The morning session on tree biology in practice was led by Andy Hirons, 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. 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, he said, 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 added.
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. 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, Hirons said as an industry we do not do well, with many professionals not visiting a nursery before they buy trees. Better written and more exacting specifications would see the better nurseries prosper.
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, Hirons said 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. Specifiers 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, the depth and diameter of the mulch should be specified. The same goes for watering – how much and how often? Hirons extolled the virtues of a mulch which can reduce plant resource competition, improve soil nutrition, reduce compaction, enhance root establishment and improve plant performance.
The afternoon’s session was led by Henrik Sjöman of the Swedish University of Agricultural Sciences who spoke on species selection.
Sjoman is particularly interested in site-adapted species us:. "What I am after is the right plant in the right place," he said.
He said 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.
In the urban environment trees are a valuable asset when it comes to reducing storm water run-off. He told delegates 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.
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. He feels that such information, along with when species come into leaf, should be given in nurseries’ catalogues.
Wind is a great problem in Sweden. 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.
Top of Sjoman's list for selecting species is hardiness and health where 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.
His 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.
The list is completed by maintenance, growth and aesthetic qualities.