Farm Forestry New Zealand

Life after wilding pine control

Thomas Paul, Carol Rolando, Matt Scott, Fiona Dean and John Meredith, New Zealand Tree Grower November 2020.

 Spreading from early forestry, shelterbelt and farm planting, around two million hectares in New Zealand are estimated to be infested, mainly with contorta pine.  Indigenous grasslands, scrublands and even forested land are under threat, especially in the South Island high country, damaging the ecology and economy of these ecosystems. Intensive control operations over the next decade aim to limit future wilding conifer spread, reduce the extent of current infestations and the long-term effect on the environment.

The National Wilding Conifer Control Programme is using mainly aerial spraying as one of its main management methods. The ‘brew’, which includes the herbicides triclopyr, dicamba, picloram and aminopyralid, is effective for killing dense stands of wildings. However, poisoning the pines is just one part of the management system. A plan to prevent reinfestation following intensive control is also essential to return landscapes to a more desirable and potentially invasion resilient land-cover, an aim which is integral to the National Wilding Conifer Management Strategy.

Natural revegetation

Scion researchers have looked at wilding pine sites which had been sprayed within the previous 10 years to put together a post-control revegetation sequence. They found the first re-colonisers tended to be exotic grasses.  In some cases, plants such as foxglove and wall lettuce followed a few years later. Generally, the ground was fully covered after nine years or so following control.  Few trees and shrubs grew, and indigenous species were mostly absent.

A return to grasslands is good for land owners who want the site to be more productive. However, exotic grasslands are low in native plant diversity, and are susceptible to re-invasion. Intervention is likely to be necessary to achieve a cover of indigenous trees and shrubs.

Planned revegetation

The most cost-effective restoration option is usually to let natural regeneration take its course, but this option is only really viable if there is a nearby seed source such as native tussocks or forest remnants. In the absence of local seed reserves, alternatives include sowing the seed of native plants or planting seedlings, often with a high proportion of faster growing shrubs, to provide a nurse crop along with a lower portion of tree species.

A further factor to consider when planning to restore sites which have been sprayed to control wilding pines is the possibility of herbicides persisting in the environment and how that could affect vegetation regeneration.

Herbicide persistence

Researchers looked at the persistence of the herbicides triclopyr, dicamba and picloram at sites in the MacKenzie Basin Southland and Hawke’s Bay following aerial spraying. These sites were sprayed as part of the National Wilding Conifer Management Programme between January 2018 and February 2019.

Soil levels of herbicides were below detection levels within one to two years. However, all three herbicides were still detectable in the forest floor litter at low levels at all sites after 18 to 24 months. The forest floor litter was mainly fallen needles.

The persistence of herbicides in the forest floor litter was found to affect woody vegetation regeneration beneath controlled wilding stands. The researchers collected soil and forest floor litter from the sprayed MacKenzie Basin site and a nearby untreated site at periods of one month, six months and 16 months after spraying. Then they attempted to germinate lodgepole pine and indigenous plant seeds in the collected samples.  The indigenous plants were red tussock, mountain beech, black matipo, manuka and twiggy coprosma.

Herbicides in the treated samples significantly affected the germination, plant form, root and shoot growth and the survival of lodgepole or contorta pine.  Even 16 months after spraying, the levels of herbicide were still high enough to cause needle curling, root deformities and lower seedling biomass.

The indigenous plant seeds also fared poorly. Four months after spraying, germination was reduced and high numbers of germinated seedlings across all the species died. A further year later herbicide levels were lower and did not affect the germination or mortality of native seedlings. However, some symptoms of toxicity could still be seen in manuka and twiggy coprosma plants.

Recommendations for revegetation

The best time for site restoration with indigenous species seems to be between two and five years after spraying to control the wilding pines. Residual levels of herbicide in forest floor litter negatively affect the germination, survival and growth of lodgepole pines for at least a minimum of 18 months after spraying.

Unfortunately, the germination and development of indigenous species seedlings is also likely to be affected.  Work looking at seeding with mountain beech and manuka showed that establishment was most likely to take place in stands controlled in the previous two to five years. It is recommended that the forest floor litter should be removed if using seeds less than three years after control, or seed sowing should be delayed until three years have passed. Depending on how much ground cover has developed, spot spraying with shortlived herbicides such as glyphosate may be necessary.

An alternative could be to plant native seedlings as part of a wider restoration programme strategically planting future seed trees across controlled sites.  Seedlings should be able to be planted after two years because herbicide levels in the soils were very low and not expected to cause problems. This would be a more costly intervention and has not yet been tested as an establishment practice.

A wilding-free future

A management plan to prevent wilding pine reinfestations is an essential part of effective control.  This makes sure the initial investment in control is not wasted, and it also helps protect native ecosystems, plant species and iconic landscapes valued by local communities and tourists. An additional benefit for land owners is that a management plan will help with being a good neighbour, preventing wilding pine spread and
possibly reducing future control costs.

Acknowledgements
This work has been funded by the New Zealand Wilding Conifer Control Group and through the Sustainable Farming Fund, Ministry for Primary Industries.