by Mark Chester - Cedarwood Tree Care. Tree health issues have been in the headlines in recent years, with Chalara in Ash, Horse Chestnut Bleeding Canker affecting that tree, and a species of the Phytophtera bacterium affecting Larch being particularly prominent. It can seem very concerning seeing trees being affected by diseases, and one can be worried about those trees near to home or work potentially becoming unsafe. However, there are ways to minimise the potential impact on our trees.
Tree safety is often cited as justification for felling trees, and it can be unnerving during stormy conditions to watch large specimen trees, with their spreading branches, moving freely with the gusting winds. To re-assure: in terms of safety, the risk of being injured, or worse, by a falling tree or branch is very low. Indeed, the tree moving with the wind is actually a design strategy which enables it to grow spreading branches and resist the occasional storm. I previously worked for a local authority in the industrial Midlands. The authority surveyed all of its street trees (some ten thousand specimens). Of these, fewer than one per cent (just eighty of the ten thousand trees) needed work to make them safe. Safety is just one aspect of tree health. Indeed, I find that it can be applied as a convenient tool, to justify the removal of trees which someone would prefer were removed.
There are two elements to tree health, proactive and reactive. Whilst the former requires planning ahead, the reality is that it is often the latter where I am called to help. I will explore proactive management later. Faced with a pathogen ‘on the march’, probably the most drastic solution is to fell all affected trees. Some pathogens are species-specific and one option to pre-empt the spread amongst a population of, for example, Larch, has been to fell all trees of that species within an area, regardless of their health. This approach is being taken by the Forestry Commission in the UK, for Larch. It is drastic!
Whilst clear-felling plantation Larch may be the pragmatic solution, and is ensuring that a crop of usable timber is being harvested, albeit earlier than was planned, such an approach is less straight forward when managing urban trees, where their contribution to the visual amenity of the locality is often highly valued. In the latter situation, there is greater scope to manage the individual tree.
A starting point is to understand how a pathogen attacks its host, and the impact that it may have. There are some pathogens which have limited influence on the tree. Tar Spot on Sycamore is an example. The fungus causes black ‘tar’ spots to form on the upper surface of the leaf, which seems to suffer little ill effect. I have ‘fond’ memories of this fungus, as it generated one of my first commissions as a freelance consultant. I was helping an oil company responding to some concerns raised by neighbours to one of their storage depots. This was in the early days of ‘Polluter Pays’ and they needed to establish whether a claim was legitimate or spurious. The question was, ‘Is Tar Spot in Sycamore an indication of pollution?’ The answer is that the tree is largely unaffected by the fungus, whose presence is actually an indication of clean air (the sulphur in smog kills the fungus).
There is also the issue of how readily a tree can respond to the pathogen attack. If we encounter a warm, wet spring, especially during May, a fungus can cause wilting of the leaves on London Plane. The leaves hang limply, and are then shed. I recall being approached, during my days at Dudley Council, by a councillor who wanted to know when the affected London Planes were going to be felled. The reality in this situation is that the tree produces a second flush of leaves during the summer, and there is little impact on the host.
Some pathogens take advantage of favourable conditions. For others, our management of trees can provide them with the platform. Cherry trees, members of the Prunus genus, are particularly vulnerable to attacks. Chondostereum purpureum is a fungus which attacks the equivalent of the nerve system of the tree (the vascular system which is just below the bark). The tree can defend itself, but only when it is actively growing and in leaf. Pruning during the winter, when it is unable to respond, leaves in vulnerable, and often sounds the death knell. The fungus decays the tree, but actually kills it first, which is an unusual turn of events. Death can be so swift that the tree is unable to shed foliage, which turns silver and hangs limply. This gives the fungus its common name of ‘Silver Leaf’.
Birch are another family of trees which can be vulnerable, again to ill-considered pruning. The trees colonise abandoned sites, and grow quickly. Compared to trees such as Beech and Horse Chestnut, which can live for two to three centuries, few Birch live to reach their first century. They are called the ‘rock and rock’ trees, living fast and dying young. As such, they focus on reaching maturity and producing seed, and do not have a strategy to respond to the attack by pathogens such as fungi. Birch can reach heights exceeding twenty metres, and are often found in residential settings, where their open crowns and small leaves cast a light, dappled shade. However, the height can unnerve residents, and there is the tendency to reduce this, usually resulting in ‘topping’. Not only is this not good practice, but the pruning wounds which result often leave the host ill-equipped to resist pathogen attack. The Birch Polypore is often on hand to take advantage. It decays wood fibres in the trunk, usually leading to the trunk failing, at a point above the area of decay. Careful management of each tree can help to avoid this problem, at least until the tree has attained a good age.
Having looked at some of the health issues where poor management has a role, I will now look at some of the fresh challenges being faced with amenity trees. The Horse Chestnut has been under attack for the past decade or so. It faces a threat from several fronts. Phytophtora Bleeding Canker (PBC) which is transported in soil water and enters via the roots, has kiiled many thousands of Chestnuts. Leaf Minor has become increasingly prevalent in recent years. Eggs of the minor are laid just under the surface of a leaf. They hatch out within the leaf and ‘mine’ the nutrients and sugars to be found there. They have become more prolific and whilst a single infestation should not prove too problematic, on-going attacks reduce the ability of the tree to generate sugar reserves through photosynthesis. It also becomes harder for the tree to resist the other attacks that it may face.
Dieback on edge – also affects the ability of the tree to photosynthesis, especially during the key summer months of August and September. In 2007, when I was managing trees in Birmingham for the local authority, one of the sites I was responsible for was a cemetery, with an avenue of Horse Chestnuts lining either side of a driveway with a gentle slope. I monitored changes in the condition of the trees as those higher up the driveway were showing the symptoms of the pathogen, and then gradually neighbouring trees lower down showed signs of infection. At the time, I pondered whether the Horse Chestnut had a future.
Ash (Fraxinus excelsior) has been in the headlines in the past several years, with the seemingly inevitable march of Chalara fraxinea, a fungus which can be particularly damaging among younger trees. As the fungus spread, there were eadlines suggesting the consequences could be as devastating as Dutch Elm Disease was among Elms. However, there is hope for the Ash, and for some of the other trees, which I shall now explore.
The main difference between Ash and Elm is that the former reproduce sexually, via seeds, which introduces genetic variation in to a given population. It is likely, within the population, that some individuals will have a natural resistance to the fungus. Elms reproduce asexually, either from cuttings or layering, where a shoot from the parent plant will produce roots, become established, and form a new plant. Asexual reproduction forms clones, which are genetically identical to the parent, and thereby vulnerable to any attacks. In the absence of genetic variation, it becomes harder to develop resistance within the population.
With Horse Chestnuts, it became evident as we monitored the response of individual trees to the various attacks, with PBC being particularly virulent, that some were more tolerant than others. This led to a focus in research. What made those who could resist different to those that succumbed more readily? Dr. Glynn Percival, a plant pathologist who undertakes research in the field of arboriculture, discussed his thoughts with me. He wanted to explore this particular issue. He started with the premise that the tree does not want to die, and will try to resist the pathogen as far as it can. A healthy tree is better equipped to resist than a tree under stress. He has explored the impact of encouraging vigorous root growth on resistance, both with regard to PBC, and to the other virulent pathogen Honey Fungus. Improving soil conditions through increased aeration (the use of an air jet can be effective here) along with the addition of compost, mulch and micchorizae, has shown that the trees involved respond well, and are equipped to resist the pathogens which would previously have been more potent.
Good biosecurity is also important, although the political need to maintain open borders to aid free trade does not help in the effort. It is not possible to enforce placing trees and other plants in to quarantine before they can cross a border.
There are, however, other ways to limit the potential for pathogens to spread. These are the proactive measures mentioned earlier. Timing pruning to the needs of the individual tree is a starting point. There is the suggestion that encouraging diversity of species within populations is a useful tool in limiting the spread of a pathogen, with a maximum of 10% of any one species (or genus when a pathogen affects species across the genus, as can be the case with Prunus and Malus) being recommended. This provides an additional challenge to those who prepare the schedules, as it will inevitably lead to the selection of trees from species previously not used.
The journey can also begin when planting the next generation of trees. Selecting species suited to the setting, and ensuring the trees planted are of good vigour, and equipped for the journey ahead, is important. It is now possible to assess vigour using the Plant Health Index via a test of a plant’s chlorophyll fluorescence. If a tree is not of sufficient vigour, it should not leave the nursery.
Ensuring a good root ball, with vigorous roots able to become established and support future growth, is important. Equally important is the situation in to which the tree will be planted, ensuring adequate space for root development through to the tree becoming mature, and providing a good supply of growing media (which need not be soil), equip the tree for its future, and help it to be able to respond to pathogen attacks.
Another factor which can be overlooked is the development of the branches which form the crown. Failure can be built in to a tree whilst it is on the nursery, if it is not properly pruned. This involves the development of a strong central leader (and not multiple leaders, which will compete for space and light, and may develop included bark, which can snap later in life). When a tree grows in open space, lateral branches can compete with the leader, and become too substantial.
I advise on the management of trees within a hospital site. Some of the trees are ornamental specimens which have not historically been managed this way. They lack central leaders, but have large lateral branches. It is possible to prune the tree to correct this, although the process can take up to a decade to fully implement, which often does not fit within the management timescales and budgets of such an organisation. A group of these trees are near to the landing pad for the helicopter, and during otherwise calm conditions during one August afternoon, the downward force of the helicopter cause a lateral branch to break. It had become long and heavy but lacked the strength to resist this unexpected event. I was asked to inspect trees nearby to see if any other trees were in a similar condition, as the hospital did not want another branch to fail. In such a scenario, it is difficult to provide the sought after assurance without implementing an intensive pruning programme which would not otherwise be sought!
I mentioned earlier about the Plant Health Index and chlorophyll fluorescence. In one sense, the technology is not new; it has been used since the 1970s to assess the condition of cut flowers, which can be affected by being chilled. The basis of the technology is that the chlorophyll within each leaf will respond the same way. The test is non-invasive and takes but a few seconds. It can detect physiological problems such as drought stress or damage from a herbicide weeks before the effects are visible within the plant.
Dr. Glynn Percival, who has been instrumental in its development, shared with me some of the more dramatic moments he has experienced testing the technology. In one case, he used it on young trees which had been planted recently. The trees seemed to be in good vigour, but the test indicated drought stress, which would take about three weeks to show. He was very relieved when his forecast came to pass. Needless to say, he is not always popular when using the technology to assess deliveries of nursery trees, especially when stock is rejected due to its condition.
He has explored the accuracy of the readings, from leaf to leaf and within a tree. He did tests on a mature oak tree, taking readings from leaves at intervals of one metre both in height and moving from the outer crown inwards. Across the readings, the variation was measured at two decimal places! In other words, the lowest reading was 20.34 and the highest 20.37. Even Glynn was impressed with these findings.
Why does this matter? Because it now enables the arborist to assess the health and vigour of established trees within the landscape while they are in situ! Previously, the only ways to assess were to look for external signs of internal decay, or to try to assess the extent of possible internal decay using invasive methods such as a drill, measuring the resistance to wood fibres. This can be useful in evaluating the locating of a cavity, or decayed wood. However, one tree can be hollow but still in good vigour, whereas another may die with sound timber. Measuring chlorophyll fluorescence against what would be expected for a species enables the health of a population of trees to be assessed, and more informed conclusions drawn.
Meanwhile, work continues to find solutions to the various pathogens we are encountering, and to exploring ways in which the trees which may be affected can be equipped to resist. Plantations of Larch may have been lost, but there is hope for the others. ?