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Wednesday, 16 January 2019 09:08

Nurse trees give saplands a headstart

Written by Stevie Waring
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Postgraduate students Stevie Waring, left, and Natascha Lewe working at Wairio.  Postgraduate students Stevie Waring, left, and Natascha Lewe working at Wairio. Dr Stephen Hartley

Stevie Waring has been looking into the benefits of nurse trees in wetlands restoration at Wairio.

Wetlands are ecologically important, biologically rich habitats that support a vast range of habitats for plants, animals and soil microbes.

Swamps are a category of wetland that are dominated by flood-tolerant trees, which thrive in soils that are nutrient-rich, but occasionally boggy or flooded.

Historically, New Zealand’s swamps were dominated by large podocarp trees,especially kahikatea andtotara, but these were targeted as valuable timber throughout the 19th century. Thereafter, land conversions for urban expansion and agriculture
continued to degrade swamps. A primary goal of wetland restoration is to revegetate sites with native trees.

However, up to 70 per cent of sapling trees can die within the first year of planting. The death of trees increases the financial costs of wetland restoration while reducing the benefits to nature and the morale of project participants. Put simply, dead trees reduce the feasibility of restoration of wetland swamps.
 
The survival and growth of sapling trees planted in wetland restoration projects depends on the interplay of many site factors including water levels, soil biology and fertility, wind exposure, herbivores and locations of nearby trees and plants.

Nearby plants can affect the earlier years of a sapling’s life. For example, highly competitive perennial grasses can shade and outcompete the saplings for light and soil resources. On the other
hand, having neighbours may benefit the sapling.
 
Nurse effects are positive interactions between plant species whereby an older ‘nurse’ tree facilitates the establishment of a sapling. Nurse trees can help saplings in a variety of ways, for example by sheltering a sapling from wind, frost, extreme heat, or intense sun. The deep root systems of nurse trees can draw up nutrient rich water from deeper soil, enhancing water availability to the shallow root systems of the saplings.
 
Nurse trees can also provide a source of beneficial fungal spores. Arbuscular mycorrhizal fungi (AMF) form mutually beneficial relationships with trees.

In exchange for sugars made by the plant in photosynthesis, AMF provide the plant with soil nutrients. Because sapling establishment is influenced by nurse trees over such a wide range of site conditions, nurse effects may be particularly important for trees.


Little is known about how nurse trees affect establishing saplings in wetland restoration. Can strategically planting saplings near nurse trees increase their survival and growth rates? Or are other
site factors such as soil moisture more important for sapling survival?

In my MSc thesis, in the Centre for Biodiversity and Restoration
Ecology at Victoria University, I sought to identify how nurse
trees improve establishment of two podocarp tree species
planted as part of a wetland swamp forest restoration.


In collaboration with Ducks Unlimited and the Department of Conservation, I monitored the survival and growth of kahikatea
(Dacrycarpus dacrydioides) and totara (Podocarpus totara) saplings planted with or without an established nurse trees at the Wairio wetland. The wetland connects a large, ecologically, culturally and recreationally important wetland complex that spans the Wairarapa valley. However, the increased nutrient inputs from livestock and chemical fertilisers and the introduction of pasture plant species have led to a weed-dominated environment.


During restoration, highly competitive perennial grasses can shade and outcompete the newly planted, slowgrowing podocarps. In addition, the complex hydrology of the site leads to species-specific spatial patterns of mortality and growth.

In my studies, I tested whether the presence of a woody nurse tree
(manuka, pittosporum or coprosma) influenced the survival and growth of kahikatea and totara over the critical first year of establishment. I monitored soil moisture, the pH and oxygen status of soils, root-available nutrients, and soil carbon content under saplings planted with and without nurses. I also quantified the abundance of spores of AMF fungi under all saplings.

Overall, I found that kahikatea saplings survived better than totara saplings, especially in very wet areas, but they were slower growing. Kahikatea did not benefit from nurse effects, rather it was frequently the only tree species capable of tolerating the wettest environments. In contrast, totara survived better than kahikatea in the drier areas of the wetland and grew on average 24cm taller in the presence of a nurse tree. The positive effect of a nurse tree on totara growth occurred regardless of the nurse species. Nurse trees increased the availability of mineral nutrients to totara. This suggests that the enhanced pull of deeper soil water by the large root systems of nurse trees increases the availability of mineral nutrients to totara saplings.
 
Finally, I found that the abundance of AMF spores varied with soil moisture, with the wettest areas having fewer spores. Moreover, the growth of both kahikatea and totara were positively related to numbers of AMF spores. These results suggest plants and  beneficial mycorrhizal fungi respond  similarly to patterns of wetland hydrology, and the availability of AMF inoculum is unlikely to limit the establishment of kahikatea and totara.  From this we have learnt that respecting the hydrology is key to revegetation 
for slow-growing podocarps; kahikatea will tolerate wetter soils, while totara should be planted in drier areas. If you want to establish totara in a restoration project, we recommend planting fastgrowing nurse trees three to five years in advance. 
 
At Wairio, nurse plants were four years old when the podocarps were planted, and totara tended to grow best where the nurse trees were largest. Planting around existing native trees and the development of expanding ‘tree islands’ over time should enhance podocarp survival during ecological restoration. • Stevie Waring and her supervisor, Dr Julie Deslippe, work at the Centre for Biodiversity and Restoration Ecology  at Victoria University, Wellington. They  wish to acknowledge the generous support of the Inshallah Trust.
Are you interested in helping with more research like this? 
 
We are looking to collaborate with Wairarapa farmers with restored or unmanaged wetlands to better understand the benefits  that wetlands provide. Please visit  our website, Wetlands for People and Place, to learn more. Contact Julie Deslippe at Julie.Deslippe@ vuw.ac.nz or Stephanie Tomscha at This email address is being protected from spambots. You need JavaScript enabled to view it. to get involved.
 

 

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