The range of views expressed recently in the Otago Daily Times on wilding conifers is in cases concerning, given the enormous threat these pests pose to the high country.
All such wilding conifers have initially escaped from trees or stands planted for one of several purposes: erosion control, windbreaks, small woodlots, commercial plantings or landscaping. To this end, several species are involved, the most aggressive being the lodgepole, or contorta, pine. The critical factors are masses of viable seed in the right place and favourable weather. Capable of rejuvenating from single needles, these trees are a daunting foe for conservation.
Often inaccessible and widely scattered, wilding pines pose a grave biodiversity risk. Accepting this, most local authorities now have policies requiring or encouraging their removal.
Professional foresters and volunteers have reduced the 80,000ha of scattered wildings downwind of the 250ha of trees initially planted by the Government to control erosion in the 1970s and '80s. These trees are now being sprayed and cut to remove this major seed source, and the Government is offsetting the carbon costs associated with their removal.
The extensive wilding pine stands to which Grahame Sydney referred (ODT, 11.1.12) and others on the rugged hills east of Alexandra are, likewise, seriously out of control. As elsewhere, these trees are economically worthless. Their spread reflects poor management of pastoral leasehold land, both by the lessee and Linz, by not fulfilling the "good husbandry" requirements of pastoral leasehold lands. Their removal, already a challenging task, will only grow more and more costly the longer it is left.
A recent decision to tackle the rapidly deteriorating situation in the Roaring Meg catchment of the Kawarau Gorge, using both ground crews and spraying, is laudable. Financial support from several stakeholders, in particular the Queenstown Lakes and Central Otago district councils, Doc, NZTA, Linz, Pioneer Generation and three affected high-country lessees, should make this a winnable operation.
Chemical mixes trialled by several collaborating government agencies throughout much of the South Island now appear to be effective with either aerial or basal stem application and will, hopefully, increase the prospects of effective control.
Otago University surveyor Dr Mick Strack's suggestion (ODT, 16.1.12.) that the tree-weed problem derives from changing government land-use policies is curious, to put it mildly. In particular, his suggestion that wilding pines are the result of stock being removed following tenure review is quite uninformed: several Central Otago high-country properties with major tree-weed problems - Matangi, Riverside, Waitiri, Lowburn and Mt Difficulty among them - have not yet entered the tenure-review process. It is well established that the stocking rates necessary to control wilding conifers on high-country land would be greatly in excess of this land's carrying capacity. Stock are, therefore, not a viable option for wilding control.
Cultural landscapes our tussocklands may be after 160 years of pastoral farming, but burning and grazing has had little to do with containing the spread of wilding conifers, as Dr Strack implies. Their establishment is more obviously a function of when and where a seed source becomes available in an area.
There is, moreover, no connection between the economic value of commercial exotic plantations and any putative value of the wilding trees themselves. Why bother with tree-weeds, which often have inferior timber, when a much higher-value product can be grown on suitable land with proper management?
Cultivated plantations are also far easier to harvest.
However, Dr Strack is correct in stating that conifers can impact on hydrology by reducing water yield. In Landcare Research's Glendhu paired catchment study on the Lammerlaw Range, the catchment under a 28-year-old pine stand recorded a 43% reduction in water yield, relative to the adjacent snow tussock-dominated catchment.
It is highly irresponsible to suggest that in several hundred years a mixed woody forest cover might develop here with increasing biodiversity. We need to be aware of the long-term consequences of wilding tree spread. There are clear indications that a dense exotic monoculture, once established, would persist with negligible biodiversity and foreclose all other options for land use.
Moreover, it would continue to spread and consolidate. By contrast, once protected, most upland areas of tussockland will increase in height, cover, native biodiversity and water production.
We know little about the role of native ecosystems, especially shrublands, grasslands and bog, in sequestering carbon. The long-term carbon sink potential may be as good as, or in excess of, either planted or wilding monoculture forests. At the same time, these wildlands provide a range of important ecosystem services that conifer plantations cannot provide. As has been said many times: "The cost of procrastination is enormous."
A national programme of eradication is urgently needed, since the battle will be won or lost by the level of commitment of this generation. The North Island Central Plateau has now been cleared of wilding pines and a major effort is under way in Queen Charlotte Sound. We should not fail future generations in the South.
•Alan Mark FRSNZ, KNZM, emeritus professor of botany, University of Otago; Anne Steven, landscape architect, Wanaka; Sue Maturin, Forest and Bird field officer, Dunedin; Matt Thompson, Forest and Bird wilding tree-control co-ordinator, Dunedin; Toni Atkinson, science editor, Dunedin; Brian Turner, writer, Oturehua; Grahame Sydney, ONZM, artist, Cambrian; Richard Reeve, poet, editor, Warrington; Janet Ledingham, chairwoman, Forest and Bird, Dunedin branch; Mike Floate, former vice-chairman, South Island FMC, Cromwell; John Turnbull, Forest and Bird, Central Otago-Lakes branch; Ines Stagers, Forest and Bird executive member, Geraldine; Marilynn Webb ONZM, artist, Dunedin.
