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 Breeding poplars and willows

Ian McIvor
New Zealand Tree Grower February 2008

New Zealand’s primary wealth depends on fertile land and plentiful, clean water. Water is essential to New Zealand’s productive economy. However water in excess can cause economic loss from soil erosion. Preventing erosion and maintaining river protection requires constant management, and is a key responsibility of regional councils. Soil must be retained on stabilised hillsides, and swollen rivers must be retained within stable riverbanks and engineered stop banks.

Widely used trees

Willows and poplars have been the most widely used trees for planting in these unstable situations for a number of reasons. They establish readily from poles and cuttings in difficult environments and in the presence of domestic stock on grazing land. New clones with superior characteristics such as improved disease resistance can be multiplied rapidly by hardwood cuttings in local authority and private nurseries and by individual farmers. Their extensive and unique root networks anchor unstable soils on slopes, and they strengthen and assure the integrity of riverbanks, thereby reducing the impacts of water movement on valuable productive land and urban settlements.

Their high evapo-transpiration rates during the growing season ease management problems in wet areas. Willows and many poplars are tolerant of flooding and periodically saturated soils. Their early growth rate is superior to all cool temperate trees with the exception of some eucalypt species.

Multi-purpose
Poplars and willows are truly multipurpose trees. They provide shade, shelter and supplementary fodder for stock. Fodder can be pruned from wide spaced trees and windbreaks and fed to stock during summer drought, or they can be produced as coppice shoots from high density plantings.  They are deciduous, allowing pasture to be retained under the canopies while the foliage provides high quality herbage similar in nutrient value to lucerne hay.

The genus Populus also represents a largely untapped resource of potential species for production forestry in New Zealand. They grow as fast or faster than radiata pine, with contrasting wood and fibre characteristics. They could provide diversity for the timber industry. This diversity could be attractive to the landowner, especially if healthy species producing high value products can be identified and grown, and especially if landowners are pressured or legislated to retire highly erodible hill country from pastoral activities.

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The importance of breeding and research

Poplars and willows are vital components of hillside erosion management systems, and willows are a vital component of riverbank protection. These management systems must be flexible, dynamic and resilient in order to respond to changing land use, changing climate and the arrival of new pests and diseases. In particular the conservation and protection values of the poplar and willow trees are at risk from diseases such as rusts and pests such as willow sawfly which have no natural controls in New Zealand.

Because of their key role in land and riverbank protection, poplars and willows require a breeding and research programme to provide the strategies needed to retain adaptability and minimise vulnerability to new diseases and climate conditions. New Zealand has benefited substantially from the international network of poplar and willow scientists that constitute the International Poplar Commission, a technical body of the FAO, to which New Zealand is a signatory.

Willow sawfly, which arrived in New Zealand in 1997, highlighted the vulnerability of our predominantly monoclonal riverbank protection plantings to biological threats. It has also prompted research into some novel plant-based approaches to complement willows in the design of future riverbank protection plantings.

The current programme
The research programme has two major goals −

• To breed new poplar and willow clones for versatile soil conservation and riverbank protection plantings, and to provide the scientific knowledge and management strategies that underpin decision making
• To develop new control or management strategies to reduce the effect of willow sawfly on current flood protection and soil conservation willow plantings.
  

Tree breeding research is long term because 10 to 15 years are required from production of hybrid seed to final bulked material being available for end users. New Zealand’s national breeding programme has provided many new genotypes for different purposes and with different adaptations suitable for particular regions over the past 30 years. The focus has been on producing trees with particular growth and productive characteristics. However the arrival in New Zealand of new fungal diseases in particular has highlighted the need to breed for resistance to new pests and diseases as well as existing
ones, such as possums.

Hybrids the mainstay
Domestication of poplars and willows internationally has been a process of interspecific hybridisation and clonal selection. The hybrids have given poplars their prominence, and it is safe to say that hybrids will be the mainstay of both poplar and willow culture in the future. The ability to cross breed among many of the species is high, vegetative propagation is easy, and juvenile growth is rapid.
These are all sound reasons for continuing to develop these species for specific uses in the landscape.

New Zealand’s poplar and willow breeding programme uses inter-specific hybridisation and clonal selection to develop willow and poplar genetic material suitable for protecting erosion prone soils such as river berms and hill country slopes. Other uses of this material include shelter, fodder, timber, bioenergy and bio-remediation.

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New clonal material
Overseas breeding programmes are largely focussed on biofuels feedstock, either fibre from poplar or energy from willow and poplar.
The clones developed for these purposes do not necessarily suit our climate or purposes. For these reasons it is advantageous for New Zealand to maintain an indigenous breeding programme and to retain and realise the genetic potential of the species and clones imported in the past which have been successful but remain susceptible to disease. However, Veronese is an imported clone that performs very well in New Zealand.

The requirement for new clonal material is continuing process. Poplars and willows are continually exposed to new insect pests and diseases. Recent known incursions include the willow sawfly Nematus oligospilus in 1997, the leaf spot fungi Marssonina castagnei in 1988, and M. brunnei in 1975. The poplar leaf rusts Melampsora larici-populina and M. medusae arrived in 1973.

Poplars and willows are also susceptible to mutations of, and hybrids between, these dominant rust strains. History has shown us that both the introduction of new diseases and pests, and the continuing evolution of new rust races can be devastating for poplar and willow plantings until new resistant varieties are available. Therefore the breeding focus has been on producing versatile and vigorous poplar and willow clones using a wide range of parent species together with importing tested clones. Selection has paid particular attention to the quantitatively determined traits −

• Excellent tree vigour and form
• Pest and disease resistance, particularly rust disease 
• Early bud burst and late leaf fall 
• Wind tolerance 
• Low palatability to possums 
• High rooting ability from unrooted stem cuttings 
• Good performance in the nursery 
• Timber potential 
• Rough bark 
• Leaf shape and colour.
  

From initial crossing to having each of the selection tests completed, then bulking up for commercial use takes 13 years.
Continual improvement in these characteristics is required as poplars and willows are increasingly used for more diverse and often multiple purposes. This includes soil stabilisation, shade, fodder, timber and shelter, and minimising pasture reduction, and unfavourable characteristics may emerge for earlier selections on maturity.

Case study for Flevo poplar
Populus Flevo  − P. deltoides NL 219’ × P. nigra NL 188 − was imported from the Dorschkamp Research Institute for Forestry and Landscape Planning in Wageningen, Holland in 1969. It was released from quarantine in 1970, and remained in the breeding nursery until the arrival of rust disease in 1973.

Due to the arrival of the rust there was not the time to carry out the necessary suitability tests. Instead a gamble was taken and it was released and distributed to all New Zealand nurseries between 1973 and 1975. It has excellent rooting from cuttings, a deep and extensive rooting system, and is highly resistant to the poplar rust diseases Melampsora larici-populina and M. medusae, and to the poplar leaf spot disease caused by Marssonina brunnea.

Flevo was planted extensively over the next 15 years. However its wavy growth form, heavy side branches, leaf palatability and smooth bark have meant this clone is susceptible to possum browsing, bark attack by browsing stock, and especially crown and branch breakage in windy situations. These characteristics were never problems when the clone was planted in a forestry context or even when the trees were up to 15 years old growing on pastoral hill slopes.

However, now old Flevo poplars on hill slopes are large, dangerous trees creating management problems for landowners. Subsequent
breeding efforts have concentrated on incorporating the disease resistance of the Flevo parentage while selecting against its smooth bark, possum palatability and wide branching brittleness. The original Flevo is no longer propagated by nurseries for distribution.

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The breeding and trialling process
Both poplars and willows are dioecious which means they are either male or female. Seedlings resulting from new crosses can be sexed in two to three years for willows, and after eight or more years for poplars. The selection process favours males in order to minimise the uncontrolled dispersal of viable seed with its potential to create a pest problem.

This is an issue with willows more than poplars so it is fortunate that identification can be made early in the selection process before new selections are distributed for field trials. Parents are selected based on their desirable characteristics, and crosses are made under strictly controlled situations. This makes possible crosses not seen in nature because of geographical, or temporal isolation, for example if flowering times are not synchronised.
Steps in the breeding process are as follows −

• Parental stock is selected and male and female flowering times are synchronised
• Pollen is collected from the male parent flowers and manually transferred to the flowers of the female parent 
• Viable seeds are collected and planted in trays in a greenhouse 
• Seedlings are transplanted in root trainers for growing on 
• After one year all seedlings are labelled and planted out in the nursery to assess survival, vigour, form, disease resistance etc 
• The best 10 per cent performers are selected after one to two years and bulked up to evaluate the inheritability of the characters 
• A further selection occurs and the best are field tested with already released clones at contrasting trial sites across New Zealand for upwards of 8 to 10 years in the case of poplars 
• Field-tested clones are released to regional councils and commercial nurseries for propagation and sale.
  

What is currently available? Tree willows include S. matsudana PN227 and three S. matsudana x alba clones with PN227 as female parent, of which only two, Tangoio and Moutere, are used extensively. Shrub willows available include the S. purpurea series bred in New Zealand such as Pohangina, Irette and Aokautere, Kinuyanagi, S. viminalis and some others. Poplars include the 1980s series of P. x eumericana such as Tasman, Argyle, and Fraser, Crowsnest, Kawa, Toa, Veronese, and others. Local suppliers can give advice on suitability and availability.

What is the future focus?

Willows
Current riverbank stabilisation programmes using tree willows are dependent on the resilience of S. matsudana and three S. matsudana × alba clones with PN227 as female parent. Only two, Tangoio and Moutere, are used extensively. This makes these programmes vulnerable to new ecological conditions resulting from pest incursions and climate change. S. matsudana is susceptible to the weeping willow rust disease Melampsora coleosporoides in its natural habitat in China.
The brittleness and poor form of other clones select against them for management reasons. The breeding programme is focussing on using a wider genetic base to breed new sawfly-tolerant non-brittle, rust-resistant tree willows, particularly for riverbank stabilisation and farm use.

Poplars
Populus trichocarpa grows in Washington and Oregon states in similar climatic conditions to those found in New Zealand. P. trichocarpa has several qualities desired in our breeding programme including faster growth, rougher bark, and resin producing properties. P. trichocarpa carries a dominant allele which is strongly associated with resistance to Melampsora medusae rust. The genetic advantage of this species will be used in developing new poplar selections for soil stabilisation, shelterbelts and timber.

Funding for the future
Poplar and willow breeding and applied activities have been running since the 1950s. Central government investment was substantial up till the 1990s, but currently poplar and willow breeding and associated research activities are not centrally funded, despite the advocacy of NZFFA and regional authorities.

Continued investment is needed to ensure that the country has a range of poplar and willow clones and a knowledge of their performance and suitability for the purposes mentioned. How else can we initiate and respond to new challenges?

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