floodwater storage, groundwater recharge, peat accumulation and biogeochemical cycling. Despite the vast benefits they provide, worldwide loss and degradation of wetlands still continues, mainly due to agriculture, urban development, population growth and exploitation. Wetland disturbance can cause altered hydrological regimes, invasive species introduction, soil and water eutrophication, habitat fragmentation, and reductions in native fauna and flora leading to an overall reduced functionality. Ecological restoration is an active practice commonly undertaken in degraded wetlands to re-establish ecosystem functioning, and most commonly includes revegetation, reconstruction of hydrology, weed control, pest management, and native species reintroductions. Wairio Wetland located on the eastern shores of Lake Wairarapa forms a part of Wairarapa-Moana, the largest wetland complex in the lower North Island of New Zealand. Wairio Wetland was historically an abundant kahikatea swamp forest, with a diverse range of waterfowl, waders and freshwater fish. However, the wetland was adversely affected by draining from the Lower Wairarapa Valley Development Scheme (LWVDS) during the 1960’s and 1970’s, the construction of Parera Road, and invasion of willow tree seeds planted in the Wairarapa Valley for erosion control. Draining of the wetland, division from nearby lagoons and ponds, nitrogen and phosphorus build-up in waterways and exotic weed invasion all contributed to the poor state of the wetland. In 2005, Ducks Unlimited (DU) in conjunction with the Department of Conservation (DOC) and various members of the local community formed the Wairio Wetland Restoration Committee, with aims to manage and restore the wetland to its natural pre-settlement state. Restoration activities undertaken at the site that have included native tree planting, earthworks, weed control, pest management and fencing sections of the site to exclude cattle, have been met with mixed success over the years. This thesis reports on two studies undertaken at Wairio Wetland with aims to inform future restoration efforts at the site. The committee have proposed to divert nutrient rich water through Wairio Wetland to increase filtration and improve the water quality of Lake Wairarapa. However, the effects of nutrient loading on established plant communities at the site are unknown. Therefore the first study, conducted between December 2012 and May 2013 in Stage 2 of the wetland, examined the effects of fertiliser addition on biomass, structure and diversity of a wetland plant community. Different levels of phosphate and nitrate fertiliser were applied to 50 plots (4m2) of vegetation at the site with percent cover, and average height of respective species recorded every four to five weeks. Results showed that the addition of phosphorous and/or nitrogen had neither a positive nor negative effect on the plant community at Wairio with no significant changes in the 15 species recorded at the site. These results contrast other studies that have reported increases in biomass, reductions in biodiversity and common/introduced species outcompeting rare/native species. The short duration of the experiment and summer drought conditions may have obscured the above-ground visual responses of the plant community to nutrient addition; therefore further continuation of this experiment is advised. Previous low success rates of native tree plantings at Wairio Wetland have significantly hindered revegetation efforts at the site. Therefore the second study, conducted between July 2011 and January 2014 in Stage 3 of the wetland, further investigates the effects of various management treatments on establishment of native woody vegetation. The study involved monitoring 2,368 planted trees of eight native wetland tree/shrub species, including; Cordyline australis, Dacrycarpus dacridioides, Olearia virgata, Podocarpus totara, Coprosma robusta, Coprosma propinqua, Leptospermum scoparium, and Pittosporum tenuifolium. The trees were subjected to various planting treatments including the excavation or retention of topsoil, presence or absence of weedmats and presence or absence of nurse trees with spacing of 0.75m or 1.5m. Survival and growth of each tree was measured every six months over the 30 month experimental period. Results showed that interspecific competition and hydrology appeared to be the main processes influencing the establishment of native plantings at Wairio Wetland, with plant mortality greatest in the first year after planting. Waterlogging, in particular, was detrimental to establishment of all species at the site except D. dacridioides. Topsoil excavation and the planting of nurse trees at 1.5 m spacing was the most effective management treatment combination promoting survival of plantings at Wairio. However, the success of management treatments varied greatly between species at the site and had different impacts on plant growth. Topsoil excavation was beneficial to survival of D. dacridioides and C. robusta but detrimental to growth of C. australis, O. virgata, C. propinqua, P. tenuifolium and L. scoparium. The concurrent planting of nurse trees with focal trees was beneficial to the survival of D. dacridioides, growth of P. totara, and survival and growth of C. australis. The planting of nurse trees further apart at 1.5 m compared to 0.75 m had a positive effect on the survival of C. propinqua and P. tenuifolium, and survival and growth of L. scoparium. Weedmats were beneficial to survival of O. virgata and growth of L. scoparium but detrimental to growth of D. dacridioides. These management treatments can be used in future revegetation efforts at Wairio Wetland, and potentially in other wetland restoration projects throughout New Zealand.