The 2014 season of arboricultural seminars at Barcham Trees Ely, Cambridgeshire, nursery began in early March with a day led by husband and wife presenters Henrik and Johanna Deak Sjöman of the Swedish University of Agricultural Sciences.
Johanna is finalising her PhD thesis on green space planning and design within the university’s Department of Landscape Architecture, Planning and Management. She began the day by talking about her involvement with the redevelopment of an old harbour site just north of Malmö into a sustainable residential urban area. In the matter of sustainable urban drainage systems (SUDS), when she asks students she teaches how they envisage them, the majority suggests something visible, such as installations or water features. These, however, often attract rubbish and sometimes involve high maintenance costs, although equally they may be attractive and encourage biodiversity, suggested Johanna.
Drainage systems often have to be installed on a small scale and face restrictions posed by car parking, refuse bins and a host of other impedimentia. “Concrete is better than tarmac, gravel is better than concrete, and grass is better than gravel”, she said. Johanna believes that urban drainage systems should try to mimic nature as far as possible.
It is important to see green infrastructure as a network because changes taking place in one area may affect somewhere else. The Swedish authorities do not recognise derelict and agricultural land as part of the green infrastructure because it cannot be accessed by the public. Sweden has not had a wet winter since 2007 when the area in which Johanna works – Lomma, Lundand Staffanstorp north east of Malmö – was flooded. Lundis expanding rapidly and rainfall here has the knock-on affect of causing flooding in nearby Lomma.
The amount of rainfall run-off is site-specific, although more occurs on a sandy soil than on a clay one. Trees intercept rainfall before it reaches the ground. Johanna told us that conifers can intercept 60 per cent of rainfall while mature planes can intercept up to 80 per cent. Sustainable drainage does not have to be visible, and it does not have to be complicated. For example, attenuation tanks can be used underneath car parks, but engineers do not see water as a resource, according to Johanna.
She then considered what makes the urban climate different from the rural one. It can often depend on what is present, such as materials which may contain energy and heat, different shapes and textures, air pollution and energy use, and the balance of water. In the urban environment, wind speeds are commonly 50 to 75 per cent less than in rural areas, but turbulence and the funnel effect are much stronger. High buildings at the ends of streets and on corners increase turbulence, although turbulence can be beneficial in dissipating pollution from vehicles.
Johanna believes that indigenous peoples around the world had a great knowledge and understanding of the buildings they created, but that in the modern world we choose to ignore this wisdom. She sees trees as a building material and feels that street trees do not have to be positioned equidistantly, but should be planted where they are most effective.
Henrik Sjöman graduated from the Swedish University of Agricultural Science in 2003 as a landscape engineer with an M.Sc in Landscape Planning. In February 2012 he completed his doctoral work with the thesis “Trees for tough urban sites – learning from nature”. In considering trees for urban landscapes, he has investigated the unique qualities of many species, constantly asking what strategies they have in their native habitat which will help them in our man-made environments? “I’m a tree nerd!”, admitted Henrik.
He believes that generally we use too few species in our urban plantings. For example, limes are used extensively in Scandinavia, Celtis inItalyand planes in centralEurope. On the other hand, Henrik feels we use conifers too little. Because they ‘invest’ so much in needles they have fewer resources for extending shoot growth and are therefore slower growing. Evergreens are flexible because they can photosynthesise when they wish, and are also very good on dry sites.
Henrik told us there are hard pines – those with one, two or three needles – and soft pines with five needles. Hard pines are pioneer species, so well suited to challenging sites, while soft pines are semi-pioneers. Soft pines grow in moist forest habitats in the northern hemisphere and are more shade-tolerant than hard pines. He believes shade-tolerant Abies koreana and sibirica are both worth considering for urban spaces, while Picea omorika, the Serbian spruce, is the perfect ‘solitary’ tree. Thuja pilcata, western red cedar, is another favourite, exuding a delightful fragrance when closely planted. The dawn redwood, Metasequoia glyptostroboides, will tolerate wet conditions for short periods, so would be good for coping with stormwater run-off. Indeed it needs a moist soil to establish successfully.
Discussion then turned to oaks, which are divided intoMediterranean, nemoro-Mediterranean and nemoral categories. Mediterranean oaks, such as Quercus ilex, are drought-tolerant and many are evergreen. The Moldavian steppes, with their warm, dry summers and cold dry winters, are a typical home of nemoro-Mediterranean oaks, which often have thick papery leaves. Henrik said that Quercus frainetto, the Hungarian oak, makes a great urban tree, comparing it to a nightclub bouncer! Another such example is Quercus bicolor, planted at Ground Zero inNew York; Henrik is conducting selection work on this species. The third type, nemoral oaks, are from New England in theUSA, where they thrive in cool, moist summers. They do not, however, flourish in hot, dry conditions.
If we are looking for a tree which does well even where nothing else does, we should look at Gleditsia triacanthos, counselled Henrik. Used extensively inManhattan, the honey locust is a nitrogen-fixer, and therefore good for paved areas. He likes the thornless cultivars Sunburst, which has yellow foliage, Skyliner as a street tree and Shademaster where shade is required. “It is always worth exploring the cultivars of a species”, Henrik told delegates.
The handsome Kentucky coffee tree, Gymnocladus dioica, is another nitrogen-fixer well suited to the urban environment. It is popular inSwedenbecause it does not come into leaf until late June, so does not block welcome sunshine, and drops foliage in September. Its crown is thin in winter, again allowing sunshine through. “The last resort” is how Henrik described Eleagnus angustifolia. “Where everything else has failed, plant this because it tolerates everything!”
Alders too are nitrogen-fixers. Henrik mentioned Alnus cordata, the Italian alder, which is grown in the Balkans in areas which experience spring floods and long, dry periods. Alnus spaethii is also very tolerant, but is rarely seen in theUK.
During the seminar Henrik asked his audience to consider an even wider range of trees for the urban landscape. For example, he invited Barcham Trees to list drought-tolerant Acer x zoeschense, which has Acer campestre as one of its parents. Several cultivars of Ulmus parviflora are available in theUSA, but only the species is used inEurope. Resistant to Dutch elm disease and with beautiful bark, Henrik would like to see this more widely used.
Koelreuteria paniculata, Pride of India, which flowers in July and August, may look rather sad when young, so he suggests planting large examples of this attractive rounded tree, which has lantern-shaped fruits in autumn.
Henrik and Johanna certainly set a high standard for the series of seminars ahead this year as they made their presentations to a full and appreciative lecture theatre. As ever, these events will be provided free of charge, including lunch and other refreshments, to arboricultural professionals