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How to Equip Trees in the Biosecurity Battles

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by Mark Chester - Cedarwood Tree Care


Tree health has been in the headlines in recent years, as various species of trees common to the UK have come under attack from what seems to be an army of pathogens. In the late 1990s, Horse Chestnuts were suffering from the onslaught of Phytophtera, which can cause Bleeding Canker and killed many trees. The English Oak has suffered from Sudden Oak Decline, Larch are being wiped from forest plantations and there are fears for the future of the Ash with Ash Dieback caused by Chalara fraxinea. With some prophecying the pending loss of these trees from the British landscape, how can we help?

Readers of a certain vintage may remember the English Elm, which dominated the landscape and was a symbol of rural life until Dutch Elm Disease arrived in the 1960s and rapidly spread. Few trees were able to withstand the attack and little remains of the original population. Do Horse Chestnut, Oak and Larch have a better chance? There are a range of tools which we can use to assist the population, and some lessons to learn from tree management elsewhere.

One of the most comprehensive tools we can use, especially here in the UK and being an island nation, is quarantine. By ensuring that plant material is
checked before it passes through customs, the risk of pathogens being brought in is significantly reduced. Dutch Elm Disease was introduced in imported timber. There is evidence that Chalara fraxinea, if not introduced by humans, has certainly been spread towards the west coast and the north of the UK by infected plant material. This has been accentuated by transporting young saplings to woodland sites. In terms of economics, it is less costly to export one – year old ash trees (known as whips) and grow them on nurseries in Holland, and then bring them back to the UK for planting a year later!

Of course, what I am describing here in terms of quarantine is the panacea. It is an ideal which not only requires support from the UK Government but also global partners. At the time of writing, Britain is preparing to negotiate her exit from the European Union. Keith Sacre, Chair of the Drafting Committee for BS8545 ‘Young Trees From Nursery to Independence in the Landscape’(2014), faced the realities of the politics of the EU market and the Single Market. Recognising the importance and value of quarantine as a tool for biosecurity, he arranged for quarantine to be included as a recommendation in this new British Standard. However, this was thwarted by the principles of the Single Market and was vetoed by other members who were reluctant to see any restrictions on free trade between member states. The document could only be published if the reference was removed.

We seem to operate a policy here in the UK of shutting the stable door after the horse has bolted. Once a pathogen has established its presence here, further imports of infected material can be banned. In fact, there can be total bans of importing, whether material is infected, or not.

Clearly, this strong weapon of prevention is not currently available. However, there are other tools in the armoury. Diversity is important. When we have a population of trees of the same species, and even the same family, pathogens, whether pests or diseases, can spread more easily and become established if provided with a larger number of hosts. So often, trees and shrubs are selected for planting schemes based on their colour, leaf shape, availability or simply personal preference of the architect or landscape specifier. The suitability of the selected trees and shrubs to the new environment, and landscape, is often a factor overlooked.

Why does this matter? Some pathogens attach members of the Maple family, genus Aceracea. This is a large family including the Field Maple, Sugar Maple and Norway Maple. Diseases such as Anthracnose and Virticulum wilt can spread among different Maples. In parts of the United States, more than 30% of the trees planted in certain neighbourhoods are Maple, and the consequences of a pathogen attacking can be devastating, actually changing the appearance of the landscape. It may come as a surprise to consider that the humble Rowan, Sorbus aucuparia, is, alongside the domesticated rose and native Dog Rose (Rosa rugose), a member of the Rose family. As such, it is vulnerable to pathogens which attach roses.

The solution isn’t to necessarily stop planting Maples and Rowans, but to ensure species diversity within planting schemes. The recommendation is that no one species should exceed 10% of the planting list, by number. Those related by family should be limited to about 35%. This is a real challenge. So many tree enthusiasts have favourites, and in some situations, where, for example, tolerance to fluctuations in water supply, or drought, or coastal conditions, may be defining limitations, the species choice can be restricted.

There are design challenges. For example, if one is designing an avenue of trees as a landscape feature, it usually consists of a single species, or at the least, species from the same genera. If one were to select from the Tilia genera (Lime, it is possible to use several different species). However, with London Plane, an ideal street tree, there are two main species widely grown: the Oriental Plane, Platanus x orien-talis and the Spanish Plane (Platanus hispanica). The Plane tree is particularly vulnerable to Plane Tree Wilt (Ceratocystis fimbriata f. platini). The additional challenge is that the Oriental Plane propagates vegetatively so there is little genetic variation within the population. The London Plane is an important part of the tree population of the capital, and elsewhere. The consequences to the landscape if this disease became prevalent are ones many arborists are dreading.

I referred earlier to the situation in the US, where the disease control approach is to fell, even if this involves 100% of the population. This approach is not preferred here in the UK. It tends to drastically change the landscape. However, the management technique has been applied to controlling the spread of Phytophtera in Larch. This does relate primarily to forestry plantations with their monoculture cropping which enables the pathogen to spread more widely than in our urban landscapes. One of the main reasons for the total clearance approach with Larch is also about timber quality. The Larch plantations exist to provide timber, and its quality deteriorates as the disease spreads. By harvesting before the disease can spread, even if the trees have not yet attained maturity, the available timber can be saved.

I was working as a local authority tree officer when the consequences of Phytophtera, which causes Bleeding Canker in Horse Chestnut, became apparent. Part of my brief included managing the trees on several cemeteries, one of which had an avenue of Horse Chestnuts which sloped downhill. The disease is passed from tree to tree in soil water. I watched over the course of several years as the disease spread from those trees uphill to the lower ones. In the end, a number of trees had to be felled as their condition declined so much and branches were being shed.

At the time, I was pessimistic about the future of the Horse Chestnut. I wondered whether any would be left in ten to twenty years’ time. Then something began to happen. Or rather, it didn’t. Some Chestnuts didn’t succumb. It became apparent that there was a degree of resistance within the wider population. Genetic variation, it seems, can help the Chestnut.

It was this that got the plant pathologist Dr. Glynn Percival thinking. Why, he mused, did some trees succumb, and others not? Was it simply genetic variation? He began to explore the differences between trees which remained healthy and those which were unable to resist the advance of various pathogens.

One thing we have known for some time is that the ability of a tree to resist a disease attack can reduce with age (the ‘hard as nails’ London Plane, which can live for 300 years, stubbornly resists Innotus hispidus whereas the Common Ash, a veteran at 100 years, is more vulnerable). Glynn decided to stop thinking of the tree as a woody structure standing helplessly as a plethora of pathogens launch a barrage of assaults until finally it is overcome. He noted that trees suffering drought stress seemed to be more vulnerable. Stress can also come from the various operations associated with construction, including compaction of soil, the removal of soil and changes to both water and soil levels.

If we can help trees to avoid stress (through the application of best practice on construction sites) and equipping them to resist stress, they will then be better equipped to resist the pathogens. There are a range of tools which the modern arborist has access to. There has been recent press coverage about the potential of biochar to strengthen Ash trees. Trials which Glynn unexpectedly found himself supervising (biochar had been used to treat the trees as part of site management rather than in a formal experiment) indicated that it could be effective in the battle with Chalara.

One thing that I value with Glynn is that he is unwilling to be drawn prematurely on research. With these trials, interviewed on the BBC Countryfile programme, he was asked whether this was the solution for Ash. More research is needed, he cautioned. However, what delights me is that, through the work of Glynn and others, an array of tools is being identified which enables me, the arborist, to be better equipped in the battle. The skill is to know what to use, and when, and I feel better equipped than I did fifteen years ago. There are times
when I have to deliver the message which my client would rather not hear; the decline has progressed too far and it is unwise to invest further in resources aimed at helping a tree to recover. It is important to be proportionate in ones’ response.

I have been working closely with Kevin Martin, the Arboricultural Manager at Kew Gardens. He manages this internationally important collection which numbers some 14,000 trees. There can be a tendency, once a tree shows signs of decline, to fell and replace. Often, in pure economic terms, this is the preferred option especially when one has very finite resources. However, for Kevin, with many historically, botanically and biologically important trees felling is often the last option, only to be explored once others have been fully explored. Fortunately, Kevin has a naturally inquisitive mind; he likes to explore.

He recently shared with me about the journey to find out why a Japanese Pagoda tree was showing signs of stress. This particular tree was part of the original collection when the Botanic Gardens were founded in the 1760s. One might naturally conclude that time had taken its’ toll. In addition, due mainly to the historic significance of the specimen, it is the most popular member of the tree collection at Kew. One can only imagine the effects of the traffic from all of those visitors.

One of the key principles in arboriculture is that what happens below ground is as important as above ground. Kevin began to explore. The tree had a mulch of gravel. This is useful in the right place, but I find it is mainly suitable for footpaths and driveways. The absence of organic matter limits its value for trees. Kevin shared with me that he removed some four tonnes of gravel from the base of this tree! Below the gravel was a weed suppressant membrane which was not only really effective in stopping weeds growing upwards, but also water filtering downwards. The ground below the membrane was dry and had little organic matter, and needed Kevin’s expert touch to bring restoration. The tree is recovering well.

The application of chipped bark mulch can be an invaluable tool in helping a tree to thrive and equip it in the battle against pathogens. However, care is needed. Fresh mulch can drain the soil of nutrients whilst too can damage the trunk. Interestingly, the same applies to water: excess can be more damaging than too little.

I have looked so far at equipping the tree within the landscape for the challenges it will face. However, it is becoming increasing evident that the successful journey actually begins on the nursery, when the tree is growing and being prepared for ‘adulthood’ in the landscape. An over view of best practice for nursery production was published in 2014, the new BS8545: Young Trees – Achieving Longevity in the Landscape. Above ground elements such as the importance of a strong central leader are covered. However, the development of a good root system is also important. I recall, years ago, a friend was growing a young oak tree in their garden. They were delighted at how the tree was developing a strong and deep tap root, and wanted to know how they could transplant this tree to me without damaging the root. The reality is that actually, to develop a successful rooting system, the tap needs to be severed early in the life of the tree, to encourage lateral roots to form. We now have access to technology enabling us to assess the vigour of the tree both in the nursery and within the landscape. This is enabling monitoring from the earliest days. One element that surprised me is that, in part because trees are usually sent from the nursery to the landscape when they are dormant, is that some are dead when they are dispatched!

I have spoken about this technology before. It uses the response of chlorophyll fluorescence to sunlight. It is invaluable because the chlorophyll shows stress within the plant much sooner than it is revealed through physical symptoms. Glynn, who helped to develop the technology for arboriculture, has shared with me how, when assessing some nursery trees which had recently been planted in the landscape, he identified one batch to be suffering drought stress. The supplier was adamant that the stock was healthy. Glynn had to wait for three weeks for the physical symptoms to appear, and was relieved when his observations became evident and the trees began to decline.

Whether one considers climate change to be fact or fiction, one thing is apparent: drought conditions are becoming more common and widespread, especially in the urban environment. We know that some trees are better equipped to thrive in drought conditions. One of Glynn’s colleagues is focusing on this. He is exploring whether we can identify the characteristics of trees with drought tolerance. If we can, then we can be more informed in species selection.

There are a range of tools now available to equip the arborist in helping trees to resist pathogen attacks, ranging from variations in species selection to good plant health and biosecurity. We are better informed than ever before in understanding how trees resist attacks. The key element remains knowing what to use, when and how. As I continue on my journey to know more, I am able to help tree owners to look after their trees and enjoy this asset in to the future.

 

by Mark Chester - Cedarwood Tree Care

by Mark Chester - Cedarwood Tree Care

Page 2

Tree health has been in the headlines in recent years, as various species of trees common to the UK have come under attack from what seems to be an army of pathogens. In the late 1990s, Horse Chestnuts were suffering from the onslaught of Phytophtera, which can cause Bleeding Canker and killed many trees. The English Oak has suffered from Sudden Oak Decline, Larch are being wiped from forest plantations and there are fears for the future of the Ash with Ash Dieback caused by Chalara fraxinea. With some prophecying the pending loss of these trees from the British landscape, how can we help?

Readers of a certain vintage may remember the English Elm, which dominated the landscape and was a symbol of rural life until Dutch Elm Disease arrived in the 1960s and rapidly spread. Few trees were able to withstand the attack and little remains of the original population. Do Horse Chestnut, Oak and Larch have a better chance? There are a range of tools which we can use to assist the population, and some lessons to learn from tree management elsewhere.

One of the most comprehensive tools we can use, especially here in the UK and being an island nation, is quarantine. By ensuring that plant material is
checked before it passes through customs, the risk of pathogens being brought in is significantly reduced. Dutch Elm Disease was introduced in imported timber. There is evidence that Chalara fraxinea, if not introduced by humans, has certainly been spread towards the west coast and the north of the UK by infected plant material. This has been accentuated by transporting young saplings to woodland sites. In terms of economics, it is less costly to export one – year old ash trees (known as whips) and grow them on nurseries in Holland, and then bring them back to the UK for planting a year later!

Of course, what I am describing here in terms of quarantine is the panacea. It is an ideal which not only requires support from the UK Government but also global partners. At the time of writing, Britain is preparing to negotiate her exit from the European Union. Keith Sacre, Chair of the Drafting Committee for BS8545 ‘Young Trees From Nursery to Independence in the Landscape’(2014), faced the realities of the politics of the EU market and the Single Market. Recognising the importance and value of quarantine as a tool for biosecurity, he arranged for quarantine to be included as a recommendation in this new British Standard. However, this was thwarted by the principles of the Single Market and was vetoed by other members who were reluctant to see any restrictions on free trade between member states. The document could only be published if the reference was removed.

We seem to operate a policy here in the UK of shutting the stable door after the horse has bolted. Once a pathogen has established its presence here, further imports of infected material can be banned. In fact, there can be total bans of importing, whether material is infected, or not.

Clearly, this strong weapon of prevention is not currently available. However, there are other tools in the armoury. Diversity is important. When we have a population of trees of the same species, and even the same family, pathogens, whether pests or diseases, can spread more easily and become established if provided with a larger number of hosts. So often, trees and shrubs are selected for planting schemes based on their colour, leaf shape, availability or simply personal preference of the architect or landscape specifier. The suitability of the selected trees and shrubs to the new environment, and landscape, is often a factor overlooked.

Why does this matter? Some pathogens attach members of the Maple family, genus Aceracea. This is a large family including the Field Maple, Sugar Maple and Norway Maple. Diseases such as Anthracnose and Virticulum wilt can spread among different Maples. In parts of the United States, more than 30% of the trees planted in certain neighbourhoods are Maple, and the consequences of a pathogen attacking can be devastating, actually changing the appearance of the landscape. It may come as a surprise to consider that the humble Rowan, Sorbus aucuparia, is, alongside the domesticated rose and native Dog Rose (Rosa rugose), a member of the Rose family. As such, it is vulnerable to pathogens which attach roses.

The solution isn’t to necessarily stop planting Maples and Rowans, but to ensure species diversity within planting schemes. The recommendation is that no one species should exceed 10% of the planting list, by number. Those related by family should be limited to about 35%. This is a real challenge. So many tree enthusiasts have favourites, and in some situations, where, for example, tolerance to fluctuations in water supply, or drought, or coastal conditions, may be defining limitations, the species choice can be restricted.

There are design challenges. For example, if one is designing an avenue of trees as a landscape feature, it usually consists of a single species, or at the least, species from the same genera. If one were to select from the Tilia genera (Lime, it is possible to use several different species). However, with London Plane, an ideal street tree, there are two main species widely grown: the Oriental Plane, Platanus x orien-talis and the Spanish Plane (Platanus hispanica). The Plane tree is particularly vulnerable to Plane Tree Wilt (Ceratocystis fimbriata f. platini). The additional challenge is that the Oriental Plane propagates vegetatively so there is little genetic variation within the population. The London Plane is an important part of the tree population of the capital, and elsewhere. The consequences to the landscape if this disease became prevalent are ones many arborists are dreading.

I referred earlier to the situation in the US, where the disease control approach is to fell, even if this involves 100% of the population. This approach is not preferred here in the UK. It tends to drastically change the landscape. However, the management technique has been applied to controlling the spread of Phytophtera in Larch. This does relate primarily to forestry plantations with their monoculture cropping which enables the pathogen to spread more widely than in our urban landscapes. One of the main reasons for the total clearance approach with Larch is also about timber quality. The Larch plantations exist to provide timber, and its quality deteriorates as the disease spreads. By harvesting before the disease can spread, even if the trees have not yet attained maturity, the available timber can be saved.

I was working as a local authority tree officer when the consequences of Phytophtera, which causes Bleeding Canker in Horse Chestnut, became apparent. Part of my brief included managing the trees on several cemeteries, one of which had an avenue of Horse Chestnuts which sloped downhill. The disease is passed from tree to tree in soil water. I watched over the course of several years as the disease spread from those trees uphill to the lower ones. In the end, a number of trees had to be felled as their condition declined so much and branches were being shed.

At the time, I was pessimistic about the future of the Horse Chestnut. I wondered whether any would be left in ten to twenty years’ time. Then something began to happen. Or rather, it didn’t. Some Chestnuts didn’t succumb. It became apparent that there was a degree of resistance within the wider population. Genetic variation, it seems, can help the Chestnut.

It was this that got the plant pathologist Dr. Glynn Percival thinking. Why, he mused, did some trees succumb, and others not? Was it simply genetic variation? He began to explore the differences between trees which remained healthy and those which were unable to resist the advance of various pathogens.

One thing we have known for some time is that the ability of a tree to resist a disease attack can reduce with age (the ‘hard as nails’ London Plane, which can live for 300 years, stubbornly resists Innotus hispidus whereas the Common Ash, a veteran at 100 years, is more vulnerable). Glynn decided to stop thinking of the tree as a woody structure standing helplessly as a plethora of pathogens launch a barrage of assaults until finally it is overcome. He noted that trees suffering drought stress seemed to be more vulnerable. Stress can also come from the various operations associated with construction, including compaction of soil, the removal of soil and changes to both water and soil levels.

If we can help trees to avoid stress (through the application of best practice on construction sites) and equipping them to resist stress, they will then be better equipped to resist the pathogens. There are a range of tools which the modern arborist has access to. There has been recent press coverage about the potential of biochar to strengthen Ash trees. Trials which Glynn unexpectedly found himself supervising (biochar had been used to treat the trees as part of site management rather than in a formal experiment) indicated that it could be effective in the battle with Chalara.

One thing that I value with Glynn is that he is unwilling to be drawn prematurely on research. With these trials, interviewed on the BBC Countryfile programme, he was asked whether this was the solution for Ash. More research is needed, he cautioned. However, what delights me is that, through the work of Glynn and others, an array of tools is being identified which enables me, the arborist, to be better equipped in the battle. The skill is to know what to use, and when, and I feel better equipped than I did fifteen years ago. There are times
when I have to deliver the message which my client would rather not hear; the decline has progressed too far and it is unwise to invest further in resources aimed at helping a tree to recover. It is important to be proportionate in ones’ response.

I have been working closely with Kevin Martin, the Arboricultural Manager at Kew Gardens. He manages this internationally important collection which numbers some 14,000 trees. There can be a tendency, once a tree shows signs of decline, to fell and replace. Often, in pure economic terms, this is the preferred option especially when one has very finite resources. However, for Kevin, with many historically, botanically and biologically important trees felling is often the last option, only to be explored once others have been fully explored. Fortunately, Kevin has a naturally inquisitive mind; he likes to explore.

He recently shared with me about the journey to find out why a Japanese Pagoda tree was showing signs of stress. This particular tree was part of the original collection when the Botanic Gardens were founded in the 1760s. One might naturally conclude that time had taken its’ toll. In addition, due mainly to the historic significance of the specimen, it is the most popular member of the tree collection at Kew. One can only imagine the effects of the traffic from all of those visitors.

One of the key principles in arboriculture is that what happens below ground is as important as above ground. Kevin began to explore. The tree had a mulch of gravel. This is useful in the right place, but I find it is mainly suitable for footpaths and driveways. The absence of organic matter limits its value for trees. Kevin shared with me that he removed some four tonnes of gravel from the base of this tree! Below the gravel was a weed suppressant membrane which was not only really effective in stopping weeds growing upwards, but also water filtering downwards. The ground below the membrane was dry and had little organic matter, and needed Kevin’s expert touch to bring restoration. The tree is recovering well.

The application of chipped bark mulch can be an invaluable tool in helping a tree to thrive and equip it in the battle against pathogens. However, care is needed. Fresh mulch can drain the soil of nutrients whilst too can damage the trunk. Interestingly, the same applies to water: excess can be more damaging than too little.

I have looked so far at equipping the tree within the landscape for the challenges it will face. However, it is becoming increasing evident that the successful journey actually begins on the nursery, when the tree is growing and being prepared for ‘adulthood’ in the landscape. An over view of best practice for nursery production was published in 2014, the new BS8545: Young Trees – Achieving Longevity in the Landscape. Above ground elements such as the importance of a strong central leader are covered. However, the development of a good root system is also important. I recall, years ago, a friend was growing a young oak tree in their garden. They were delighted at how the tree was developing a strong and deep tap root, and wanted to know how they could transplant this tree to me without damaging the root. The reality is that actually, to develop a successful rooting system, the tap needs to be severed early in the life of the tree, to encourage lateral roots to form. We now have access to technology enabling us to assess the vigour of the tree both in the nursery and within the landscape. This is enabling monitoring from the earliest days. One element that surprised me is that, in part because trees are usually sent from the nursery to the landscape when they are dormant, is that some are dead when they are dispatched!

I have spoken about this technology before. It uses the response of chlorophyll fluorescence to sunlight. It is invaluable because the chlorophyll shows stress within the plant much sooner than it is revealed through physical symptoms. Glynn, who helped to develop the technology for arboriculture, has shared with me how, when assessing some nursery trees which had recently been planted in the landscape, he identified one batch to be suffering drought stress. The supplier was adamant that the stock was healthy. Glynn had to wait for three weeks for the physical symptoms to appear, and was relieved when his observations became evident and the trees began to decline.

Whether one considers climate change to be fact or fiction, one thing is apparent: drought conditions are becoming more common and widespread, especially in the urban environment. We know that some trees are better equipped to thrive in drought conditions. One of Glynn’s colleagues is focusing on this. He is exploring whether we can identify the characteristics of trees with drought tolerance. If we can, then we can be more informed in species selection.

There are a range of tools now available to equip the arborist in helping trees to resist pathogen attacks, ranging from variations in species selection to good plant health and biosecurity. We are better informed than ever before in understanding how trees resist attacks. The key element remains knowing what to use, when and how. As I continue on my journey to know more, I am able to help tree owners to look after their trees and enjoy this asset in to the future.

 

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