Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Wilding pines – the exotic invasion

Wilding conifers, also known as wilding pines, are invasive tree species in New Zealand’s high country. Each year public agencies spend millions of dollars trying to control the spread, while some individual farmers spending hundreds of thousands of dollars.

Wilding conifers are a problem primarily in the Marlborough Sounds, the South Island high country and the central plateau of the North Island, but are also invading natural habitats in Otago.

Wilding pines at Craigieburn.Because these trees grow in dense stands, and are not able to be managed as a plantation, they have no economic value. Instead, wildings can reduce the value of managed pasture, displace native biodiversity, reduce water availability, and alter the character of the landscape.

While the impacts of wildings on productive land and on conservation values are well documented, control programmes present a number of challenges. As well as the sheer scale of the problem (more than 1.5 million hectares are affected – the total area has more than tripled since 2000) some of the areas impacted are remote and difficult to access, making control a dangerous and costly exercise.

There are ten main species that have become wildings including Lodgepole pine (Pinus contorta), Radiata pine (Pinus radiata), Douglas fir  (Pseudotsuga species) and European larch (Larix decidua).

Understanding the issues

In the past year we have undertaken fundamental research to improve understanding of wildings and their wider role in ecological systems. This includes publishing papers on co-invasions – understanding what they are, knowing where they occur and the biological interactions involved.

In the case of wildings, a co-invasion occurs when more than one species interact to assist the spread of the weed, and our research has determined that wilding conifers co-invade with invasive mycorrhizal fungi (i.e., beneficial fungi found on tree roots), and these fungi in turn are eaten and dispersed by wild mammals including deer and possums. This set of positive interactions amongst invasive species is known as ‘invasional meltdown’ whereby multiple non-native species benefit one another.

Until now land management has traditionally focussed on one of those factors – weeds or animals – but by managing them together in the same setting land managers may have the opportunity to get more ‘bang for their buck’ when dealing with limited resources. In other words, controlling both weeds and pests in the same area may provide more benefits or than carrying out these management activities independently because this overcomes ‘invasional meltdown’.

All this information provides land managers with sound information on which to base possible future management options.

In the past year we have also assisted in the development of new technology that has involved developing a suitable cocktail of herbicides that can be sprayed onto individual trees through a specially designed gun or wand.

We sub-contracted this work to Scion as part of our Beating Weeds Programme.

Scion’s herbicide combines the herbicide triclopyr and paraffin oil, has been field tested, and is proving effective - up to 85% mortality has been seen in trees treated this way.

Another advantage of this herbicide is that it can be applied outside of the growing season of the pines. The key to applying it is in the specially designed gun with a 1.5-m wand that can be operated from within a helicopter. The wand enables operators to spray the crown of the tree thus minimising the effects on surrounding vegetation. By eliminating the need to fell the trees, there is no physical disturbance to the surrounding vegetation, therefore reducing the chances of secondary weeds moving in such as gorse or broom.

We have worked with groups as diverse as the Department of Conservation, Scion and community control groups to address the wilding problem and more specifically, highlighting how the research and associated knowledge can be more effectively utilised back into the management of wildings.

More cost-effective

The work undertaken by Scion has seen them develop the aerially-applied herbicide into a highly cost-effective method of control. The Department of Conservation is already successfully using the new spraying system and early studies suggest it is about 2-5 times more cost-efficient than traditional approaches.

For groups like the Waimakariri Ecological Landscape Restoration Alliance (WELRA) our assistance is more practical. WELRA is a not-for-profit incorporated society devoted to controlling wilding tree spread in the upper Waimakariri River in Canterbury. The alliance is a formal amalgamation of the key parties involved in wilding tree control efforts and includes recreation groups, Transpower, Transit, local farmers, Ngai Tahu and local councils.

Groups like WELRA need to know if they’re effectively using their funding to manage the area and that every time they go back to undertake control work that there’s fewer trees. We are beginning to apply our expertise in experimental design and monitoring to helping them achieve that goal by providing information on WELRA’s work that can provide credible evidence that progress is being made toward the desired outcomes.  More generally, anyone managing wilding conifers, and their funders, need to know if their considerable efforts are halting the invasion, reversing the invasion or whether the landscape is being reversed to its traditional form.