This indicator identifies the impact of biotic processes and agents have on forests. Where change due to these agents and processes occurs beyond a critical threshold, forest ecosystem health and vitality may be significantly altered and a forest’s ability to recover could be reduced or lost. Monitoring and measuring the effects of these processes provide information helpful in the formulation of management strategies to mitigate risk.
In 2013, less than 1 percent of the total planted forest area was affected by insects and about 10 percent of the total planted forest area was affected by diseases, the most important of which are: Cyclaneusma needle cast, Dothistroma needle blight, Armillaria root rot and Nectria flute canker.
The introduced Australian brushtail possum continues to be widespread in indigenous forests. Forest health records indicate that, in 2013, possums affected eight percent of the planted forest estate.
Historically, very few insect problems have been noted in planted forests, apart from one species of bark beetle (Hylastes ater) that can be associated with seedling death. All major exotic planted forests are inspected at least once a year for signs of newly established pests or diseases: forest health assessments are undertaken, and damage by biotic and abiotic agents estimated and recorded.
The most important fungal diseases affecting pine planted forests are: needle cast (Cyclaneusma minus), needle blight (Dothistroma septosporum), root rot (Armillaria spp.) and flute canker (Neonectria fuckeliana (syn. Nectria fuckeliana). Other species are affected by a number of diseases.
The most recent estimated of the economic losses from diseases affecting planted forests is a cost of about $83 million per annum; slightly more than the estimate for 2008 ($82 million).
Damage to radiata pine due to Nectria flute canker remains confined to the lower South Island of New Zealand and management regimes have reduced the disease incidence in stands. This disease was first recognised in the early 2000s but may have been established in New Zealand for some years prior.
Incidences of both cypress canker and diseases in eucalypt species have increased. Dutch elm disease affecting urban trees remains confined to the Auckland area and control involves a trapping programme for the disease vector beetle species.
Kauri dieback disease caused by the pathogen Phytophthora taxon Agathis (interim name) affects kauri of all ages and is present in a number of the northern kauri forests. Work is underway to detect the extent and pattern of spread of the pathogen, and more formal identification of the fungus and methods of control. Surveys of distribution are continuing. Publicised hygiene measures to limit physical spread include footwear cleaning and forest access management.
Four species of social wasps, accidentally introduced to New Zealand, are now established and classed as pests. Two of the four species are vespulid wasps (common and German); the other two species are paper wasps (Asian and Australian).
Social wasps are a pest of urban, rural and natural ecosystems. They pose a health risk; affect the profitability and safety of industries such as beekeeping, horticulture, forestry and tourism; and upset the ecological balance in native ecosystems. Wasps are a significant pest in forest areas, especially in beech forests where high populations can develop due to their attraction to honeydew – a sweet exudate of tree-dwelling scale insects. In forests, wasps can displace birds by competing for food such as honeydew, or by driving them from the habitat. Control methods are largely through chemical use either by direct destruction of nests or through bait carried back to nest by foraging wasps.
The introduced Australian brushtail possum (Trichosurus vulpecula) is considered a major forest pest in New Zealand. Possums are widespread, can attain high densities and browse on some canopy and sub-canopy trees. Possums are also significant predators of some indigenous native birds and invertebrates.
Possums living in or adjacent to planted forests commonly use radiata pine as a seasonal food source, including foliage and cones, which subjects the trees to browse and secondary physical damage. Historic reports of possum damage in planted forest indicate the greatest reported damage coincided with significant cycles of replanting and establishment, such as in the 1960s and then again in the 1990s, indicating possums’ preference for young stands.
Trappers’ control of possum numbers in accessible areas, with the currently high prices for possum fur, and the effective outcomes of the bovine tuberculosis control programmes (see below) are two key factors helping control possums numbers across the New Zealand planted forest estate. Forest owners and managers report that possum numbers have remained constant over the last five years and damage is generally minimal due to both factors. However, while trappers are controlling possum numbers in accessible areas, populations are still high in some remote areas. Some report isolated damage in planted forests adjacent to some indigenous forest areas where control measures have been limited.
Possums can carry bovine tuberculosis (Tb). TBfree New Zealand, a government-industry partnership (previously the Animal Health Board), supports research into, and treats forest areas for, control of possums where the spread of Tb into livestock is a problem. Sodium monofluoroacetate (1080), applied across forest areas in aerial spread baits to control possums and other vertebrate pests, remains a core method for large-scale control. This attracts both opposition and support from the public, but is currently regarded as the most practicable extensive control method, especially over terrain with limited ground access. Aerial 1080 operations covered approximately 432 000 hectares of land in 2012.
The term “wildings” refers to the seed-sourced natural regeneration of introduced conifer tree species (particularly pines, Douglas-fir, redwood and larch) originating from stands planted in New Zealand for a variety of purposes over many years. 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 and the Mackenzie Basin. Wildings can grow in dense stands. They reduce the value of managed pasture, displace native biodiversity and alter the character of the landscape.
The New Zealand Wildling Conifer Management Strategy 2015–2030 is a non- regulatory strategy supporting collaborative action between land occupiers, researchers, regulators and communities to address the critical issues facing wilding conifer management. The Strategy has been developed in collaboration with a multi-stakeholder working group and identifies actions for key parties involved in wilding conifer management under four principles: individual and collective responsibility; cost-effective and timely action; prioritisation; and co- ordination. The overall aims of the working group are to prevent the spread of wilding conifers, to contain or eradicate established areas of wilding conifers by 2030, and to seek the following outcomes:
In both 2003 and 2008, Cyclaneusma needle cast and Dothistroma needle blight were the pathogens that caused the most loss, but outbreaks of these diseases have been less extensive in recent years. Drier conditions and more effective copper spray formulations have reduced Dothistroma incidence, while the reduction in forest areas in the central North Island through land conversions has diminished the spray requirement. The production of resistant clones has helped to reduce Cyclaneusma incidence.
Based on forest health inspection records in the forest health database, the area affected by possums has continued to decrease. Over the five-year period from 1998 to 2002 just over 2900 (580 per year) records of possum damage were made. Numbers declined to 260 per year during 2003 to 2008, and since then (2009 to mid 2014) about 150 records of possum damage have been logged. With the average area affected at 200 hectares per record, this equates to 30 000 hectares over the entire estate.