WILDING PINES A GROWING PROBLEM

Forest and Bird, Volume 21, Issue 2, 1 May 1990, Page 38

WILDING PINES A GROWING PROBLEM

by

Mike Harding,

Forest and Bird South Island Conservation Officer

HILE NEW ZEALANDERS in their thousands protest at the logging of rainforests, and internationally the plea is heard for extensive reafforestation to combat the greenhouse effect, calling for the felling of trees may sound like a cry from the wilderness. And indeed in a sense it is, because scattered throughout the country are forests that pose a significant threat to wilderness areas and to native plant communities. These are forests that no one wants and in some case forests that are costing the country money in lost agricultural production. The spread of self-sown, or wilding, exotic trees affects something like 10,000 hectares in the South Island and 30,000 hectares in the North Island. From the open tops of Mt Tarawera to the hillsides around Queenstown, and at dozens of other locations in between, are self-sown forests standing stark on the open landscapes. In places it may be only the odd pine that has seeded from a shelter belt or roadside planting, but in others it involves thousands of wilding trees at densities of over 100 trees per hectare gradually advancing over areas of protected native vegetation or productive grazing land. Members of conservation groups are not the only people oppposed to wilding pine spread. High country farmers recognise the threats to their livelihoods from wilding pine spread across the open tussock grazing lands. High country Federated Farmers chairman, Hamish Ensor, believes those who have planted exotic trees should be responsible for controlling their spread. Few would dispute the importance of exotic trees in the New Zealand context for shelter and amenity purposes, and for many other values, not to mention their significance as an economic resource. But these are benefits gained from planned and managed plantings. While trees that establish themselves in the wild may be ideal in some circumstances, unplanned exotic afforestation rarely results in a harvestable plantation. Trees tend to be of different ages and sizes and yield timber of inferior quality. Management of a self-sown forest is more difficult and less financially rewarding than a plantation. Even if the site is in fact a preferred site for forestry, it makes far better economic sense to plant the trees at the desired density and in the most appropriate pattern.

Aggressive Colonisers

Exotic trees, particularly those planted at higher altitudes, are usually far more competitive than the existing native vegetation. Several species have been used widely in plantings in hill and high country and the more successful of these have shown themselves to be aggressively capable of colonizing the open country of grass and shrubland communities. They can displace

or overwhelm slower growing native species and therefore have seriously threatened protected native plant communities and unique landscapes. The hardier exotic species such as lodgepole pine (Pinus contorta) will grow at higher altitudes than native woody species, posing a significant threat to herbfields, grasslands and screes. Even native beech forests are not spared from this alien invasion. The shade tolerance of Douglas fir, a species commonly planted in the South Island high country, enables its seedlings to establish themselves within existing natural forest. The problem of wilding tree spread is not new. It was first recorded in the South Island at the turn of the century. But because the species involved are common pines, firs and

larches and because the most dramatic infestations, with one or two exceptions, are in remote hill country far from the public eye, we have been slow to recognize the implications of wilding spread. Trees that were planted for shelter or amenity are now spawning vast plantations, and trees that were hand planted or aerially sown in afforestation trials on steep lands have successfully established and threaten to spread like a carpet across broken mountainous country, crowding out the existing native vegetation. Take the case of Pinus contorta in the central North Island, where wildings occur in Tongariro National Park, the Waiouru Army land, the Ruahine, Kaimanawa and Kaweka Conservation Parks. (see accompanying box

on Tongariro National Park). Another good example is the spread of Corsican pine (Pinus nigra) on the Amuri Range near Hanmer Springs. Seedlings were first noticed around 1940, next to forests planted at the turn of the century. However it was the change from sheep to cattle farming on the adjacent pastoral run country in 1974 that really boosted the spread of pines. By 1976 landholders were expressing concern at the invasion of Corsican pine which in places had formed a closed canopy completely eliminating the grassland cover beneath. In 1979 Corsican pine was declared a Class B noxious weed in the Amuri Range area and fire was used to remove stands of unwanted trees. This was only effective when followed by pasture improvement and grazing. Where oversowing with pasture species did not occur, and where stock pressure was light, re-establishment of seedlings was prolific, with densities of 8,575 per hectare recorded — seven times that of a plantation forest. Now Corsican pine covers about 6,000 hectares of the Amuri Range and six other species of wilding trees are also present. The only areas that will remain free of pines are those already covered in beech forest and those potentially productive areas where the expense of tree removal can be justified for pasture establishment. Meanwhile local farmers bemoan the loss of grazing land and the local authorities ponder the difficulties and expense of continued control work.

Research Plots

Another source of wilding tree spread is the research plots established by the former New Zealand Forest Service to determine which species were most suitable for high country revegetation. These plots have spawned untidy downwind forests and threaten to overwhelm huge areas of native grassland or shrubland. Many are hidden from the public gaze, such as that in the Jolliebrook Catchment of Lake Sumner Forest Park where Douglas fir patches are too dense to walk through despite the removal of 20,000 trees in 1987. Some Forest Service plantings were a little more ambitious. In the Branch and Leatham Catchments of inland Marlborough over 700,000 conifer seedlings were hand planted on steep country and, as if that were not enough, the steeper more inaccessible country was covered by sowing 2 tonnes of conifer seed from the air. Today sparse but healthy pines emerge from regenerating manuka and beech while at higher altitudes stunted twisted pines grow from precipitous rock bluffs and scree slopes. When questioned about wilding spread from trial plots at lower altitudes in Marlborough the Forest Service promised to de-cone the trees to prevent seeding!

The succes0000000 of these introduced trees rests on their ability to shed large quantities of wind-borne seed and to grow in harsh conditions that our woody native species are less able to tolerate. In the mountains of New Zealand frequent and strong winds favour wind dispersed species such as conifers over species that do not normally spread far from the parent tree as with native beech trees. Pines are generally regarded as a pioneer species, favouring bare sites or sites with low grass or shrub cover. The hardier of the pines thrive in dry cold conditions, growing faster than natives and producing seed at an early age, a mere 5 years in the case of Pinus contorta.

Control Problems

The control of wilding spread will not be an easy task. The first problem is perhaps one of awareness. Wilding trees are often well established and already seeding before people recognize that they pose a problem. By this time control is usually time consuming and expensive. The obligation of individuals and agencies to meet cost recovery targets engenders a cautious reluctance to commit funds to what is potentially a very expensive exercise. Advocates of exotic trees insist that careful siting of plantings and choice of species, and appropriate management of downwind areas, will limit wilding spread. Using less aggressive species and avoiding "take-off sites" such as ridge tops or exposed slopes are suggested strategies. However, in the harsh climate of the mountainous areas where tree spread is a problem the most successful trees are also the most aggressive invaders. One seed can lead to the establishment of a tree that grows on to be a prolific seeder. Most seed falls near the parent tree but seedlings have been recorded 10 kms from the nearest seed source and winged seeds of pines aid in wind dispersal. Accurately predicting the direction and occurrence of strong winds or storms is impossible, making the identification of downwind areas for management very difficult. Suggested management includes mob-stocking with sheep and pasture improvement or cultivation. The many areas of uncontrolled wilding spread in the South Island high country testify to the failure of this management technique. Seeding trees know no boundaries. Many wildings originate from plantings on adjoining properties or lands administered by different agencies or government departments. This makes effective management and control of wildings extremely difficult. Farmers with tree spread problems and agencies with conservation or weed control responsibilities are becoming increasingly concemed at the implications of wilding spread on conservation and agricultural land. At a recent planning hearing considering a proposal to plant Douglas fir in the Rangitata Valley, the Strathallan County Council insisted on the lodging of a $30,000 guarantee by the applicant to ensure wilding control work would be undertaken and required that this guarantee be recorded on the title of the property.

While agencies and individuals debate the spread of trees and who should be held responsible, we live with a legacy of past New Zealand Forest Service zeal which established exotic species trial plots throughout the country. Many of these plots are the seed sources of current wilding spread and, now that the Forest Service no longer exists, no one is willing to take responsibility for cleaning up the mess. And, to accentuate the problem, the ideology of the Forest Service has re-emerged within the new Ministry of Forestry where some staff are actively promoting afforestation of high country lands.

Tussock Grasslands Threat

Foresters continued advocacy for exotic tree planting, particularly in the South Island high country, poses a continued threat to the open tussock grasslands that form such a distinctive part of this landscape. Concern over wilding spread has been expressed by botanists and conservation agencies such as the Nature Conservation Council for many years. Control efforts by volunteer groups such as Forest and Bird continue but in most cases have only a local effect on what is an extensive problem. While the arguments rage over who is responsible, the trees continue to grow and produce seed, the seedlings grow into seeding trees themselves, and the spread slowly advances across scenic landscapes, overwhelming vulnerable native plant communities and important agricultural land. It is important that the concern over wilding spread is not seen as an anti-tree campaign. The planting of trees is obviously a worthy activity especially while the removal of forest cover continues throughout the world and atmospheric carbon dioxide levels increase. The debate is not about the planting of trees but rather about the type of trees, the appropriateness of the site, and the subsequent management of the plantings. Managed plantations are a resource. Unwanted and uncontrolled trees are weeds. Identifying a problem is one thing; finding a solution is another again. While there have been some successes in controlling wilding pines, most notably Tongariro National Park, the area involved was relatively small and the commitment by park staff and volunteers was high. However, an entirely different approach is required in areas not perceived as important, or difficult of access. Former Forest and Bird executive member David Appleton, recently remarked on the problem in the Kaweka Conservation Park: "Our members attempted to totally clear contorta from several hectares of ridges but were leaving many small onés behind as they moved up. On bare tops they were creating mini avalanches as small trees were pulled from the

soil, the surface ripping away. The entire exercise seemed pointless." Mob stocking and management of downwind areas are often cited as the best ways of controlling wilding spread. However mob stocking (the concentrated grazing of large numbers of animals) is just as likely to threaten the ecological values of native grassland or shrubland communities as is wilding spread. In such areas control is often limited to the physical control of trees. This has been carried out by volunteer groups from Forest and Bird branches for many years and involves pulling out seedlings by the roots. If the trees are so large that they have to be cut, all the green needles have to removed and the bark should be stripped to ground level. Sites should be checked again for at least five years for seedlings. Tree age can be estimated from the number of whorls of branches. All wilding control efforts should be documented and landowners and local authorities lobbied to accept responsibility for wilding control. In some areas exotic conifers have been declared noxious weeds; funds may then be available for control. Efforts to conserve the unique habitats that make this country so special are fraught with many difficulties. The introduction of exotic plants and animals has already wreaked havoc amongst vulnerable indigenous communities. The high mountains, forested hills, native grasslands, lakes and rivers, and scenic coastlines make up the landscapes that people see, the visual character of the land that makes this country distinctive from any other. The uncontrolled spread of exotic trees is one influence we can control — if we care enough. ~

Spread of Introduced Trees in N.Z. Major areas of concern.

MAIN PROBLEM TREES Common Name Lodgepole pine Scots pine Douglas fir European larch Corsican pine Radiata pine Maritime pine Ponderosa pine Muricata pine Macrocarpa Sycamore Blue gum

Tongariro — A Case History

bie BEST documented case of wilding pine infestation in New Zealand is that of Tongariro National Park. Between 1927 and 1935 Pinus contorta was planted in Karioi Forest, on the southern slopes of Mt Ruapehu. Every summer — and this still occurs — seeds can travel 12 kms from the parent tree, aided by the fact that Pinus contorta seeds are very light and winged. DSIR botanist Dr Ian Atkinson first alerted park staff to the magnitude of the problem in 1962. Unless action was taken, he said, most of the upper slopes of Mt Ruapehu between 1300 and 2000 metres would be covered by pine forest by the end of the century. Despite the opposition of foresters, park staff began an eradication programme; in 1967 volunteer pine weeders began arriving on a regular basis, usually either tramping groups or from conservation organisations. One tramping club’s record would be hard to beat: it has not only supplied volunteers for weekends, but the club sets aside a week each year when its members carry out weeding. They have been doing so for the last 20 years. Thanks to these efforts, the problem within the park has been contained. However, on the slopes below the park

towards Karioi forest is a substantial area of Maori land which has also been infested. All the work in the park will be for nothing unless this other area is cleared. It is here that volunteers are now operating. Pinus contorta’s spread has not been confined to the park and its immediate environs. To the east, the military have also been learning to live with the weed. Some scientists view the military-con-trolled land around Waiouru as ecologically more significant than the park itself. This huge landscape of open vegetation — almost 100,000 ha — is much older than the area in the west, and is able to tell us about the evolution of New Zealand's plants and animals. When Pinus contorta initially spread into the Army land, it was welcomed by some officers because of the shelter and shade it provided. However, once tanks found it difficult to move through the by-now dense pine shrub/forest, the Army saw the sense of eradication. But eradication is an expensive business. In 1987 the Army spent almost $500,000, and the programme is destined to go on well into the 1990s. One area providing a headache is the 2500 ha firing range, where the presence of live ammunition makes it dangerous for soldiers to enter by vehicle or on foot. Here, for the moment, Pinus contorta flourishes.