The factors that influence the reestablishment of Podocarpus totara (totara) and Dacrycarpus dacrydioides (kahikatea) in a freshwater New Zealand wetland

Abstract

Wetlands are productive transitional lands between terrestrial and aquatic systems. They provide social, economic and cultural values, while providing valuable services such as carbon storage, water purification, flood abatement and biodiversity support. While wetlands only cover ~3% of the globe, they contribute up to 40% of these global renewable ecosystem services. Worldwide degradation of wetlands through urbanisation, conversion to agriculture and flood management schemes has resulted in a 50% loss of the worlds original wetlands, with New Zealand being one of the most extreme examples of this with >90% of the original extent of wetlands being lost.

Wetlands unique hydrology results in distinct plant zonation and community composition and seedling survival is the primary factor that influences stand structure and community composition. However, restoring degraded wetlands is challenging because the alterations to the hydrology through filling or diverting water will impact the effect of physical, chemical and biotic environmental variables on native plant establishment. The use of facilitation in restoration through successional planting using nurse trees, and arbuscular mycorrhizal fungi (AMF) is common in restoration, however research into the effectiveness of these techniques in wetland systems is lacking.

This thesis is comprised of two studies with aims to determine the factors that had the biggest impact on the survival and growth of kahikatea and totara in Wairio wetland and inform future restoration. Wairio wetland has large isolated remnant kahikatea trees, so my first study focused on how these established trees and a connection to fungal hyphae networks influenced the survival and growth of newly planted saplings. One sapling of each species was planted at the dripline of the remnant tree, and another sapling of each species was planted 2 metres from that point. Out of the 16 remnant kahikatea trees used, 8 were ‘disturbance’ plots where saplings were planted in 35μm mesh bags that excluded roots but fungal hyphae could penetrate, in slotted pots which were turned every 3 months. In the 8 remaining ‘undisturbed’ plots, saplings were planted into the ground. I assessed the influence of distance and disturbance on sapling survival and growth using a chi-square test of independence and general linear models.

Results showed that kahikatea trees survived better than totara overall. The survival of totara was significantly reliant on a close proximity to the remnant kahikatea, and a connection with mycorrhizal networks. Kahikatea had greater biomass than totara, however they suffered strong conspecific competition with the remnant trees, with kahikatea saplings planted at the dripline having 51.28g greater biomass with regular disturbance of AMF mycelium. Kahikatea trees are light demanding species, and therefore growing under the canopy of a parent tree has a negative impact on kahikatea saplings growth.

Knowing that kahikatea and totara trees respond differently to nurse trees and AMF, my second study focused on how nurse effects and AMF association changes with chemical, physical and environmental stressors. The survival and growth of 5-year-old kahikatea and totara trees, with or without a nurse across 10 blocks in Wairio wetland were analysed. At each tree, soil moisture and root available nutrients were measured and soil cores were taken to determine gravimetric soil moisture, reduction-oxidation reaction (redox), pH, soil carbon content and I counted the presence of AMF spores in two size classes. I assessed tree survival and growth against these variables using a chi-squared test of independence and general linear models. The results of this study showed that kahikatea survived better than totara trees in the very wet blocks, where no other species survived. Totara trees survived in the upland sites of the wetland and had better growth than kahikatea trees. Moreover, totara trees grew 24cm taller in the presence of a nurse tree, and were strongly positively associated with spore number. Nurse trees further benefitted totara trees by increasing plant available soil nitrate and potassium by almost double.