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Macrocarpa – The fatal flaw

Ian Brown, New Zealand Tree Grower February 2010.

The Waikato is a graveyard for macrocarpa. Here in warmth and humidity we live in fungal paradise. The old spreading farm trees that are a feature of our rural landscape are slowly yielding to canker, and more recent plantations soon disintegrate. It has been known for some time that this is not a safe place for macrocarpa.

Of concern is the speed with which plantations in cool southern locations previously considered safe have now become infected. We are running out of space. After the last of the plantations and ancient shelterbelts are milled, and global warming boosts the fungal populations, it is likely that macrocarpa will become of historical interest. Its future will lie with its hybrid offspring.

We should have seen this coming. The clues have been out there for some time.

The homeland

The homeland of macrocarpa on the central coast of California is worth a visit. It will be a short visit − it is limited to two small colonies of trees flanking Carmel Beach, south of the Monterey Peninsula. On this narrow strip of coast the trees have two distinct forms. The most striking of them are the wind-sculptured macrocarpas that cling to the rocks along the shoreline. Here they bear the same relationship to the coast as pohutukawa in New Zealand.

These guys are seriously tough, sourcing their nutrients from rocky crevices, and tolerant of incessant battering from salt-laden winds. They are magnificent trees, worthy of their venerated status, and iconic. They survive for hundreds of years, much longer than the neighbouring radiata pine, which crash out over there at about 120 years. The Lone Cypress, perhaps the world’s most photographed tree, is believed to be over 300 years old. If you visit, it would be unwise to check its age with a core sample, unless you do not mind staring down the barrel of a Colt 45 – that tree, and much of the land around it, is the property of Clint Eastwood.

Lone Cypress at Monterey

A few hundred metres inland the remaining trees are quite different, and are clustered in groups that look like plantation grown macrocarpa, except that they were not planted. These look even-aged, and are likely to have germinated after fire. Macrocarpa, like radiata pine, have a fire-adapted ecology, and like radiata have serotinous cones that liberate their seeds after fire. As a bonus, I suppose the fires clean out any fungi before germination.

That is all there is. Although there are no physical barriers, or obvious constraints to spread, they have not extended further inland. What then limits their spread? According to the locals, it is fungal canker, which thrives in the warmth. The trees are like an invading army that over the millennia has been confined to a narrow beachhead. As they push forward into warm territory they are driven back in waves of attack by fungal pathogens.

Survival

So why do these trees survive? I think there are two factors. The first is a low spore level. These coastal trees are exposed to constant on-shore winds that have travelled across the Pacific, so should be as spore-free as you can get. The second is temperature.

Inland on the central coast the summer temperature is scorchingly hot. Along the coastal margin it is much cooler because of the sea fogs that drift ashore on most days in summer. This is fortunate, because without these fogs we would not have macrocarpa, or radiata pine, or possibly redwood further north. Fog is the great unsung saviour of New Zealand forestry. If you want to keep cool in summer on the central coast, head for the beach, and if you want to get seriously cold, try swimming.

Diaspora

Macrocarpa were planted inland on the central coast of California in the 19th century. Fungi were first located on them in 1928, and reported in the following year. Within 20 years, plantations in the central valley were described as being devastated by canker. Seridium cardinale seems to have been the chief offender, together with S.unicorne and S. cupressi.

Amenity plantings were hit, such as the Golden Gate Park in San Francisco. On the Stanford campus, there were few survivors out of 200 macrocarpa that were planted. Macrocarpa planted in the south-eastern states went down, and infections then spread to Leyland cypress.

Offshore, wherever macrocarpa were planted, infections were reported − in Japan, Hawaii, Australia, Africa from Kenya down to South Africa, England and France where fungal infection on macrocarpa was recognised 50 years ago. The problem is severe in Europe, where infections have spread to the native C. sempervirens.

So we are in good company. Consistently, high temperature seems to have been the common factor. We know that from local experience.

What explains the crippling vulnerability of macrocarpa to fungi? My guess is that it has to do with its evolutionary path. What follows is largely speculation and I am aware that travelling in uncharted waters carries the risk of an unexpected reef.

Ice age survival

I think macrocarpa could be regarded as an ice age specialist. Over the last two million years, macrocarpa have evolved in ice age conditions. During that time there have been over 20 periods of glaciations. Over the last 600,000 years each of the glacial periods lasted 100,000 years, and were followed by a warm interglacial of about 10,000 years. The present interglacial is exceptional, and looks set for about a further 10,000. We live in the best of times – a brief respite in a long sequence of ice age periods.

As the earth cools and the glaciers head south, for trees it is a case of migrate, hunker down, or perish. In the US, the north-south alignment of the main ranges does not impede southern migration, therefore a healthy number of over 600 tree species have survived. These vast movements of entire ecosystems must be among the great migrations of history. In contrast, in Western Europe, the east-west alignment of the Alps, and the Mediterranean coast form natural barriers to southern migration, which might explain the relative poverty of tree species in Europe.

A third option

Not all species head south. Some hold on in small enclaves. Ponderosa pine, for example, joined the southern migration, while the more hardy lodgepole pine stayed put.

There is a third option. As the glaciers form, the sea level falls, and an exposed continental shelf is available for colonisation, and is glacier-free. On the east coast, this has extended out for up to 60 km and is known to have been occupied by spruce, hemlock and eastern white pine. On the west coast, the shoreline was closer, extending out for up to 30 km. At Monterey it extended out about 10 km. These coastal plains have been occupied by spruce and fir in the north, and further south Douglas fir has reached out to the Channel Islands offshore from Santa Barbara.

My guess is that these exposed coastal plains near Monterey and further south form the natural habitat of both macrocarpa and radiata pine. We know that both species grow well on sand. There is a fine macrocarpa growing on sand just above the high tide mark on Carmel beach. Closer to home they have colonised sand bars on the North Island east coast. On the North Island west coast, Denis Hocking has some good examples on his sand dunes.

A coincidence

We know that they remained in the territory. In California, Constance Millar has studied fossil pollen in deep ocean core samples. During the glacial periods, the dominant species are in the Cupressaceae family, comprising junipers, cypress, and perhaps redwood. During the interglacial warm periods they are replaced by oaks. Radiata pine is intermediate, thriving at times of climate change transition during the glacial cycles.

The close proximity of macrocarpa and radiata pine at Monterey may not be coincidental. They may have been linked together during glacial periods by a common dependence on fire. Millar’s study of fossil radiata pollen showed that for over 20,000 years, periods of relative abundance of radiata pollen have been associated with charcoal deposits. The fires would have been ignited by lightning strikes in storms at times of climatic disturbance. In addition, we know that macrocarpa grow well in proximity to radiata, possibly as a result of an interaction between their mycorrhizae.

If this is correct, macrocarpa have spent most of their recent evolutionary history on these bleak coastal plains, exposed to strong icy winds from the west and sweeping down from the northern glaciers, nutrient poor soils, dust storms and salt spray. There is one consolation – there were no fungi to contend with. The trees had enough problems without spending their scarce photosynthetic resources in chemical defences against pathogens that did not exist in that environment. It is possible that resistance to fungi might have evolved by mutation during a previous warm period. If so, it has been lost. It would have been redundant in the subsequent ice ages, and eliminated by natural selection.

Macrocarpa is a battle-hardened ice age survivor with a fatal flaw. When we introduced it into our temperate climate and exposed it to a suite of fungal pathogens to which it had no natural defences, we were asking for trouble, and now we have it.

The story with lusitanica is quite different. They are unlikely to have drifted far from the sheltered valleys of Mesoamerica. In that warm environment they would have been constantly exposed to fungal pathogens.To survive they were obliged to engage in an arms race with them, and the outcome is a high level of resistance.

Where to next?

Coastal cypress, Waiheke Island

It seems to me that there are three options – site selection, single tree selection or hybrids.

Based on the recent spread of canker I do not think that there are any sites in New Zealand that should be considered safe for plantation macrocarpa.

Single tree selection

There is no doubt that genetic resistance to canker varies among individual trees in macrocarpa. This has been one of the lessons from the kukupa clone, another is the folly
of releasing clones before they have been comprehensively field tested. For a small native population, macrocarpa have a surprising degree of genetic diversity in growth rate and morphology, and presumably their metabolic products. I have no idea what factors influence susceptibility to fungal attack, but it could involve a number, including bark structure, branch size and occlusion, and chemical products of photosynthesis.

If apparently resistant clones are identified, it is worth considering some possible explanations −

  • The canker-free trees may have simply been lucky. Every earthquake has a survivor or two, a result of good fortune, rather than the possession of an ‘earthquake-resistant’ gene
  • Interlopers in the pack. I have one survivor in my small disintegrating plantation of macrocarpa, a tree with excellent form, branch architecture, and it is canker-free. It turned out to be a lusitanica.
  • Hybrids might appear by chance in any plantation. They may not be pure macrocarpa, but if they have the right attributes, there is no reason why they should not be included in field trials.

I would like to be proved wrong, but am not confident that we will identify pure strains of macrocarpa that have a significant level of inherent resistance to canker. My argument is as follows −

  • The native population is confined to a tiny coastal strip in California
  • The limiting factor to inland spread is their susceptibility to fungal infections
  • Over 10,000 years, none have managed to escape and establish a population inland from that coastal boundary, where they are protected by spore-free winds and cooling summer fogs.
  • The coastal population contains the entire genetic resource available to macrocarpa
  • A natural trial extending over 10,000 years should be long enough to test whether natural resistance to canker exists, and it has not happened.

Hybrids

It seems to me that the future of macrocarpa in forestry will depend on one attribute. It is not their tolerance of wind, cold or exposure. It is their promiscuity. The current assortment of Leyland hybrids is the product of chance encounters with a distant cousin from Alaska – one night stands in English country gardens.

We can improve on that.A controlled hybrid breeding programme is now under way. Of the progeny there will be some dogs, some that are adequate, but more than likely some winners, bearing the vigour, toughness, and wood qualities of macrocarpa, but with added resistance to canker.

Where will macrocarpa be found in 50 years from now? In its pure form I would not expect to find more than remnants in the countryside. However they are likely to persist in the same territory that they have captured in their homeland in California, as stunning windswept trees scattered along our coastal margins.

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