City Trees is a premier publication focused on urban + community forestry. In each issue, you’ll learn how to best manage the trees in your community and more!
Issue link: https://read.dmtmag.com/i/128847
Populus trichocarpa in fall color in Reykjavik, Iceland • Photos by Halldor Sverrisson An extremely hardy and toler- ant species for difficult climates, the black cottonwood (Populus balsamifera subsp. trichocarpa) is now one of the most commonly used urban trees in Reykjavík, Iceland. Climatic circumstances in Iceland do not favour most trees and there is a very short history of using trees in cities and towns in this almost treebarren and windswept country. In short, the Icelandic climate can be described as extreme coastal, having cool summers (10-17°C/5063°F) and mild but very unstable winters with temperatures fluctuating between minus 10°C (14°F) and plus 10°C (50°F) from October until the beginning of May. There are not many species that can tolerate this climate along with all the difficulties that urban trees and especially street trees have to cope with. Black cottonwood comes to Iceland from the south-central coast of Alaska where climate is in many ways similar to that of Iceland. This is a vigorous, fast-growing deciduous tree that reaches 30Black cottonwood is one of a few species stalwart enough to thrive in the harsh climate of Reykjavik. www.urban-forestry.com 50 meters (98-164 feet) in Alaska and has reached 25 meters (82 feet) in Iceland. Many clones are available in production, some extremely salt and wind tolerant such as 'Keisari' (the Kaiser), a relatively small male clone; 'Brekkan', a medium size male clone with broad crown; and the slender female clone 'Súla' with beautiful olive green spring foliage. New cultivars are being bred to create hardy and wind-tolerant, well-shaped trees, with resistance to the cottonwood rust (Melampsora larici-populina) and favourable male clones to prevent troublesome midsummer "cotton snowing" from the female seed pods. This durable American native is widely used as a windbreak in Iceland. It became popular in the 1970s as a garden tree, although people have since discovered it grows a little too large for this purpose, often blocking out precious sunlight and causing disagreement among neighbours. Since the late 1980s it has been widely planted as street tree in Reykjavík and has coped well in that tough environment. It fits also well with the Icelanders' mentality, as it is easy to establish. However, everything has its drawbacks and the black cottonwood has many; it has become notorious because of its great vigour and intrusive root system, destroying pipes and pavement. In the year 2010 the newly elected mayor of Reykjavík and the City Council decided that cottonwoods in central Reykjavík should be uprooted and something else planted instead. After a huge debate about the pros and cons of the black cottonwood, arborists agreed that even though the species has its liabilities, no available tree species in Iceland can, in the short term, replace it due to lack of size, vigour, or tolerance towards stress and pollution. Until a better street tree shows up for extreme circumstances, the black cottonwood will be the king of the urban trees in Iceland. – Samson Bjarnar Hardarson, Assistant Professor in Department of Landscape Planning and Architecture, Faculty of Environmental Sciences at the Agricultural University of Iceland, Reykjavík. Hardarson runs a research programme on garden and landscape plants in Iceland: http://yndisgrodur.lbhi.is. 39