Failure rates among newly planted trees can range from 10 to 80 per cent within a year and that the average tree mortality rate is somewhere around 25 per cent. Henrik Sjöman and Andy Hirons addressed the issue at a seminar organised by Barcham Trees .. Colin Hambidge reports
The morning session on tree biology in practice was led by Andy Hirons, (pictured below left) who began by reminding his audience that failure rates among newly planted trees can range from 10 to 80 per cent within a year and that the average tree mortality rate is somewhere around 25 per cent. Why, he wondered, should we find this acceptable in trees while we would be horrified by such a figure when it came to brakes on vehicles? He also mentioned that urban tree life expectancy is between 19 and 28 years and that in paved areas this figure is often less.
Tree ecophysiology, plant quality, planting and post-planting care plus the rooting environment are, Andy believes, the four major factors in tree establishment. When selecting trees for urban sites the primary considerations should be constraints, such as those relating to the site, those of a biological nature and practical concerns and tree ecophysiology. The secondary considerations involve aesthetics and the functions which the tree is expected to provide. “Planting a tree and getting it to establish is not always easy!”, he pointed out.
The rooting environment must provide resources such as water, oxygen, nutrients, anchorage and a habitat for vital organisms. Roots take up water and nutrients, offer mechanical support and storage, regulate plant growth and provide habitat for micro-organisms which can produce chemicals capable of modifying soil properties. Soil volume is also important, and this can be increased by soil vaults. Structural soil can also sometimes be an answer. Andy Hirons likes load-bearing structural cells, but they are expensive.
He cited the urban plaza experiment conducted by Bartlett Tree Research Laboratories in the USA, where trees were grown in various media and planting environments, and those grown in suspended pavements did best of all. The University of Manchester has conducted trials of Pyrus calleryana growing in paved areas, grass verges and Amsterdam soil. Those in Amsterdam soil had a faster growth in height, diameter at breast height (DBH) and crown diameter than trees in paving or on verges. Amsterdam soil also seems to reduce the impact of compaction.
When it comes to plant quality above ground, Andy feels that as an industry we do not do well, with many professionals not visiting a nursery before they buy trees. If we wrote better and more exacting specifications then the better nurseries would prosper. He gave as an example regarding stem taper that point A should be thicker than point B, which in turn should be thicker than point C as it ascends.
Root pruning in successive stages on the nursery is important when considering plant quality below ground. We should not, however, prune at the same diameter. Looking at rootball trees, Andy believes that tree spades can remove up to 98 per cent of a root system when a tree is lifted from the field for this treatment. And when specifying for rootball trees, we should insist the diameter is greater than the depth and that root development is apparent in each quarter. Other specification criteria could include vitality, freedom from pests, disease and injury, the ability to self-support with good stem taper and sound branch attachment.
Trees should be planted at nursery level and, while a mulch is beneficial at planting time, we should also specify the depth and diameter of the mulch. Once agreed, this can easily be checked. The same goes for watering – how much and how often? Andy extolled the virtues of a mulch. It can reduce plant resource competition, improve soil nutrition, reduce compaction, enhance root establishment and improve plant performance.
He went on to stress the importance of care when transporting trees, saying it is imperative that roots should not be exposed to the air for hours before planting and that they should always be covered when being transported. Going on to look at more mature root systems, soil compaction causes many problems for a tree, preventing adequate aeration, diminishing soil porosity and causing nutrient deficiency.
Why do we prune trees? It can be for a variety of reasons – to promote health, reduce conflict, increase light, encourage flowering or fruiting, control future growth or to mitigate a risk. But pruning has a down side, Andy reminded us. It increases both physiological and hydraulic stress, makes a tree vulnerable to pests and pathogen colonisation, and it exposes heartwood. In addition, the leaf/stem biomass is reduced, as is the leaf area and habitat. Pruning and not pruning should therefore both be management options.
We should consider the time of year, the species and the tree’s form when pruning. Spring pruning can sometimes remove bark, while sap bleeding can be a problem for genera such as Acer and Betula. Summer pruning causes water stress, but trees are actively growing and respond immediately to the wound. Autumn pruning may impede shoot development and reduces nutrient resorption. It may also stimulate late shoot growth which would be vulnerable to frost damage. In the case of winter pruning, there is a diminished wood response, but there is low pathogen activity and the tree is less likely to suffer xylem damage. The tree will also have completed its nutrient resorption.
The afternoon’s session was led by Henrik Sjöman of the Swedish University of Agricultural Sciences. His presentation concerned species selection. Introducing himself as a ‘black sheep’ among landscape architects, he told delegates he both researches and teaches, and is particularly interested in site-adapted species use. “What I am after is the right plant in the right place!”, he told us.
He said he was pleased that trees are no longer regarded just as ‘green fluff’ in Sweden, although in most planning processes they are considered later than just about everything else. It disappoints him when he hears landscape architects talking about ‘trees’ as though they were all the same, regarding them simply as circles on a diagram. When it comes to cars, we are much more careful, Henrik believes.
In the urban environment trees are a valuable asset when it comes to reducing storm water run-off. He told us that due to its great amount of paving in the city Copenhagen usually floods twice a year. Trees can help in two ways because not only do their roots take up moisture from the soil, but tree canopies intercept a great deal of rain and ensure that much of it never reaches the ground. He told us that a 10 per cent increase in tree cover would mitigate the 4°C rise in temperature attributable to climate change.
On the matter of shade provision, some trees are better than others. Gleditsia, for example, is poor in this respect, but hornbeam is very good because it has a much higher leaf area index. Henrik feels that such information, along with when species come into leaf, should be given in nurseries’ catalogues.
Wind is a great problem in Sweden and one with which they struggle constantly. The green infrastructure is important, but it continually battles the chilling effect of the wind. Trees can break the wind, but they can also cause turbulence. It is, however, important to have some ventilation underneath the tree crown.
So what should we look for when making our selection of trees for the urban environment? Henrik believes there are seven aspects, which he discussed in order of their importance. His main consideration is hardiness and health. In his opinion, hardiness involves performing well and not just an ability to withstand frost. In second place is site-adapted species use, and this is where he feels we must get to understand the ‘personality’ of trees, such as their tolerances and the type of growth they typically make. He feels we should look for ecological matching in our selection of trees, rather than forcing them into situations which they cannot handle. For example, most Scandinavian street trees come from a meadow system, but they would be better using hornbeams and pines, which originate in the steppes system. A tree needs the right weapons and the right strategy if it is to survive.
Henrik’s third consideration is function and his fourth is succession and an understanding of which trees are naturally pioneers, for instance after a landslide, and which take longer to establish. This can help us to decide which species to plant first and which to plant later in a scheme. And yet, said Henrik, succession is something which is never discussed in relation to street trees. The list is completed by maintenance, growth and aesthetic qualities. The ‘take home’ message with which Henrik concluded his presentation was – Do not talk just of ‘trees’, but go into detail.
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The 2013 Barcham season of arboricultural seminars began at its Cambridgeshire nursery on 13 March with presentations to a full lecture theatre from Andy Hirons of Myerscough College and Henrik Sjöman of the Swedish University of Agricultural Sciences.