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PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND

Whorl cankers and multiple leaders: a long term effect of drought

Scion is the leading provider of forest-related knowledge in New Zealand
Formerly known as the Forest Research Institute, Scion has been a leader in research relating to forest health for over 50 years. The Rotorua-based Crown Research Institute continues to provide science that will protect all forests from damage caused by insect pests, pathogens and weeds. The information presented below arises from these research activities.

From Forest Health News 87 July 1999.

The immediate effect of drought on Pinus radiata in Canterbury is commonly top and leader dieback. This has certainly been the case over the years to the point where a peppering of red and brown tops in a sea of green forest is an accepted part of the plains forest landscape during summer droughts. However, by the following spring the forest canopy is green again and all looks in good order, but is it?

Unfortunately from a forestry perspective the effects of a drought event persist beyond the 'greening up' of the canopy. Typical leader dieback and associated Sphaeropsis sapinea infection, resulting from moisture stress, tends to affect trees around eight years and older. The severity of the dieback can range from the top few centimetres to several meters of the main stem. Often the apical leader is affected, fungal development halts at the junction of the previous seasons growth, the top dies, resulting in the development of one or more new leaders in the following spring. In other cases, infection is initiated lower in the stem and often a persistent canker will form resulting in resinosis and continual dieback at the whorl, and leaving no leader at all.

The  forest  manager  probably   overlooks   this phenomenon to a large degree as the bottom log is usually well formed and things look all right from the ground. The main loss from this type of disorder however, is above four meters. Any double or multi-leader that develops will be of a small diameter, in many cases rendering it unsuitable even for chip. In addition multi-leadered trees, and those with whorl cankers, are far more likely to suffer wind breakage.

A quick assessment of stands over ten years old found between 5% and 55% of trees with double leaders and whorl cankers in the mid to upper stem. In one 23 year old stand an area left untended (1000 trees/ha) had a 50% incidence of whorl canker/multi-leaders where as an area thinned to 300 trees/ha had a 15% incidence. It is well documented that trees stressed from lack of moisture are considerably more susceptible to infection by S. sapinea. Admittedly the high overall level of S. sapinea/whorl canker was atypical but the effect of competition for moisture in the higher stocked area was interesting. Soil conditions aside, the general trend for higher incidence of whorl cankers and multi-leaders with higher stocked stands (after age eight) appears consistent across the Canterbury plains. It is certainly these stands which suffer most from top dieback in drought years.

Dry summers are a feature in Canterbury and NIWA records show that since 1975 there have been major drought events about every ten years (1978, 1988, 1997-1998). These major droughts, as well as lesser events will always bring increased S. sapinea infection and dieback to the pine forest. Unless there have been recent advances in rain dance technology the task for forest managers is to try and minimise the level of dieback as best they can through more contemporary management techniques. This may be by maintaining adequate boron levels from an early age to encourage root development and water uptake, or settling on an optimum stocking level which delivers an acceptable volume despite losses resulting from double leaders.

(Paul Bradbury, Forest Research, Christchurch)
 

The report written by Paul Bradbury in Forest Health News no. 87 generated the following correspondence (Forest Health News 89, September 1999)

BORON: WHORL CANKERS
The article on whorl cankers of radiata pine (FHNews 87: 1, July 1999) raised some interesting questions. The article indicates that Sphaeropsis sapinea is causing the dieback as a consequence of drought, and that adding boron at an early age might alleviate the problem by promoting adequate root development and hence water uptake. Is this really the current thinking? Or, is it possible that adding boron ensures adequate meristem development so that even during drought, when water uptake, and hence boron uptake drops, the trees still have adequate reserves to avoid dieback and subsequently double leaders.

As for settling on an optimum stocking to ensure an acceptable volume despite losses from double leaders - this suggests planting higher stocking which will put the trees under even more stress. I suggest the opposite advice would be more appropriate - ie, plant fewer trees where drought stress is a concern.

(Bill Dyck, Tamahere
)
 

BORON: A REPLY
The reference to the role of boron in reducing the incidence of the leader dieback associated with drought stress and subsequent Sphaeropsis sapinea infection is anecdotal and I apologise for not stating this in the article. However, in the years since boron application has become a widespread practice in Canterbury plains forests, there has been a noticeable reduction in the amount of leader dieback of Pinus radiata during spring and early summer droughts. With regard to my reference to managers settling on an optimum stocking rate to deliver an acceptable volume due to multi-leaders, I was implying that the stocking rate should go down and not up. But in these days of a 'plant and leave' approach in some quarters a happy medium will have to found, where there will be enough stems per hectare but not so many that moisture stress and dieback accentuated by inter-tree competition becomes an issue.

(Paul Bradbury, Forest Research, Christchurch)
 

BORON: IDEAS AROUND THE ''MYTHS"
Boron (B) has a key function for maintaining cell division in the apical meristems of both shoots and roots. Any interruption in the supply of B can cause the meristem to die. In radiata pine, death of the meristem tissue (leader die-back) is reflected in a lateral branch taking over the "leader" role, resulting in subsequent stem malformation.

There is a wide range of anecdotal evidence implicating B in a number of conditions ranging from resin pocket formation, internal checking, bending and bowing of wood, to improvements in nutrition as a result of "improved" root properties. Definitive experiments and trials to put these ideas in a rigorous context have been lacking until recently.

Work in the early 1990s at Forest Research at one site only showed that where the B nutrition of pines was restricted (<10 ppm B) stems showed both reduced lignification and changes in cell morphology (cell wall thickness). As part of the Site Management Cooperative Programme a series of 4 trials is being installed to investigate the effect of B supply on both radiata pine productivity and wood quality. The trials investigate the effect of rates of application of B (from 0 to 32 kg B/ha) on pine B nutrition, and the effect of radiata genotype on B* uptake and wood quality. To date 2 trials have been installed (Balmoral Forest, and Lake Taupo Forest) and a further 2 trials are scheduled for installation in June and July 2000.

Alongside this programme PGSF and NZ/USA Cooperative Science Programme funds are being utilised to study the role of B in cell development and wood morphology at a fundamental level. We are anticipating that the fundamental work around the physiology and biochemistry of B will be undertaken in a PhD programme to begin in early 2000. This will be followed by additional studies (internal, PhD and post-doctoral) examining the B nutrition of pines and the long-term consequences of B biochemistry and physiology on wood properties.

(Malcolm Skinner, Forest Research)

LESS DIPLODIA DIEBACK IN BORON FERTILISED STANDS
From Forest Health News No. 50, February 1996

After a good spring and early summer growing conditions in North Canterbury forests a prolonged dry spell with associated warm north westerly winds was experienced after Christmas. These conditions provided ideal conditions for entry of the wound pathogen Diplodia pinea through stressed tissues. Interestingly there has been only limited dieback in boron treated stands and even when occurring has not extended much more than 20-30cm down the leader. As one of the reputed benefits of boron fertilising on marginal or deficient sites is improved root development, there is a possibility that the trees are better equipped to scavenge water in dry soil conditions reducing stress which might otherwise facilitate the entry of Diplodia.

(Paul Bradbury, MOF, Christchurch)

 

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