by Mark Chester - Cedarwood Tree Care
Trees are generally recognised to be one of the longest-lived features within our landscape, whether this be the urban environment or rural settings. Get it right and we can have an asset for decades and centuries to come, to be enjoyed by those who plant them, and future generations. However, with the newly-planted urban tree typically failing to live for two decades and as many as 25% of all newly-planted trees dying within 2 years, something is evidently wrong. If a tree is a focal point, then with its decline, all of the investment in infrastructure is lost or has a reduced impact.
We increasingly live in a world of short-term priorities, in which planning for five years hence can be viewed as the long term. Indeed, in the world of planning, one which frequently involves trees, there is often consideration for only the first five years of a development’s life. However, trees function in a timespan of decades and centuries. Many of today’s mature specimens were here before we were born, and will out-live us. A longer-term perspective is required in order to secure the mature trees of tomorrow.
One factor to be considered is how a tree will relate to the landscape around it in the coming decades. Will it have sufficient space to fully develop? Will it be able to adapt to the climate? Will the characteristics it presently presents remain beneficial to those living within its sphere of influence? How often does a land owner desire a quick growing evergreen hedge to screen an eyesore, not appreciating that the hedge will continue to grow rapidly after the desired height has been reached? The tree planted as a 2-3m tall specimen, or even as a larger semi-mature specimen several times this size can increase significantly further. Allowance needs to be made for this growth if the tree is not going to out-grow the setting and either be felled or require regular pruning to ensure it fits the available space. This may sound obvious, but I find that it is a factor often overlooked by planning professionals who are less familiar with trees than the arborist.
A key consideration is the timescale of a development, and how this may affect trees being planted. Once completed, residential developments can remain relatively unchanged for many decades and even centuries, allowing time for trees within the landscape, even the slower growing species, to mature and become established. In such situations, it can be effective to plant species which live for 100 or more years. Selecting ornamental trees, which typically live for only several decades, is generally a lost opportunity, as it is less likely that dead specimens will be replaced.
For a retail environment, where major changes to landscape and infrastructure can be anticipated sometimes within a decade or less, impact is more important, and a tree which takes fifty years to fully mature is unlikely to be allowed the luxury of such time.
The setting in which a tree will be planted and become established should be a major factor in species selection. Alder is fast growing, can attain heights of more than twenty metres, tolerates both drought and flood, and the harsher environments typically found in urban settings. It is ideal for car parks and industrial settings. However, the shading caused by a mature specimen makes its use in residential areas a more delicate proposal.
There are situations where a larger tree would detract from the local enjoyment of the setting. In such situations, selecting smaller trees such as Rowan and the Whitebeam, whilst not as long-lived or as substantial as the Alder, can contribute locally. If a tree is to remain as a longer-term asset, enhancing the local environment, then the contribution needs to remain positive. Shading in a car park or communal area can be welcome. Heavy shading in gardens, and damage from roots, especially near to homes, can lead to requests to prune or fell, an investment of unfilled potential, a wasted opportunity.
The process of enabling a tree to realise its potential as an investment should begin before the tree has been purchased. An appraisal of the site is needed to establish constraints and define the desired outcome. Why are we planting this tree? What outcome are we seeking? What will the tree need in order to become
established? Something as seemingly obvious as irrigation being overlooked can render all other considerations irrelevant.
One site where I have surveyed the trees for several years has seen major redevelopment during this period. Surveying the trees at the end of a particularly hot and dry September (2014), I noted some newly-planted Birch in raised beds. Birch are sensitive to the roots becoming dry. They have shallow roots and focus on growing rapidly and producing vast quantities of seeds. These trees were already showing signs of drought stress and there was no evidence of watering infrastructure. The effect of gravity on drainage also seemed to have been overlooked.
Birch can recover, and with prompt intervention, a second flush of leaves can be grown. For this group, however, the damage had already been done. An excess of water can be as damaging, if not more, than the roots drying out. Water displaces oxygen and causes anaerobic conditions to develop. Tree roots do not contain chlorophyll, and so need oxygen to function. A system that enables carbon dioxide to be removed and replaced with oxygen in the soul will better equip roots to thrive.
There are two elements to securing the long-term investment with trees: creating the opportunity and conditions, and ensuring that the trees selected are suited to the situation and structurally and physiologically equipped for the challenges they are likely to face. Trees contribute so much to the local setting, providing shade, cooling the air through the shading and evapotranspiration, infiltrating water to reduce surface run-off, screening eyesores and providing focal points. Research by Professor Roland Ennos in Manchester, where Birch trees were planted on a road frontage of a busy local road, showed a significant reduction in air pollutants from passing traffic reaching homes nearby. They can reduce run-off by as much as 60% and temperatures can be reduced by up to 20C.
Trees can make a financial difference. They are associated with urban regeneration and enhancing the quality of life in urban settings. The i-Tree project which has surveyed trees across urban areas such as Birmingham, London and New York, has demonstrated that the tree population in these areas can be worth hundreds of millions of pounds. The survey in London, completed during 2015, found some 8.4 million trees which provide some £132 million in annual benefits such as carbon sequestration and reduced energy costs from cooler air. This does not include the capital value of the trees or the cost of replacing them. In New York, the i-Trees survey found that for every dollar invested in managing the tree population, there was a return of more than four dollars.
One of the challenges is that trees are often unseen, and their benefits not fully appreciated. As a result, we as tree enthusiasts take every opportunity to share
the message of the benefits. One element I look to promote is how many situations can be enhanced by tree planting. There are the settings that may seem more obvious, such as amenity spaces including gardens. There are also many places where trees can add considerably, but which may not seem so obvious. These include car parks and pedestrianised areas. I call it ‘planting trees in impossible places’.
In the past decade, much work has been done to develop subterranean infrastructure which can sustainably support trees and other plants within hard surfaces. The growing media (it doesn’t necessarily have to be soil) needs to be protected from compaction. There also needs to be space for sufficient media; it will need to sustain the trees for the long term. It also needs to be sufficiently robust to support the traffic above ground. This enables trees to be planted in the middle of car parks and high streets. They can enhance new developments and contribute to a better environment.
Last year, I was able to assist a factory owner working to convert an area of waste ground to a car park for his expanding business. The site contained some trees which had been planted as part of landscaping back in the 1970s, and whilst they were mature, and contributed to a setting with little other green cover,
showed the signs of limited management. The trees were crowded and there were few individual specimens I considered worth retaining. It wasn’t practical to retain existing trees within the site and provide optimum parking spaces.
The local planning authority, understandably keen to enhance the tree cover in a very built up area, sought to have a new tree planted for every good one which
needed to be removed. This could have become a problem, which may have threatened the viability of the proposals, with space being limited. However, combining careful design with the subterranean infrastructure, it was possibly to incorporate new trees within the layout, and keep all of the parking spaces required. The trees were carefully selected to enhance both the immediate site and the local setting. I do encourage tree planting wherever possible, with the use of smaller, ornamental specimens where space is at a premium. Larger trees are often more valuable in terms of longevity and contribution, for example, being more visible. However, as mentioned earlier, there may not be the space for them, and they are not always welcome. In such scenarios, I find it is preferable to plant smaller trees that fit the setting than no tree.
This brings me to the second key factor, that of ensuring the tree is suited to the setting. There are two important elements: design and production. If either of these elements has not been fully considered, the best of intentions can easily be thwarted. For example, a colleague of mine who specialises in selecting the optimum tree for each setting, recalled a development in a coastal setting, where trees were afforded a high priority, with care being taken to allocate space, generous provision for growing media and irrigation, and designing the trees to complete the locality.
One key issue had been overlooked. Beech trees were chosen as the main feature. Unfortunately, they dislike salty conditions, and should not be chosen for coastal settings. These particular trees were unable to become established in the setting, and died in their first year, rendering all the investment in infrastructure, and of purchasing, planting and maintaining the trees, void. Choosing trees which thrive in coastal settings would have ensured the aim of establishing good tree cover was met.
My experience is that, all too often, the process of selecting the tree is given lower priority than other elements, such as site design and street furniture. One thing to remember when a tree dies is that the cost of the lost is often more than just the physical cost of the tree concerned. This would typically be several hundred pounds. When a tree has been planted in a hard surface, whether a footpath, car park or similar setting, there is the additional cost of creating the
planting pit to accommodate the tree. The cost of opening up a tarmac surface can be several times greater than that of the tree, and it is not always possible to plant a replacement.
To be suitable for the setting, we first need to evaluate the site, looking at:
• Ground conditions (is there open ground for trees, or will they need to be accommodated within the new built infrastructure?)
• What is the aspect? Where is the prevailing wind coming from?
• Will the site experience extremes in temperature and soil moisture? Some sites can be flooded
periodically, and then experience droughts.
• Is there existing tree cover? Is this going to be retained, in part or in all?
• Are there key views to protect, screen or enhance?
• Are any focal points to be created?
• Are there local limitations, such as coastal air, frost pockets and strong prevailing wind? These can require tree selection from a list of those particular suited to thriving in such settings.
The final factor, which is arguably the most important, is the condition of the trees to be planted. As I mentioned in my opening paragraph, as many as 25% of newly planted trees die within the first two years following planting. One of the factors is the quality of the tree that leaves the nursery. It is shocking that some trees are dead when they leave the nursery, and others are utterly ill-equipped for the conditions they will face when planted in the landscape.
Guidelines identifying best practice for nursery production were published in 2014 (BS8545:2014, ‘Young Trees from Nursery to Independence in the Landscape’). There is guidance in the development of healthy roots, the formation of a good, open crown with a strong central leader, the requirements of a sustainable planting pit and options for staking. The British Standard benefits from best practice ideas and research from the past decade or so guiding the recommendations.
One of these has been the development of chlorophyll fluorescence data for assessing vigour in trees. Whilst this technology has been used since the 1970s especially for the cut flower industry, the recent development has focused on nursery trees. Collaboration by the research specialists Bartlett’s and Barcham Trees has produced a benchmark library of data covering more than 200 species of trees. Equipment has been developed which enables readings to be taken in the field in a non-invasive manner. These readings can then be compared to the benchmark for the species to identify the chlorophyll activity of a healthy tree.
The reading measures the efficiency with which chlorophyll in the tree leaf is able to absorb sunlight. Like a human, whose alertness and productivity can be reduced by ill health such as flu, a tree with physiological problems will operate at reduced efficiency. What is exciting is that the chlorophyll assessment can detect changes which are not evident externally. A tree which has been sprayed with a herbicide may not show signs of the damage for 48 hours, which can be too late for treatment. Drought stress can take up to 21 days to show.
Dr. Glynn Percival, who runs the research centre for Bartlett’s, shared with me an experience where he ran the test on nursery trees delivered to a site for planting in the landscape. The trees looked healthy, but the chlorophyll test indicated drought stress. Glynn concluded that the trees lacked the vigour to become established, and suggested that the order be returned to the supplier, who, needless to say, was not happy with the proposal. Glynn had to wait for 21 days before the effects of the drought stress showed in leaf senescence. This was in the early days of product development, and he was relieved to be proved right.
Not all problems are hidden away in the tree. Some issues are very evident, and I encourage clients to inspect trees either prior to despatch or on delivery. I have encountered issues relating to trees that should never have left the nursery, including several involving trees purchased at some cost from a local garden centre. Structural weaknesses do not necessary become problematic until a tree has become mature.
Several years ago, I was asked to assess a specimen conifer which had broken at the base. It was situated on a boundary, and relations with the neighbour were
not as amicable as my client wished, and there was concern that the damage may have been malicious. Before taking the matter further, he wished to establish whether there was a natural cause to the damage. The tree had been purchased in a container from a local garden centre. I recognised that root girdling had occurred. If a plant is kept in a container for too long, it becomes root-bound. Roots then begin to grow around the inside of the container, and then around the trunk. If allowed to continue, the roots can restrict the growth of the trunk, which can then snap. This particular tree was doomed to fail before leaving the garden centre. Whilst my client lost a tree, he had the reassurance that the cause was natural and avoided aggravating the situation with the neighbour.
I mentioned, when referring to BS8545:2014, that a tree on the nursery should have a strong central leader. This does not necessarily form naturally. Whilst in a forest setting, where there is competition for light and resources, there is pressure to gain height in order to reach the canopy, for many trees grown for amenity, there may be little competition for resources, and space for side branches to become established. This often leads to poorly developed branch unions. These are structurally weak, and vulnerable to breaking, especially during inclement weather. The consequences of failure can be significant, and not just the potential loss of the tree. The good news is that restorative pruning can be undertaken on even established trees, ensuring that the investment is not necessarily lost due to earlier oversight.
Trees contribute to our quality of life for generations ahead. They can cool the air, clean the air and enhance our general well-being. Much of the cost occurs in the early years. Whilst many trees do fail, with care and an informed approach, those suited to the situation will readily mature to become established specimens providing a return on the investment for decades and potentially centuries to come.
Tree Consultant, Chartered Arboriculturist