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Freshwater yields and why wildings matter

Date: 28th April 2023

Water is the lifeforce that powers the land, whether it be pastoral or horticulture farming, forestry, hydro generation, towns and of course our natural environments. The presence of large wilding pine and conifer forests can severely impact on the volume of water in catchments.

“Wilding conifers reduce surface flows and aquifer recharge in water-sensitive catchments. Less flow means less water for farmers’ irrigation needs, hydroelectric generation, or outdoor recreation use. And less water for those plants and animals that live in and around our rivers. This reduction of water due to wilding conifers spread impacts on water availability for irrigation and electricity generation in water sensitive areas like Southland, Otago, Canterbury and Marlborough.

Wildings – Jollies Pass Road – Hanmer April 2023

Climate change is likely to increase wilding conifer spread and reduce water yields in regions that are experiencing hot and drier, and water shortages. As well, water quality and water quantity are required to sustain native plants, animals and ecosystems. Controlling wilding conifers to maintain water yield is a compelling argument”. This from the 2018 Cost Benefit Analysis (CBA) for Phase 2 of the Wilding Pine Programme produced by the Sapere Research Group.

The Sapere report indicates that impact on water yields is significant. The CBA reviewed various studies. Where pastureland becomes covered in wilding conifers, this reduction in annual water yield has been shown to range from 30-81% (with the upper end of that range recorded in dry South Island sites). For example, one study at Glendhu Forest in Otago measured the effects of converting tall tussock grassland to radiata pine. Results showed a water yield reduction of 40-45%, 22 years after the trees were planted. It is expected wilding conifers will have similar impacts on water yields compared to planted trees.

At the time of writing the Sapere report it was acknowledged that most wilding conifer infestations are currently sparse and do not have a significant impact on water yields. As wilding conifers grow and spread, they will form dense stands and there will be increasing impacts on water yields over time. Travelling around the country it’s obvious this has changed. The Branch-Leatham catchment in Marlborough is such an example at scale with its expansive pure Douglas fir dominated wilding tree forest from the valley floor to the mountain tops and beyond.

The extent of the problem in places like the Branch Leatham with no end in sight prompted Paul Williams from the South Marlborough Landscape Restoration Trust to write a submission on freshwater management for the Marlborough Environment Plan as it relates to National Policy Statement for Freshwater Management 2020. The submission highlights concern that invasions of wilding conifers already will be affecting water yields and water quality from infested catchments and if left unchecked, existing infestations will continue to spread from ‘seed rain’ at an exponential rate with consequential dramatic effects on downstream water quality. Vulnerable south Marlborough rivers include the Clarence, Awatere and Wairau.

The Branch Leatham catchment extends over some 52,000ha of coning trees and much of that is at high elevations with the infestation covering indigenous alpine rock lands and also smothering indigenous tussock land, scrublands and bush at lower altitudes.

Paul references the Glendhu study but also two more recent studies which predict the effects of water yield reduction should the Mackenzie basin become dominated by wilding conifers. One study indicated a reduction of 53.5 cumecs of water in the Waitaki (S.M. Thomson, NIWA) and the other estimated the reduction at 50.8 cumecs (Mason 2016, Landcare Research. This has major implications for power generation let alone the many native plants and animals that rely on minimum flows in these waterways and equates to hundreds of millions of dollars in lost hydro generation.

Peter Hore from Glenshee station in the Maniototo area of Cental Otago shares these concerns. Peter relies on irrigation for some of his farming operation and has lived in the area all his life. He has watched wilding trees emerge and dominate landscapes over the last 40 years or so years. He says he has seen the trees take up to 70% of the water out of a catchment and is concerned they could dry up all the waterways if not removed – you can’t farm without water.

It is a particular issue in periods of low flows – the Sapere CBA suggests 16% was the average reduction in water yield based on work done by Scion in 2015 based on three South Island catchments. Scion noted this assumption was possibly conservative because the reduction in water yield from dense wilding conifer stands could be higher than from afforestation as wilding stands might have a much higher interception effect, because of their rougher canopy surface. In addition, wilding conifers might occur in the far upper reaches of catchments and therefore can affect low flow yields more significantly than mid-altitude afforestation’s.

Bottom line is wilding pines and conifers, and water catchments don’t mix whichever way you look at it. We have moved on from past use of these trees for erosion protection and land stabilization in the top of catchments. We endure the costly legacy of wildings from these plantings in many areas today.

Lake Tennyson on a still clear April 2023 day

Although there are some people that say any tree is a good tree in these environments as they sequester carbon and protect erosion prone hillsides – our natural environments are under siege and our unique native species of plants and animals are still in decline and even in 2023 with all our knowledge and experience some are close to extinction.

Removing ALL invasive exotic species from these places is key to changing this as is ensuring that the life force – water – that powers these places is protected in a manner where sufficient flows and yields are maintained year-round. For wilding pines and conifers that means permanent removal and continued proactive vigilance to stop them re-establishing. I for one have a sinking feeling when I visit places like Jollies Pass (refer above photo) but feel uplifted and privileged to be part of a project that is keeping places like Lake Tennyson in the St James Valley as they are – iconic, breathtaking and worthy of protecting for the role they play in our natural world but also to ensure future generations can enjoy them in this state forever.


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