Looking first at temperate trees, these generally experience 120 – 250 frost-free growing days a year with a mean winter temperature range from -5°C to 10°C and a mean summer temperature range from 20°C – 27°C. Precipitation in such areas is around 50-75cm, with trees usually growing in relatively deep, fertile soils. Looking at the canopy morphology of temperate trees, Andy pointed out they receive diffuse light from cloudy skies and that a domed canopy allows a tree to take light from all sides on cloudy days.
As canopies become more complex the problem of shading increases, with reduced photosynthesis limiting energy production. He said there are structural differences in the leaves adapted to shade, with many tree canopies having both ‘sun’ and ‘shade’ leaves - Tsuga and Fagus being two examples. Branching patterns and leaf arrangements minimise overlapping with leaves arranged in a spiral sequence (phyllotaxy). The upper leaves may be smaller and on shorter stalks, while leaves in a flat plane fit together rather like a jigsaw puzzle, exposing the maximum leaf area to light.
Monolayer trees, such as beech, hemlocks, suagar maple and sycamore produce a single shell of leaves over the entire canopy. They are suited to shaded environments and have inefficient light interception if
growing in the open. Multilayer trees, such as poplar and birch, are more adept at growing in high light environments because their leaves are stacked above each other and they tend to have a more open canopy
But self-shading is a significant problem with flattened petioles, shedding of leaves and the separation of leaves.
Looking next at wood anatomy, Andy told delegates there is a huge variation in wood structure across species. Angiosperms have two types of structure – they are either ring-porous, such as Quercus robur, or diffuse-porous, such as Acer saccharum. Eighty to 90 per cent of all tree roots occur in the top 1.5m of soil, with very few roots found below 3m in temperate zones, although conifers tend to be more deep-rooted than deciduous trees.
Moving northwards there are significant differences in the environment. Boreal trees are those between the lines of latitude 45° and 70°N, being Canada, Alaska, Siberia, some parts of Russia and Scandinavia. They can expect a 30 – 150 days’ growing season with a temperature greater than 10°C, with temperatures capable of falling below -25°C. Precipitation varies between 38 and 50cm, and soil is generally acidic and poor in nutrients. Firs and spruces are typical here where light angles are low, even in summer. Tree physiology inevitably slows down in such regions. Alders, poplars, larch, birch and willow will also be found in these northern areas.
As regards wood structure, there are no ring-porous trees in the boreal belt. The roots are generally much more shallow than those of temperate trees. Not surprisingly, permafrost is a real problem here, with little root growth occuring when temperatures drop below 5°C.
On to mediterranean or ‘chaparral’ trees, their habitat is characterised by winter rainfall and summer drought. Rainfall can be very variable, with a range of 100-2000mm. There will also generally be several months without any rain at all and there is a mean average temperature of 13-17°C. Sclerophyllous (leather-leaved) trees such as olive and cork oak thrive in a mediterranean climate.
Andy concluded by telling us that tolerance of the local climate and environment is a key factor in tree establishment, preceded by how it is treated prior to planting. There are of course many differences between the woodland and urban environments, but it is important to try and get the tree physiology right by mimicking woodland conditions as closely as we can.
The afternoon’s session was led by Henrik Sjöman of the Swedish University of Agricultural Sciences. His presentation, entitled Selection of Urban Trees – Learning from Nature and based on his recent PhD thesis, dove-tailed neatly with what he had heard from Andy Hirons during the morning. Introducing himself as a landscape engineer and plant technologist, he told delegates he would be talking from a Scandinavian perspective, where planners are increasingly appreciating the importance of trees in the urban landscape. 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.
Henrik began by drawing attention to the lack of diversity among urban trees. For examples, Tilias make up 44 per cent of all trees in Helsinki and 25 per cent in Oslo, with Acer platanoides making up 12 and 18 per cent respectively in these cities. With such concentrations, pests and diseases are a worry. Whether we believe in climate change or not, city climates have changed significantly in the last 100 years, Henrik said, becoming ‘heat islands’. His university has trial areas of trees from different countries, such as Japan, to see how they perform in the Swedish climate. Bur what guidance is there for urban tree planners? There is apparently plenty of information available, even for rare species, but it is piecemeal. This makes acquiring an overview both difficult and time-consuming. Dendrological and taxonomic literature is frequently too general, while scierntific literature tends to be too specific. Information in nursery catalogues and in literature on plant use in cities is generally presented without references to other sources, which gives the impression that the information is mainly based on the authors’ own experiences and qualitative observations of existing urban plantings.
Henrik Sjöman believes it is important to start by trying 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.
Competitive species such as Populus and Acer saccarinum have a ‘live fast, die young’ outlook – acording to Henrik they are the rock ‘n’ rollers of the tree world! Stress-tolerant trees are good as street trees. Quercus cerris and Pinus nigra are good examples of this. Ruderal (pioneer) species thrive in disturbed ecosystems, such as an environment following a fire or other clearance. Scandinavia has few native tree species, many having been wiped out by the last Ice Age. Henrik feels they need to look at exotic species, bearing in mind Sweden benefits from the Gulf Stream – a fact of which I was unaware.
As part of his studies Henrik visited the Qinling Mountains in a remote part of central China. There are very many species here, but the climate is mild and so many species are not hardy enough for Sweden. On the other hand, trees from northern China are almost too hardy for Sweden, suitable only for Uppsala northwards. Qinling was chosen because it is the meeting point for the two types of trees and it has not been studied before. It also happens to be the home of the giant panda, resulting in a high level of governmental protection and great difficulty in accessing it. While in the Qinling Mountains, Henrik was on the look-out for trees growing on south-facing, dry slopes as those with potential for becoming street trees in Sweden.
Myths sometimes grow up about the conditions a tree requires if it is to thrive. Paeonia suffruticosa var. ostii is a good example of this, as its native habitat is dry and rocky. Small-leaved trees are valued in Sweden, as they do not block out the sun during the short summer. Koelreuteria (Pride of India) is well regarded, but Ailanthus is invasive in the wild and Henrik is not keen to introduce potential weeds.
He is keen to hunt for genetic variations within a species and feels that with some trees there are many forms in nature which are better than those currently in cultivation. As an example, he showed us the considerable variations which occur within Magnolia biondii in central China.
In his quest for potential street trees, Henrik has also conducted research in Romania and Moldavia. The further east one travels across Europe the drier the cliamte becomes. The climate of the steppes is actually similar to that of Copenhagen, so one might expect it to be a good source of potential street trees for Scandinavia. The soil of the steppes is fertile and dry, and was formerly forested. Sorbus torminalis, Tilia tomentosa and Quercus frainetto are native here and, unlike many Chinese trees, they are already known in the west and are good trees for the urban environment, with Tilia tomentosa generally doing much better than others in the genus. Its late flowers provide a welcome source of nectar, but Henrik feels it requires further selection work. Carpinus orientalis is, he believes, a species with a big future, while he is similarly enthusiastic about Cornus mas, although its fruits may be considered a problem in paved areas.
His quest for stree trees has also taken Henrik to North America. Here he found that Platanus occidentalis handles both very wet and very dry conditions equally well, while Catalpas speciosa and bignonioides and Quercus palustris are trees of the river plain. Alnus cordata, from the wetlands of the Balkans, is being used increasingly in southern Sweden.
Both speakers were well received in a packed lecture theatre, prompting much interaction with delegates. The Barcham lecture series of 2012 promises to be as stimulating and revealing as it has been in past seasons.
Andy Hirons graduated from Myerscough College with BSc (Hons) Arboriculture in 2003. After pursuing an opportunity to work as a climber and plant health care technician in the USA, he returned to England and joined the Arboriculture Department at Myerscough College, Lancashire. His main teaching responsibilities and academic interests are in tree physiology and arboricultural practice, particularly the challenges of establishing trees in the urban landscape. In addition to his lecturing, he is currently working on a PhD that explores the response of temperate trees to water deficit and the potential application of those responses in tree production.
Henrik Sjöman graduated from the Swedish University of Agricultural Science (SLU) in 2003 as a landscape engineer with an M.Sc in Landscape Planning. After finishing his master studies, Henrik worked on research and teaching at the SLU with a focus on plant material and vegetation construction. He began work on his PhD 2008, beginning with a year in China, studying the Qinling Mountains in search of potential urban trees. During his PhD, Henrik has also conducted several field expeditions in Romania, Moldavia, China and North America in search of future species and genotypes for urban environments. In February 2012 Henrik completed his doctoral work with the thesis "Trees for tough urban sites – learning from nature".