Lessons from Insect and Disease Impacts on Radiata Pine (Pinus Radiata D.Don) Plantations in New Zealand Over the Last Hundred Years

HIGHLIGHTS Radiata pine remains the greatly preferred medium density softwood in New Zealand although with climate change it may become more marginal on some sites. Past biotic attacks on New Zealand's radiata pine plantations have either abated or are easily managed, but while the probability of a biotic catastrophe appears low, it is not zero. Despite their impact on growth rates, radiata pine productivity has increased since it was widely planted. The opportunity costs and the associated risks make major species diversification currently inadvisable, despite New Zealand's very high reliance on radiata pine. A broad defensive strategy, including strong biosecurity infrastructure, is strongly recommended. SUMMARY The impacts of past and potential insect pests and diseases in New Zealand's radiata pine plantations are reviewed. Invariably their impacts have decreased with time or can be easily managed. Despite past biotic impacts. growth rates have increased over the last 100 years. Pitch pine canker (PPC) is perceived as the greatest new threat. PPC's impact in California, Spain, Portugal, South Africa and Chile suggests that in New Zealand it would become a nursery problem. Radiata pine remains the best medium-density softwood for New Zealand although climate change may alter the site limits. While a biotic catastrophe, despite its low probability, remains an important risk, this risk is outweighed by the opportunity costs and risks associated with diversifying into alternative species. A strong biosecurity infrastructure is vital, as is maintaining a broad genetic base from which to breed resistance. Large plantation estates should develop defensive strategies against new biotic invasions. Les impacts des maladies et des insectes passés et potentiels dans les plantations de pin radiata en Nouvelle-Zélande sont examinés. Leurs impacts ont invariablement décru avec le temps ou peuvent être gérés facilement. Les taux de croissance ont augmenté au cours de la dernière centaine d'années, malgré les impacts biotiques passés. Le chancre résineux du pin (PPC) est perçu comme la nouvelle menace la plus grande. L'impact du PCC en Californie, en Espagne, au Portugal, en Afrique du Sud et au Chili suggère qu'il pourrait devenir un problème dans les pépinières néo-zélandaises. Le pin radiata demeure le meilleur bois tendre à densité moyenne pour la Nouvelle-Zélande, bien que le changement climatique puisse altérer les limites des sites. Alors qu'une catastrophe biotique demeure un risque important, malgré sa faible probabilité, ce risque est mis en veilleuse par les coûts communautaires et les risques associés à la diversification vers des espèces alternatives. Une forte infrastructure de biosécurité est vitale, ainsi que le maintien d'une large base génétique à partir de laquelle une résistance puisse être développée. Les grandes domaines de plantations devraient développer des stratégies de défense contre les nouvelles invasions biotiques. En este artículo se revisan los impactos de las plagas de insectos y enfermedades pasadas y potenciales en las plantaciones de pino radiata de Nueva Zelanda. Invariablemente, los impactos han disminuido con el tiempo o pueden gestionarse fácilmente. A pesar de los impactos bióticos del pasado, las tasas de crecimiento han aumentado en los últimos 100 años. El cancro resinifero del pino (PPC) se percibe como la mayor amenaza en el presente. El impacto del PPC en California, España, Portugal, Sudáfrica y Chile sugiere que en Nueva Zelanda se convertiría en un problema de vivero. El pino radiata sigue siendo la mejor madera de coníferas de densidad media para Nueva Zelanda, aunque el cambio climático puede alterar los límites del lugar. Aunque es una catástrofe biótica, a pesar de su baja probabilidad, sigue siendo un riesgo importante, que se ve superado por los costos de oportunidad y los riesgos asociados a la diversificación con especies alternativas. Es crucial contar con una sólida infraestructura de bioseguridad, así como mantener una amplia base genética a partir de la cual generar resistencia. Las grandes patrimonios de plantaciones deberían desarrollar estrategias de defensa contra las nuevas invasiones bióticas.

rates, good wood properties and general amenability to plantation culture.Part of their fast growth may be related to introduced species being, at least initially, largely free of diseases and pests.This implies that a species is in ecological disequilibrium when planted as an exotic, compared to in its native habitat (Crous et al. 2017).Further, New Zealand, Chile, Australia, and South Africa, where radiata pine has been widely planted, do not have indigenous species that are closely related; this means that opportunities for pathogen host shifts from indigenous plants are less likely and if they do occur will usually be with polyphagous or opportunistic species (Brockerhoff et al. 2016, Crous et al. 2017).Brockerhoff et al. (2016) noted that most common insects found on radiata pine in New Zealand had been introduced from Europe or North America.Furthermore, when non-native insects become established in exotic plantations, they may not have their natural parasites and there can be explosive outbreaks.
There is also the temptation to push favoured species beyond their inherent site tolerances and this can bring increased risks of disease and pest outbreaks (Evans 2009).Radiata pine's ecological niche and site limitations and the dangers of planting 'off-site' have been reviewed by Mead (2013).There is some evidence that risks of new biotic damaging agents to exotic plantations are increasing, primarily driven by global trade and climatic change (Ennos 2015, Ramsfield et al. 2016, Wingfield et al. 2015).In some exotic plantations native pests have become problems, although as we will see later, this has not been a major problem with radiata pine in New Zealand.
There will always be a risk of a catastrophic outbreak of a new pest or disease to New Zealand's plantations.While this risk is difficult to quantify, especially as any vulnerability created by local environments cannot be manifested in the absence of such agents, New Zealand's long-term experience has some pointers for the future.Nevertheless, Wingfield et al. (2010), Brockerhoff and Bulman (2014) and Payn et al. (2015) have all called for greater vigilance and preparedness.Radiata pine exemplifies these issues.
In New Zealand some tree-growers, farming groups and environmentalists are questioning if further planting of radiata pine should be encouraged.This is partly because of perceptions of biotic dangers, but also because of perceived adverse Lecciones del impacto de insectos y enfermedades en las plantaciones de pino radiata (Pinus radiata D.Don) de Nueva Zelanda durante los últimos cien años INTRODUCTION Worldwide, there are 294 million ha of planted forests of which 131 million ha are plantations grown for wood production, the latter plantations making up about 3% of total forest area (FAO 2020).However, these plantations currently supply 46% of total industrial wood production, so they have huge economic and social significance (Payn et al. 2015).The global review of Cubbage et al. (2020) concluded that industrial plantations are good investments.Of the planted forests, only 17%, or 50 million ha, employ introduced, non-native, species and these are predominantly industrial plantations in the Southern Hemisphere (FAO 2020, Payn et al. 2015).Of the several thousand tree species only about 30 have been widely planted as exotics, despite often extensive testing (Carle et al. 2009).Radiata pine (Pinus radiata D.Don) at 4 million ha worldwide, is one of the most planted exotic species (Burdon et al. 2017, Mead 2013); it is imputed to contribute around 20% by volume and value of the international log trade (FAO 2021).
New Zealand, the country with greatest economic dependence on radiata pine, has 1.57 million ha of this species which is 90% of the commercial forest estate (MPI 2021, NZFOA 2023).In 2021 new land planting was 34 000 ha of which 44% was on improved pasture with most of the remainder on unimproved pasture hill country.In 2020-21 radiata pine provided 93.4% of the 37.2 million tonnes of harvested logs (MPI 2021, NZFOA 2023).Native forests contributed only 10 000 tonnes.About 60% of this wood was exported in log form to Asia.Forest products exports in 2021 were $NZ6.25 billion and accounted for 13% of the food and fibre exports from New Zealand (NZFOA 2023).Internally, forestry and wood processing employ around 41 000 people (NZFOA 2023), Also, radiata pine plantings represent a huge carbon sink, reducing New Zealand's greenhouse gas emissions by about 30% (NZFOA 2023).
There are strong economic attractions of using monoculture plantations, as they are easier to manage than natural forest, and they produce more uniform products suited to large-scale industrial use (Carle et al. 2009).In some countries, particularly in the Southern hemisphere, exotic softwood species are preferred over natives because of their inherent high growth social or biodiversity implications that extend to plantation forestry in general (Payn 2021, Villamor et al. 2023).In contrast, in 1992 Burdon and Miller (page 15) wrote: "Although there have been past alarms over insect attacks.fungal diseases are currently regarded as a more serious hazard for radiata pine in New Zealand.Overall, however, the species now suffers less from pests and diseases than almost any alternative tree species.Thus diversification into large plantings of other less profitable species of similar role to radiata pine, which were once recommended to reduce catastrophic damage or disease within the monoculture, appears to be unwarranted".
The purpose of this paper is to review New Zealand's long experience of insect-pest and disease impacts with radiata pine plantations to determine: • if there are reasons to be concerned about possible future impacts of insect pests and diseases, • how to balance the risk-reward equation of planting a preferred species against possible major losses from a new biotic invasion, • and the importance of having a defensive strategy against biotic risks.
For the review, we examined the occurrence of pathogens and insect pests in radiata pine in New Zealand over the last 100 years and the subsequent experience and research on them.We also consider whether there is evidence that such agents have reduced stand growth rates, and the possible impacts of future climate change on the effects of those agents.
Other biotic threats, from overseas, are evaluated based on experience outside New Zealand and their invasion potential.Of them, pine pitch canker (PPC) is considered the most potentially dangerous.Thus, for PPC, we contacted researchers and forestry experts in California, Chile, Spain, and South Africa, to assess the current status of this disease in these countries.We also considered the broader forestry perspective for such threats.
Finally, the risk-reward relationship for new biotic threats to the radiata pine plantations is reviewed, with special reference to the New Zealand situation, and we link this to having a set of measures that collectively represent a long-term defensive strategy against new invasions.

NATURAL OCCURRENCE OF RADIATA PINE
There are three natural areas of radiata pine on mainland California, USA, historically covering about 10 000 ha, and two other small areas on offshore islands of Mexico (Burdon et al. 2017, Mead 2013) 1 .All but one (Cedros Island, Mexico) have been substantially impacted by human activities (Burdon et al. 2017, Burdon and Miller 1992, McDonald and Laacke 1990).In 2013 radiata pine was listed as endangered in its native habitat (Farjon 2013).Almost all planted radiata pine in New Zealand has been derived from the Monterey and Año Nuevo mainland populations, the genetic contribution of the latter disproportionately high in relation to its extent.Notably, radiata pine in its natural range is often found as pure, evenaged stands that arose following fire (Figure 1).
These stands may have various understory plants depending on location and site (McDonald and Laacke 1990).The most common species are bracken fern (Pteridium aquilinum (L.) Kuhn), poison-oak (Toxicodendron diversilobum (Torr.& A.Gray) Greene), manzanitas (Arctostaphylos species); salal (Gaultheria shallon Pursh), coyote brush (Baccharis pilularis DC), blueblossom (Ceanothus thrysiflorus Eschsch.),California wax myrtle (Myrica californica Cham.& Schltdl.),California coffeeberry (Frangula californica (Eschsch.)A.Gray), California blackberry (Rubus ursinus Cham.& Schtldl.),California sagebrush (Artemisia californica Less.) and California huckleberry (Vaccinium ovatum Pursh), plus native grasses, sedges and rushes.However, there are situations where uneven-aged stands occur, particularly at boundaries FIGURE 1 Natural mature radiata pine at Monterey, California.This stand has relatively small, flattish-topped pines and a shrubby understorey (Photo: D.J. Mead, taken in 2012) with other forest types where radiata pine may be associated with other tree species, or where long periods have elapsed since the last fire.These include coastal live oak (Quercus agrifolia Née), coastal redwood (Sequioa sempervirens (D.Don) Endl.), Douglas fir (Pseudotsuga menziesii (Mirb.)Franco, Cupressus macrocarpa Gordon (also called Hesperocyparis macrocarpa (Hartw.)Bartel) and other closed-cone pines (P.attenuata Lemmon and P. muricata D.Don).The future management of native mainland stands is challenged by the difficulty of using fire in semi-urban situations, and the arrival of exotic weed species (Rowland Burdon observation 1995).One concern is that the reduction of fire and increase in pine pitch canker in some stands may allow the coastal live oak to dominate over the radiata pine, while other stands may become mixed-aged rather than even-aged (Zander Associates 2002).

INTRODUCTION OF RADIATA PINE IN NEW ZEALAND
Radiata pine was first introduced to New Zealand as a three-year-old seedling from Australia, in 1859 (Burdon et al. 2017, Burdon and Miller 1992, Mead 2013).That tree is still standing as of 2023.In the 1860s to 1882 there were further introductions from the USA, Great Britain, and Australia, with at least 24 kg of seed known to have come directly from California.Again, some trees grown from these introductions, often in parks, are still growing healthily.By 1882 New Zealand was self-sufficient in seed (Burdon et al. 2017, Burdon andMiller 1992).The first large-scale plantations of 240,000 ha were established between 1921 and 1936 with a substantial second surge of tree planting over the 35 years from 1968 to 2003 when a further 1.4 M ha were planted (MPI 2021).Most of these plantations were radiata pine, so that in 2021 there was 1.57 million ha of radiata pine in New Zealand.A further expansion of plantations is expected over the next decade driven largely by Government climate-change initiatives but also by other environmental needs (Payn 2021).Radiata pine is a fast-growing medium-density softwood, and now typically grown in pure stands on 26-30-year rotations.Thus, over the past 100 years there have been three completed rotations on some sites.

CHANGING IMPACTS OF DISEASES AND INSECTS ON RADIATA PINE IN NEW ZEALAND
The following discussion is in chronological order within major groups.

Aphids
Pineus pini Macquart.(pine woolly aphid) became established in New Zealand before 1884 and became of concern in the 1930s (Figure 2).It is now of no importance because it is controlled by other insects, both native and introduced (Rawlings 1955, Scion 2009).It was worse where radiata pine was planted on droughty sites.In an experiment with 10 radiata pine clones, on a summer-dry site in Canterbury, it was found FIGURE 2 Pine woolly aphid (Pineus pini) on young radiata pine on a droughty site in Canterbury, New Zealand.In this experiment of monoclonal plots and clonal mixtures, the aphids only attacked one of the 10 clones (Sharma et al. 2008. Photo: E.G. Mason;taken in 1995) to severely attack one clone (2 years after planting) whether this clone was planted in clonal blocks or as a mixed clonal stand; this illustrated that some genotypes may be more susceptible than others (Sharma et al. 2008).
In 1998 the Monterey pine aphid (Essigella californica Essig.) also became established in New Zealand, but has not been a problem to date (Watson et al. 2008).Like the pine woolly aphid, it is also most common on drought-prone sites, where its incidence varies greatly from year to year.Based on Australian research, this pest could become more of a problem where climate change results in drought-prone sites (Watson et al. 2008, Watt et al. 2019).

Sirex woodwasp
Sirex noctillo F, first seen about 1900, caused a spectacular epidemic in the late 1940s following several warm, dry summers, and caused heavy mortality in well over 100 000 ha of overstocked radiata pine stands (Figures 3 and 4; Burdon et al. 2017, Eldridge andSimpson 1987).From the 1950s this insect has no longer been a problem in New Zealand, and it is now seldom seen because of biological control by introduced insect parasites (primarily Ibalia and Rhyssa species -Nuttall 1980), by a nematode (Deladenus siricidicola Bedding) and improved silviculture (Bain et al. 2012, Rawlings 1955, Slippers et al. 2015).The source of the nematode into New Zealand and North America is unknown.S. noctillo primarily attacks stressed trees, including drought-stressed or overstocked stands, so lower stockings and thinning can be an important measure for control.Interestingly, Ayers et al. (2014) found that radiata pine was not a preferred pine species in the insects' native habitat, even though it is considered a suitable host.Climate modelling shows that pine exotic plantations in New Zealand are in areas considered moderate or highly favourable to Sirex (Ireland et al. 2018).
S. noctillo is a good example of an insect that came from overseas without its natural parasites and this eventually led to a major outbreak until parasites brought them into ecological equilibrium.

Shoot dieback pathogens
In 1932, severe shoot dieback over large areas of radiata pine caused considerable alarm (Rawlings 1955).Two pathogens were implicated.One was Sphaeropsis sapinea Fr. (Diplodia pinea (Desm.)Kickx), commonly called Diplodia (Birch 1936), and this has caused some subsequent, lesser alarms  (Burdon 2011, Chou 1976a).It was probably in New Zealand since 1900, and is presumed to have come from Europe (Brokerhoff and Bulman 2014).It is best recognised as a wound pathogen, attacking after damage by agents such as hail, frost, drought, etc.However, with sufficient warmth and humidity, it readily attacks soft, unwounded shoot tissue (Chou 1976b).It has thus continued to manifest itself occasionally, on localised humid sites (Currie and Toes 1978).In stands, its most spectacular and troublesome symptom is leader dieback (Figure 5).Note, the native stands in California do not have the same species of Diplodia found in New Zealand but have D. scrobiculata J. de Wet, Slippers & M.J. Wingf.(Burgess et al. 2004).D. scrobiculata has been found on radiata pine in Spain (Manzanos et al. 2017).Overseas, Diplodia has prevented the use of radiata pine in humid tropical and sub-tropical regions (Burdon et al. 2017, Mead 2013).The application of a Trichodema atroviride P. Karst.isolate as a drench has been reported to prevent Dipliodia infection on radiata pine seedlings (Regliński et al. 2012).
The other dieback pathogen associated with the 1932 alarm was Phomopsis pseudotsugae M. Wilson (Phomopsis strobi Syd.) (Birch 1935).It can not only attack wounded tissue but also frosted shoots that show no visible injury.
In hindsight, the 1932 alarm reflected a combination of an extreme climatic event (the severe, exceptionally late frost) and a large area of radiata pine at a vulnerable stage of development

Other insect outbreaks
Small outbreaks occurred in Canterbury in 1933Canterbury in , 1949Canterbury in , 1951-52 -52 but all subsequently abated (Rawlings 1955).Two of the three species were native defoliator moths and the third (Lecanium hesperidium L.) a scale insect.Of these, the native looper caterpillar Pseudocoremia suavis Butler (Syn.Selidosema suavis Butler) resulted in a few significant outbreaks in Canterbury, but was naturally controlled by a virus and once by spraying with DDT (Berndt et al. 2004, Dugdale 1958, Rawlings 1955).Thirteen primary parasitoids have been recorded from this native species (Berndt et al. 2006).The other native species was the winter moth (Hybernia indocilis Walker), but it did not cause significant damage (Rawlings 1955).Rawlings also noted other small passing outbreaks.

Phytophthora root rot
In Northland forests, particularly at Riverhead Forest, very slow growth and dieback caused concern in the 1930s and some pathologists argued this was partly due to Phytophthora cinnamomi Rands root rot (Newhook 1970).Research found that phosphate deficiency was the real problem, and so the on high-hazard (flat, high-altitude) sites.Planting on such frosty sites was then avoided (Rawlings 1955), until nursery practice and site preparation were improved (Burdon et al. 2017).Burdon (2011) suggested that first-rotation trees may have lacked protective symbionts against severe dieback, as outbreaks did not occur to the same extent in later rotations.However, this has not been supported by the most recent consideration of the evidence (Burdon and Low 2022).

Stem cankers
Diplodia has caused concern by entering via pruning wounds causing cankers, but this can be managed by pruning lightly and avoiding summer pruning (Chou and Mackenzie 1988).Nectria flute canker disease (Neonectria fuckeliana Booth) was detected in 1996, and has been an occasional problem in the South Island of New Zealand (Figure 6; Brokerhoff and Bulman 2014).It is often associated with pruning and can cause substantial timber degrade.However, its impact can be reduced by appropriate seasonal timing of pruning in the region (Hopkins et al. 2012).(Photo: D.J. Mead;taken in 1984) disease could be managed by applying fertilizer (Mead and Gadgil 1978).Phytophthora root rot can be a localised problem on waterlogged sites and in some nurseries (Regliṅski et al. 2009).They (op.cit.) discuss how phenylamides and phosphite may be used to control outbreaks of this disease, particularly after root pruning.

Cyclaneusma
The needle cast fungus Cyclaneusma minus (Butin) DiCosmo, Peredo & Minter was first detected in 1952 (but likely present since 1900), and prefers warm, humid and higher-altitude areas (Watt et al. 2012).Trees under age 3 years are unaffected, with symptoms being most pronounced in stands between 9 and 20 years (Ridley and Dick 2001).Chemical control is considered uneconomic but thinning at age 7−10 years and tree breeding can reduce its impact (Ismael et al. 2020).

Dothistroma
Dothistroma septosporum (Dorog.)M. Morelet (previously D. pini Hulbary), first seen in 1962, initially caused great concern as it had severely defoliated radiata pine in East Africa (Mead 2013, Ridley andDick 2001; Figure 7).In New Zealand it was more severe in many other pines (Figure 8), as it only affects radiata pine up until mid-rotation.Fortunately, it has proved to be controllable with copper (Cu) sprays, through tree breeding, site selection, and to a lesser extent by pruning and thinning (Bulman et al. 2016).It is worse when pine is grown in higher-rainfall areas or high humidity.In New Zealand the fungus has a narrow genetic base, and studies have found its virulence has decreased over 50 years, and that there is no evidence of increasing resistance to Cu sprays (Bradshaw et al. 2019).Recent research has found that breeding against both Dothistroma and Cyclaneusma together is simplified as their symptoms are very highly correlated (Ismael et al. 2020).In Chile, Cu spraying against Dothistroma has largely been abandoned (Mead 2103).

Phytophthora needle casts
In the 1960s a severe needle cast, historically called physiological needle blight, was first observed in the Northland region following long spells of wet, cloudy weather in winter.Its significance was limited by it being only regional and sporadic.The cause was obscure, but it now appears likely to have been caused by the apparently native Phytophthora kernoviae Brasier, Beales & S.A. Kirk (McDougal and Ganley 2021).Since then, a similar sporadic needle cast, called red needle cast (RNC), which follows similar weather conditions.but with a somewhat different and wider geographic distribution, was attributed in 2008 to P. pluvialis Reeser, Sutton & E. Hansen (Dick et al. 2014, McDougal andGanley 2021).RNC usually passes after 2−3 years and does not lead to tree death (Figure 9).It can be controlled by copper sprays at low application rates (Fraser et al. 2022, Rolando et al. 2019), and tree breeding for resistance holds promise (Dungey et al. 2014, Graham et al. 2018).Both Phytophthora species also are known to be able to infect fine roots of radiata pine although the significance of this finding is unclear (Scott et al. 2019).

Armillaria root diseases
Root decay caused by fungi of the genus Armillaria (Fr.) Staude were first reported in 1930s (Ridley and Dick 2001).They have been found to kill small groups of young radiata pine, particularly where planted on recently felled native forest sites (Hood 1989, 2008, Kimberley et al. 2002).Stand growth can be reduced by a quarter, primarily due to incomplete site occupancy after planting on cleared indigenous forest.Most tree deaths occur early in the rotation, but trees can show infection through the whole rotation.Further, from the 1940s it has been known that second-rotation plantations in the Central North Island can have a low level of infection (Hood 1989), with growth reduction from infected trees being under 3% (Hood andKimberley 2009, Kimberley et al. 2002).Of the four native species only two, A. novae-zelandiae (G.Stev.)Herink and A. limonea (G.Stev.)Boesew.attack radiata pine plantations with A. novae-zelandiae being the more common of these two (Hood 1989(Hood , 2008)).
However, Armillaria is not now considered to be especially important as there is now no planting of felled indigenous forests, with almost all new planting being on pasture sites (MPI 2021).Control measures are expensive, and include  Hood 1989).There appear to be no prospects for breeding radiata pine for resistance to Armillaria (Hood et al. 2009).In heavily infested stands it may be wise to forgo heavy thinning (Hood and Kimberley 2009).

Important lessons
Several key messages come from this overview for radiata pine in New Zealand.
• When an outbreak is first seen there is often panic, with hasty, often expensive responses that prove unwarranted, but later the problem seems to dissipate or can be managed.• With insect pests, which are often introduced without their natural predators, there is sometimes a period where they build up to an outbreak, followed by collapse as natural/introduced parasitoids take over or because other factors are involved (Strayer et al. 2017).• With foliar diseases biological control is less certain, but virulence may change over time.
• Wound parasites are dependent on damage to the tree.
• In New Zealand the native Armillaria species are generalists; while they are difficult to completely eliminate, they are not a major problem because of the changing afforestation practices.• Site selection can often reduce the impact of the pests and diseases.• Silvicultural techniques, breeding strategies, biocontrol and copper sprays have allowed radiata pine to be successfully grown on a wide range of climates and sites in New Zealand.

RADIATA PINE STAND GROWTH RATES
Over time there have been improvements in growth rates of radiata pine in New Zealand (Table 1).This has been due to several factors.There was the initial huge increase in growth rates relative to the growth rates of radiata pine in California (Mead 2013) which was probably due to higher site productivity compared with where radiata pine is found naturally.
Other factors contributing would have been an absence of some pathogens and Western dwarf mistletoe that occur in the natural range (Offord 1964, Roy 1966), the use of planted nursery stock rather than natural regeneration, and a release from the 'neighbourhood inbreeding' that tends to occur in natural stands.After the first generation, land-race formation would begin, in response to natural and silvicultural selection for adaptation to the new environments, with some contribution from a release from natural neighbourhood inbreeding.
Countervailing influences, however, could be poor seedcollection practices and inappropriate regional seed transfers.Burdon et al. (1992) reported a modest but definite land-race effect, although in later papers Burdon et al. (1997Burdon et al. ( , 1998) ) showed that this effect was variable among New Zealand sites.During the first major planting period (1921 to 1936) local seed was used, and from the 1950s improved seedcollection practices and then intensive selective breeding and new site-amelioration practices were adopted.In a comparison between first-and second-rotation sites, without intensive silviculture or genetically improved trees, Woollons (2000) also found a small improvement in growth rates.One might expect that the gradual accretion of injurious biotic agents over the last 100 years would be reflected in reduced growth rates.However, any such reduction in New Zealand has been more than offset by more intensive silviculture and tree breeding, as productivity has increased in later rotations (Mead 2013; Table 1).This is supported by several other studies.Kimberley et al. (2015) found that compared with unimproved seed from 'bulk' collections, seedlots of moderate and high genetic improvement (denoted by growth and form (GF) ratings) have increased growth rates by 11 and 25.5%, respectively.Thus, on an average site, mean annual increments (MAIs) were 26, 29 and 33 m 3 ha -1 yr -1 for the unimproved, GF 17 and GF 22 plants, respectively.Beets et al. (2019) found that MAI was 18% higher (26 vs 31 m 3 ha -1 yr -1 ) when radiata pine was planted on former improved ryegrassclover pastures, because of their elevated soil fertility.Incidentally, the greatest growth rate reported by Shula (1989) for radiata pine in New Zealand was 52 m 3 ha -1 yr -1 , while the FIGURE 9 Red needle cast (Phytophthora pluvialis) defoliating radiata pine in a higher rainfall area of the Central South Island, New Zealand (Photo: D J Mead; taken 2022 after a wet spring) mean for the 106 plots with over 40 m 3 ha -1 yr -1 was 43 m 3 ha -1 yr -1 .Improved growth rates in later rotations of pine plantations have also been measured in other countries where good silviculture has been applied, despite changes in pests and diseases (Evans 2009, Fox et al. 2007, O'Hehir and Nambiar 2010).In contrast, as discussed later, PPC and other factors have reduced the importance of radiata pine as a plantation species in Spain and South Africa.
Modelling studies on climate change suggest that this may result in faster radiata pine growth rates in future (Watt et al. 2019).Their study also suggested that climate change may result in some changes to diseases; for example, Dothistroma and Cyclaneusma needle casts may be reduced in the North Island but increase in the South Island.They suggest the greatest biotic threat may come from making New Zealand plantations more suited to warm-tropical or sub-tropical insect pests and pathogens that are accidentally introduced.For abiotic threats their model suggests greater wind damage due to taller, more-slender radiata pine trees, plus increased fire risk.However, recent research predicts that climate change would have to be very pronounced to increase fire danger in New Zealand, as it has been decreasing in many places in the last 20+ years (Dudfield et al. 2021).

OVERSEAS THREATS TO RADIATA PINE IN NEW ZEALAND
There is the ever-present possibility that new pests or diseases will be introduced to New Zealand, some known to be troublesome, some known but with no history of being troublesome, or some completely unknown (Brokerhoff and Bulman 2014, Brockerhoff et al. 2023, Herron et al. 2020).Many suggest pine pitch canker (PPC) is the greatest threat because of how it impacted the native Californian stands and the Spanish and South African radiata pine plantations and its spread to many other countries (Amaral et al. 2022).It is currently not in New Zealand or Australia, but has been present in radiata pine nurseries in Chile since 2001.It is a notifiable organism under New Zealand's biosecurity act (Biosecurity New Zealand 2022a).
After considering PPC we consider several of the other threats to New Zealand's plantations that have been identified.

Pine pitch canker
Pitch canker is a fungal disease (Fusarium c ircinatum Nirenberg & O'Donnell) that is spread by spores, often assisted by insect vectors and human activities (Amaral et al. 2022, Biosecurity New Zealand 2022a, Gordon et al. 2015, Wingfield et al. 2008, Zamora-Ballesteros et al. 2019).In nurseries PCC causes a root disease that kills seedlings (damping-off), while in older trees it forms cankers that degrade or may eventually kill (Figure 10).Insect vectors are important when the canker invades established trees, as their damage provides a place of entry.Humid and wet conditions aid dispersal and thus often the stands most affected are where these conditions exist, as in coastal California and NW Spain (Ganley et al. 2009, Wingfield et al. 2008;Philip Cannon, pers. comm. 2022).The disease is reportedly also favoured by good nutrient conditions, as well as by environmental stresses (see review by Wingfield et al. 2008).
Pitch canker was first detected in pine plantations in the south-east USA in 1945 although it is thought to be Mexican in origin (Drenkhan et al. 2020).According to their comprehensive study, it is known to infect 106 different species -67 pines plus 18 pine hybrids, six non-pine conifer species including Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco), and 15 grasses and herbs.Radiata pine is highly susceptible, in both nurseries and established stands.Douglas-fir and several other conifer species grown in New Zealand are susceptible to PPC, but fortunately ryegrass and clover, the major pasture species used in New Zealand, are not.In nurseries, many Fusarium species can infect a wider range confirmed it should be largely confined to the coast (Ganley et al. 2009).
Research has indicated that radiata pine seedlings and trees can develop systemic acquired resistance to PPC (Gordon et al. 2011, Reynolds et al. 2016, Swett and Gordon 2017).This suggest that acquired resistance may allow radiata pine to adapt to the exotic pathogen relatively quickly, and it has been suggested that this was behind the Californian outbreak subsiding (Gordon et al. 2015, Wingfield et al. 2008), although lower rainfall may have also contributed (Philip Cannon, pers. comm. 2023).Further, recent observations suggest that PPC is still active in natural radiata pine stands located in the coastal fog-drip area and is causing severe damage to Pinus muricata stands in coastal California (Philip Cannon, pers. comm. 2023).Individual trees in highly infected stands often show no symptoms, suggesting that it may be possible to breed against the disease.However, Wingfield et al. (2008) noted that the genetic base of the pathogen is very narrow in California compared to elsewhere, and this may complicate breeding for resistance if several PCC strains are present.

Spain and other European countries
In 1995 PPC was identified in nursery seedlings in northern Spain, where radiata pine is concentrated (Drenkhan et al. 2020, Wingfield et al. 2008).The first report, in a 20-yearold established stand, was in 2004.In 2006 Spain began a government-enforced eradication programme, and infected areas were not able to be replanted in susceptible species (Jorge Martín García pers.comm.2021).Closely monitored 1-km-wide buffer zones were established around infected stands (Drenkhan et al. 2020).This has been partially successful, so that PPC is now confined to the coast, where further efforts are underway to control it.Detailed effects on the plantation forest industry and the loss of radiata pine area are unavailable (Jorge Martín García pers.comm.2021).There will have been costs associated with monitoring programmes, destroying infected trees, increased nursery costs and restrictions on wood exports.Quite unknown are opportunity costs, if any, from possibly over-cautious elimination of radiata pine.It is not known if radiata pine is developing increased resistance in the field, because of the eradication programme (Jorge Martín García pers.comm.2021).
The EU also mandated a similar eradication plan to prevent the further spread of PPC.In Portugal PPC was found in radiata pine and Pinus pinaster Aiton in 2007, and an eradication plan was undertaken.In 2016 PPC was found in an established stand which was destroyed, and this seems to have been successful (up to 2020) in eliminating the disease (Drenkhan et al. 2020).
Pitch canker has also been found in France and Italy, but in both countries the disease has been eliminated (Drenkhan et al. 2020). of tree genera and species (Gordon et al. 2015).Fusarium oxysporum Schlecht.is a widespread damping-off fungus in New Zealand nurseries but is seldom problematic (Dick and Dobbie 2002).Nurseries tend to be of higher hazard because of their microclimates and management practices, as well as being a way of spreading PPC to plantations (Drenkhan et al. 2020, Wingfield et al. 2008) Californian natural stands Pitch canker (Figure 10) was first recorded in Californian radiata pine trees in 1986 (Gordon et al. 1986).Several insect vectors were identified, and these were the major factor in the infection and death of older trees there; the spread by spores without any insect vector is minor (Drenkhan et al. 2020) 2 .Cankers are most prevalent close to the coast and in isolated trees such as on golf courses.The disease has been worst in areas of higher humidity and fog, and climate modelling also

South Africa
The first record of PPC in South Africa was in 1990, in a nursery of Pinus patula Schltdl.et Cham.(e.g.Drenkhan et al. 2020, Wingfield et al. 2008).It is still found in P. patula nurseries and soon after planting.In 2005 PPC was recorded in 5-and 7-year-old stands of radiata pine that had been attacked by weevils (Coutinho et al. 2007).In 2008 South Africa had 57 000 ha of radiata pine (Mead 2013), and while this area has decreased very substantially, the current area is not known (Mike Wingfield and Josua Louw, pers. comms. 2021).This decrease has been primarily driven by planting other species on marginal radiata pine sites, to reduce wilding spread onto the important fynbos biodiversity areas, mitigate fire risk, and allow increased water yields from catchments.The impact of PPC in nurseries or stands may also have been a factor.It is not known if radiata pine is gaining induced immunity, nor how PPC has affected industries based on the species.

Chile
Pitch canker was first recorded in radiata pine nurseries and clonal-propagation hedges in 2001 (Wingfield et al. 2002).Strict measures were implemented to control its spread, but 2013 and 2019 data found it was in 39 and ~50 nurseries, respectively (Ahumada and Rotella 2020, Carrasco et al. 2016;Rodrigo Ahumada pers. comm. 2021).However, it has not been found in established stands of radiata pine, presumably because of a lack of insect vectors and its sub-optimal climate (Ganley et al. 2009), although Wingfield et al. (2008) has suggested it may do so in future through insect spread.Nursery control measures have been expensive and are Government-mandated.The bark beetle Hylastes ater Paykull, which is present (Mausel et al. 2007) and can feed on the bark of young, live seedlings, is a potential vector from any infected nursery stock.Hylastes ater has been recorded occasionally in Chile as having PPC associated with its damage on young trees, but has not been considered an important vector there or potentially in New Zealand (Brockerhoff et al. 2016, Wingfield et al. 2002).

PPC threat to New Zealand
The likelihood of PPC reaching New Zealand is considered to be low, with the greatest threat of introduction coming via global trade and international travel (Brokerhoff et al. 2016).Even if it reached Australia, it is unlikely that the thin-walled spores would survive being blown 2000 km over the Tasman Sea to New Zealand.Further, the likelihood of insects getting spores from the plant surface alone is low; it is more likely they would transmit the disease when they have been in close contact with diseased tissue (Brockerhoff et al. 2016).The greatest threat for forestry in New Zealand, if PPC was introduced, would be in nurseries, with a potential for field vectoring by Hylastes ater as in Chile.The risk for established plantations is mitigated by there being no known insect vector; indeed, Ganley (2007) and Brockerhoff et al. (2016) suggest New Zealand is unlikely to have insect dispersal to established plantations.Climate modelling suggest the greatest danger is currently in coastal areas of the northern North Island, where there would be less cold stress.With climate change the susceptible area might expand to most of the North Island and to parts of the east coast of the South Island (Watt et al. 2011, Watt et al. 2019).If PPC did get established in stands its impact could be reduced through silviculture, control of any insect vectors that might appear, avoiding very humid sites such as fog-or mist-prone areas, and in the longer term through breeding (Carrasco et al. 2016, Gordon et al. 2015, Zamora-Ballesteros et al. 2019).Perhaps, based on Californian experience, radiata pine could become resistant over time if the disease became established (Gordon et al. 2015).
Finally, New Zealand has a well-developed surveillance program for forest pest imports and this includes an action plan if an incursion of PCC is detected (Biosecurity New Zealand 2022a,b).
Several other potential threats to New Zealand's plantations have been identified.These include:

European pine tip moth
The European pine tip moth (Rhyacionia buoliana Denis & Schiffermuller) became epidemic in Chile from 1989, but has since been less important because of a complex biological control with introduced and native parasitoids and hyperparasitoids (Mead 2013).The moth attacks shoots, particularly younger trees, leading to malformation and growth reductions.This pest is also found on radiata pine in Spain.

Western gall rust
The Western gall rust (Endocronartium harknessii (J.P. Moore) Y. Hiratsuka) is common in Californian natural radiata pine stands (Figure 11;Offord 1964, Old et al. 1986, Mead 2013).Although this fungus is widespread through North America it has not spread elsewhere, and is considered of low risk of being introduced to New Zealand (Ramsfield et al. 2007).This rust does not need an alternate host, but it is only a problem for young trees.Cedros and Guadalupe Island provenances have shown higher resistance to this rust than mainland populations (McDonald andLaacke 1990, Old et al. 1986).Although this disease would likely be troublesome if it were introduced (Ramsfield et al. 2007), it might well be controlled by removal of infected trees, sprays, tree breeding and using post-juvenile cuttings.

Pine processionary moth
The pine processionary moth (Thaumetopoea pityocampa Denis & Schiffermüller) from southern Europe could potentially do considerable damage to radiata pine plantations in New Zealand, as this species is a preferred host (Brockerhoff et al. 2006, Kriticos et al. 2013).However, it has not spread outside its native habitat.There are several potential parasitoids and control methods available if it did appear, although these could be costly to implement (Kriticos et al. 2013).

Invasive wood-boring and bark beetles
Of the many non-native bark beetles, only Hylastes ater and Hylurgus ligniperda (Fabricius) have become established in New Zealand (Mausel et al. 2007).The burnt pine longhorn beetle (Arhopalus tristis F.) has also been introduced to New Zealand.None of these three is currently an issue for growing radiata pine in New Zealand, but can spread sapstain to logs and create problems for wood exports (Brockerhoff et al. 2017).That study suggested they may become more important with climate change.Although, as has already mentioned, H. ater is unlikely to be a dangerous vector for PPC, it has been recently found in a small area of dead radiata pine trees in Kaingaroa forest in the Central North Island where it was associated with the nematode Bursaphelenchus hildegardae Braasch (Zhao et al. 2021).It is unclear if this was a secondary infection, but the paper suggests it and a closely related species (B.eggersi Rühm.) have probably been in New Zealand for some time.Ips grandicollis Eichhoff has been a problem on some dryer sites in Australia, and has been caught by New Zealand border controls, but has not become established; it and related Ips species may pose a significant risk (Brockerhoff 2009, Brockerhoff and Bulman 2014, Brockerhoff et al. 2006, 2023).Other secondary bark beetles known to attack weakened radiata pine outside New Zealand, and which might become vectors of PPC if this fungus appeared in New Zealand, are Orthotomicus erosus Wollaston, Pityophthorus pubescens Marsh., Dendroctonus valens Leconte and Tomicus pineperda L. (Bezos et al. 2018, Brockerhoff 2009, Brockerhoff et al. 2023).Finally, although Brockerhoff (2009) suggested that radiata pine appeared to be resistant to the nematode B. xylophilus Steiner & Buhrer, that is spread by bark beetles, it has been found, after inoculation, to cause pine wilt disease in radiata pine in Spain (Menéndez-Gutiérrez et al. 2021).These authors found that it should be possible to breed trees resistant to these nematodes.
Thus, there is a long-term risk posed by introduced bark beetles although they are likely to be troublesome mainly in low rainfall areas where growth rates make commercial forestry less attractive.Nevertheless, on-going surveillance programmes are vital.

Lymantria moth species
The European spongy moth (Lymantria dispar L.), which was formerly known as gypsy moth, is a noxious species and can rapidly expand into new areas, causing serious, large-scale defoliation (Boukouvala et al. 2022).In their review they note the caterpillars have been recorded as attacking over 500 tree species, mostly deciduous, although they have been reported as damaging radiata pine in Portugal and Spain (Castedo-Dorado et al. 2016).There are three subspecies, two of which are Asian (Asian spongy moths) (Boukouvala et al. 2022).Populations are often low but are capable of rapid expansion causing major damage, particularly in wetter conditions.A small invasion from Asia was eliminated in New Zealand in 2005.
A related serious defoliator, the nun moth (Lymantria monacha L.) is also considered a threat and has been intercepted in New Zealand ports (Brockerhoff and Bullman 2014).

THE WIDER FORESTRY PERSPECTIVE
Recent reviews have found that the recent upsurge of introduced forest diseases can be attributed to human influences, primarily through trade and climate change (Panzavolta et al. 2021, Woodward et al. 2022).For example, in the USA about 450 species of non-native tree insects and diseases have become established since 1800, of which 15 species were considered high-impact (Aukema et al. 2010, Fei et al. 2019).Of these 15, nine invasive species caused significantly greater mortality than in uninfected areas, and only one of the 15 was a generalist, the European spongy moth (Fei et al. 2019).As discussed above, for isolated New Zealand the rate of actual FIGURE 11 Western gall rust (Endocronartium harknessii) on the stem of radiata pine in Northern California (Photo: D.J. Mead;taken in 1992) importations of significant pests and diseases has been much lower, although they are possibly becoming more frequent as global trade has expanded (Brockerhoff and Bulman 2014, Brockerhoff et al. 2023, Herron et al. 2020).
Nevertheless, the complete devastation of a forest tree species from introduced pests or diseases is very uncommon provided the species is adapted to the site.Failures of radiata pine in humid, summer-rainfall areas simply reflect the fact that it is not adapted to these conditions (Burdon et al. 2017, Mead 2013).In the continental USA, where 881 native tree species have been recorded (Carrero et al. 2023), there have, despite the large number of introductions, been only five tree species where >90% of the host has been invaded (Fei et al. 2019).These cases were the widespread devastation by Dutch elm disease (Ophiostoma ulmi Buisman and O. novo-ulmi Brasier) on Ulmus americana L.: chestnut blight (Cryphonectria parasitica, (Murr.)Barr. on Castanea dentata (Marshall) Borkh.; white pine blister rust caused by Cronartium ribicola J. C. Fisch; butternut canker, Ophiognomonia clavigignentijuglandacearum Broders & Boland on Juglans cinerea L. in Wisconsin (and also Canada); and Phytophthora lateralis Tucker & Milbrath on Chamaec yparis lawsoniana (Murray) Parl.(Boyd et al. 2013, Broders et al. 2015, Budde et al. 2016, Ennos 2015, Hansen 2015, Hunt et al. 2010, Schlarbaum et al. 1998, Woodward et al. 2022).
Dutch elm disease, chestnut blight and butternut canker affected almost all the native trees of the species, even though they were often in mixed stands.Tree breeding for resistance has been difficult with most of these diseases; it has taken 100 years since first recorded to develop resistant varieties of chestnuts (Woodward et al. 2022).Hypovirulence (the infection of the fungus with a virus) has been found with chestnut blight (Budde et al. 2016, Woodward et al. 2022).With Dutch elm disease, the main control method is killing bark beetles with chemicals as these insects spread the fungus.White pine blister rust, which needs Ribes as an alternate host and was introduced on seedlings from Europe in about 1910, has spread over North America infecting 5-needle pines in natural forests and plantations, some of which are considered keystone species (Hunt et al. 2010).It has also severely affected commercial forest management, although it has not completely wiped out any white pine species and there has been some success in breeding resistant varieties (Pike et al. 2021).Butternut canker has killed trees even though it is usually a minor component of stands; it has also successfully colonized several species of Juglans and Carya (Broders et al. 2015).There have been several introductions of this disease, but the recent virulent strain has spread faster than chestnut blight or Dutch elm disease.Phytophthora lateralis was first introduced around 1920 to horticultural nurseries from Asia and about 1950 it appeared in mixed conifer stands (Hansen 2015).It quickly spread via roads, machinery and waterways and is now common in southwest Oregon and northwest California where it has devastated lower-altitude stands of Chamaecyparis lawsoniana.The fungus infects the roots allowing the weakened trees to be attacked by bark beetles.Sanitation and tree breeding for resistance have been the main methods of combating this disease (Hansen 2015, Pike et al. 2021).
There are also several other introduced biotic agents that are causing great concern in the continental USA, affecting mixed natural forests, urban trees and some tree crops (Fei et al. 2019, Pike et al. 2021).These include the emerald ash borer (Agrilus planipennis Fairmaire), the red bay ambrosia beetle (Xyleborus glabratus Eichhoff) and its associated fungus (Raffaelea lauricola T.C. Harr., Fraedrich & Aghayeva), which causes a wilt disease in the Lauraceae family (including avocados), and the hemlock woolly adelgid (Adelges tsugae, HWA) in the Eastern states.
Of the 232 indigenous tree species in New Zealand, there have been no extinctions from any cause since human occupation (Wardle 2011).Currently, the most concerning forest disease in New Zealand is kauri dieback (Phytophthora agathidicida Weir, Beever, Pennycook & Bellgard).It often kills remnant kauri (Agathis australis (D.Don) Loudon) forest (Bradshaw et al. 2020).It infects kauri growing in mixed kauri/podocarp/broadleaf forests, and is spread by animals, including humans.This fungus attacks other native species as well.Similarly, Newhook (1970) found P. cinnamomi kills kauri and other native species, and a recent survey has found this pathogen can be widespread in kauri forests (Froud et al. 2022).The research effort on P. agathidicida has been summarised by Bradshaw et al. (2020), but it is too early to say if this disease can be mitigated.Recent studies have shown that this disease, which was initially assumed to have been imported about 1945, has probably been here for several hundred years or even longer (Winkworth et al. 2021), and it is only spreading slowly, often associated with human activities (Froud et al. 2022).The latter report also suggests that much of the ill-thrift seen in kauri forests is not associated with Phytophthora species.
Recent reviews on the resilience of mixed forests to abotic and biotic disturbance have concluded that mixed-species forests are often more resilient, but are not always better than monocultures (Bauhus et al. 2017, Jactel et al. 2017, 2021, Roberts et al. 2020, Staab and Schuldt 2020).Often the outcome depends on the species' properties and whether they are being impacted by specialist or generalist organisms, and how the impacts change over time.In particular, specialist insect herbivores often cause less damage in mixed stands compared to monodominant stands when they include tree species from very different genera or families (Jactel et al. 2021).The use of mixtures to reduce the impact of diseases can be helpful with Armillaria and similar fungi but may be less so with foliar diseases or generalist pathogens, and also often depends on the species in the mixture (Jactel et al. 2017, Roberts et al. 2020).For example, for Dothistroma, which has been a problem in radiata pine plantations in New Zealand, a metaanalysis of world data found that mixed stands were just as susceptible to the disease as single-species stands (Drenkhan et al. 2016).Further, as discussed above for the worst disasters in the USA, mixed forests did not reduce their impact.Roberts et al. (2020) also suggests that there are very few rules regarding how to approach control, and that forest managers need to manage trade-offs of increased resilience against other benefits obtained from forests.
These reviews do not consider the implications that some tree species, such as radiata pine, in their natural habitat often occur essentially as single species stands as they usually regenerate following major disturbance events.Nor do we know of any studies where biotic stresses have been investigated in mixed stands where radiata pine is a major component, such stands being difficult to find on account of radiata pine's fast growth rate.

RISK-REWARD RELATIONSHIPS FOR RADIATA PINE
It is clear from our review that monoculture radiata pine plantations in New Zealand, while gradually getting more biotic pests over the last 100 years, are not currently being threatened by any devastating pests.That, however, does not mean that biotic risks are absent or even nearly so.Indeed, they are very significant because of the scale of the radiata pine plantations, despite the low probability of a biotic catastrophe (Burdon 2010, Burdon andDungey 2015).But such risks must be weighed against the costs and risks of growing alternative tree species or even of resorting to alternative land uses, along with the net mitigation of risks achievable through such alternatives.
With alternative tree species, the most relevant costs are opportunity costs in profitability relative to that of growing radiata pine.Because of lower productivity, or need for longer rotations, or much greater sensitivity to site variation, and often problems of utilisation and marketing their wood, alternative species are typically far less profitable to grow than radiata pine (Maclaren 2005 3 ).Site sensitivity can effectively reduce a species' wide use, often making them more suited to small growers of speciality woods.Considering Douglas-fir, the second most widely planted plantation species in New Zealand, it is limited to cooler areas, has 35+ year rotations, often suffers from needle cast and has some-wood property limitations, even though it may sometimes be a good investment and has an active breeding programme (Cown 2014, Suontama andDungey 2018).Coast redwood (Sequoia sempervirens), while potentially extremely productive and can produce very valuable timber, is far more site-sensitive and requires longer rotations (Watt et al. 2021, Watt andKimberley 2022).Prior knowledge of its site sensitivity, by driving local choice of sites, could easily lead to an upward bias on a country-wide productivity assessment.Cypresses (Cupressus species) are sometimes suggested as an alternative to radiata pine in New Zealand (Bulman and Hood 2018).While they can produce valuable timber, their use as an alternative to radiata pine is currently severely limited by cypress canker (Seiridium species); they are also more site-sensitive and are often less productive than radiata pine, and require longer rotations (Hay et al. 2005, Watt et al. 2023).There is a suite of Mexican and Central American pines that promise complementary spectra of disease resistance (Burdon and Dungey 2015), but they are currently not competitive with radiata pine for growth rate and proven site tolerances, while their silviculture and utilisation potential in New Zealand are greatly under-researched.Eucalypts (Eucalyptus species), while they can be very fast-growing, tend to be site-sensitive and are not regarded as an alternative to radiata pine on a large scale (Hay et al. 2005).In New Zealand, they have tended to be affected by insect pests arriving from Australia (Kay 2005) without their natural enemies.
It is possible to draw up a list of contingency species (Burdon andMiller 1995, Hay et al. 2005), which might be used to replace radiata pine if a biotic catastrophe prevented its use over much or all its present commercial range.Some of those species, however, have not been researched in depth because radiata pine has been so strongly preferred, or their available seed sources are very inadequate.Big knowledge gaps involve the site categories on which species would be the best replacements for radiata pine.More specifically, these gaps involve site tolerances, productivity, biotic vulnerabilities, the appropriate propagation and silviculture, and the commercial potential of the wood.There are certainly niches for other species, but these are usually appropriate for small growers or where specific wood properties are sought such as ground-durable eucalypts (Millen et al. 2018).
In contrast to many countries, few native forests are currently managed or planted primarily for wood production; currently their total annual production in New Zealand is only 10 000 tonnes of logs (Bergin andGea 2005, NZFOA 2023).Podocarpus totara D.Don and kauri show most promise but as their rotation lengths would be twice or more that of radiata pine and they are not considered as replacement species for radiata pine plantations.They, and other native species, are more suited to specialist wood markets than as generalpurpose timbers, and are primarily promoted for their biodiversity, cultural roles, and long-term carbon sequestration.
Climate change may alter some risk profiles, abiotic as well as biotic, of alternative species.However, net changes in overall risk levels for individual species are often unclear.Predictions of climate change and its impacts are usually coarse and can lead to different interpretations as illustrated above for future fire risk.On balance, climate change is unlikely to substantially change the risk-reward relationship.
In New Zealand alternative land uses are often highly problematic.For meeting carbon sequestration obligations farming and even horticulture cannot compete.Reversion to native vegetation can be the long-term optimum for carbon sequestration but, importantly, gives slower early sequestration, and may not offer sequestration through timber products.Pastoral farming over large areas is less profitable than commercial forestry, gives limited carbon sequestration, and can incur more erosion and degradation of waterways.On much of this land radiata pine is often the clearly preferred species for commercial afforestation.
Despite a risk exposure to radiata pine in New Zealand, in a context of changing climates, the case for major preemptive diversification from this species is weak.This reflects radiata pine's: • biotic resilience historically shown in New Zealand, • broad site tolerances, • usually much greater commercial attractiveness compared with alternative species, • great uncertainties, which include biotic risks, surrounding some possible alternatives, • and lack of preparation for recourse to alternatives.
In our opinion, extra security from pre-emptive diversification is very uncertain, and on present knowledge would involve very high opportunity costs.

A DEFENSIVE STRATEGY
Despite the low risk of a biotic catastrophe for radiata pine in New Zealand, the heavy reliance on it, largely for want of suitably competitive alternatives, demands a comprehensive, multi-pronged defensive strategy.Strict border controls and contingency plans must remain a key plank of the defensive strategy, minimising the likelihood of further incursions (Biosecurity New Zealand 2022a, Biosecurity New Zealand 2022b, Brockerhoff and Bulman 2014).Fortunately, those controls have broad political and institutional support.To cope with incursions that will doubtless still come, a range of preparations are needed.Strong technical infrastructure is needed to attempt eradication of incursions, or to develop control measures for uncontained incursions.
Genetic defences are another key plank for coping with uncontained incursions.The current inclusion of resistance to existing foliage diseases in breeding goals needs to be complemented by other genetic defences.A broad genetic base (germplasm) within radiata pine in New Zealand is required to enable breeding for resistance (Brockerhoff and Bulman 2014Burdon 2010, Burdon and Dungey 2015, Budde et al. 2016).And gene technology capabilities need to be available, including genomic selection and genetic transformation and/or gene editing.A further line of genetic defence would be to identify and prepare for using contingency species that are available within the country (Burdon and Dungey 2015).However, the attractiveness of genetically improved stocks of radiata pine makes meeting the opportunity costs of hosting and managing back-up genetic material a severe test of institutional discipline (cf Pike et al. 2021).

SUMMARY
This review of the past and possible pests and disease threats to future radiata pine plantations in New Zealand highlights: 1.There have been many biotic scares in New Zealand over the last 100 years, but these pests and diseases have generally subsided or are able to be managed.Some will have reduced growth rates and incurred additional costs.2. However, growth rates of these plantations have increased over the 100 years since the species was widely planted.
3. We can expect further incursions from outside New Zealand.Prevention of these at the border must be a priority.4. If an incursion occurs, attempt eradication.5. Avoid planting radiata pine on ecologically marginal sites, as these often increase risks.6. Tree breeding and various silvicultural treatments, aimed at ensuring healthy trees, have often proven to reduce the impact of epidemics.Actual breeding, however, is only part of a comprehensive package of genetic defences.7. Despite the seemingly reasonable contention that monocultures are particularly susceptible to severe outbreaks, this has not always been borne out in practice.Further, radiata pine in its natural habitat grows as a single species and there is no proven advantage of growing it in mixtures.8.For pine pitch canker, the experience from California, Europe, South Africa, and Chile does not suggest it should prevent future use of radiata pine plantations in New Zealand.However: a.We do not want this disease to become established in New Zealand.b.But the likelihood of it establishing in New Zealand is considered low, even if it reached Australia.c.If it became established in New Zealand it is likely to be a problem in nurseries or perhaps in foggy, very humid areas.9. We should prepare contingency plans, with gene conservation and breeding strategies given priority.
In summary, we agree with Burdon and Miller (1992) that radiata pine is still New Zealand's best option for a mediumdensity softwood species for large-scale commercial forestry.We would, however, add one caveat.Climate change may mean that the optimum zones to grow the species will shift, particularly if parts of the country experience higher summer rainfall.But climate change is likely to similarly impact other ecosystems including native forests and agriculture.New Zealand's long experience with radiata pine should provide guidance to other plantation growers.

DECLARATIONS
No funding was received to assist with the preparation of this manuscript and the authors have no relevant financial or non-financial interests to disclose.
Open access publication of this work was funded by the New Zealand Ministry of Business, Innovation and Employment Strategic Science Investment Fund (C04X1703).

FIGURE 3
FIGURE 3 Patches of Sirex (S. noctillo) damage in radiata pine at Matahina in the Central North Island of New Zealand, at the peak of the epidemic.(Photo:Scion, Rotorua; taken about 1950)

FIGURE 4 FIGURE 5
FIGURE 4Severe killing of semi-mature radiata pine at the peak of the Sirex (S. noctillo) epidemic at Matahina in the Central North Island.(Photo:Scion, Rotorua; taken early  1950's)

FIGURE 6 FIGURE 7 FIGURE 8
FIGURE 6 Neonectria fuckeliana sometimes enters via pruning scars causing stem cankers on radiata pine in the SouthIsland, New Zealand (Photo: D.J.Mead; taken in 2007)

FIGURE 10
FIGURE 10 Pine Pitch Canker (Fusarium circinatum) on Pinus muricata D.Don at Salt Point State Park, California.PPC often results in resin flow on the stem (Photo: L. Pior, CA State Park; taken in 2023)

TABLE 1
Estimated changes in mean annual increments of radiata pine stands over time in New Zealand (NZ) and compared to growth rates in the Californian natural stands.This is based on published reports and are not from designed studies MAI = mean annual increment about age 30 years.MAI figures are inside-bark total stem volumes and are NZ averages.Some figures converted from harvested to total volumes.** Change from natural stands (assumed 10 m 3 ha -1 yr -1 ).*** Californian stands are natural regeneration and untended.The seed for the 1921-36 plantings came from small earlier stands and shelterbelts in New Zealand and were mainly Año Nuevo and Monterey provenances.The seed for the 1950s was largely bulk collections from existing stands although some came from selected trees.From the 1980s on, improved establishment, fertilization of deficient stands and improved seed were being used. *