Journal of Arboriculture 31(2): March 2005 95 Research Note PRELIMINARY EVALUATION OF A LIVING WILLOW (SALIX SPP.) SOUND BARRIER ALONG A HIGHWAY IN QUÉBEC, CANADA By Michel Labrecque and Traian Ion Teodorescu Urban populations are becoming more concerned about air and noise pollution caused by motor vehicle traffic and are demanding solutions. Conventional concrete walls built to mitigate the effects generated by motor vehicle traffic have a negative impact on the urban landscape and quality of life. During the last decades, private companies in diverse countries, notably in Germany and the United Kingdom, have developed a new technology based on the use of living stems to build green walls as noise barriers. In Canada, a private company, The Living Wall Inc., owns a patent on living noise barriers (Freitag 2001) and promotes their use in the province of Ontario. However, to our knowledge, no serious scientific study has been conducted to evaluate the potential and the limits of these structures, especially in regions characterized by rigorous winters. The principle of construction of living walls is based on the use stems of willows (Salix spp.), plants characterized by very high growth and photosynthetic rates (Cannell et al. 1987, 1988; Labrecque et al. 1993). Willow living walls can be established quickly and require easily accessible organic materials (plants, soil, and wood). They are aesthetic, are green, and contribute to the improvement of the landscape in urban areas. Their organic porous structure (plants and soil) might be more effective than concrete walls in attenuat- ing noise. In addition, these structures can have a positive impact on carbon sequestration and on the diversity of fauna habitats, particularly birds. The objectives of this study were to verify whether living walls can be established and grown in the climatic condi- tions of Québec province (eastern Canada). MATERIALS AND METHODS Experimental Site The living wall set up for this study was established in the city of Saint Bruno, located about 15 km (9 mi) southeast of the city of Montréal, Québec, Canada. The climate of the area is continental, characterized by cold, snowy winters and hot, humid summers. The annual average temperature is 6.4°C (43.5°F), and annual average precipitation is 954 mm (37.5 in.) (Ministère de l’Environnement du Québec 1991). The experimental wall, 30 m (98 ft) long, was established on a marginal north site of Highway 116, in a residential sector of Saint Bruno. It was oriented parallel to the road on an east–west axis. Plant Material A European basket willow clone (Salix viminalis L.), identified by the Québec Ministry of Forestry as clone 5027, was used in this experiment. This clone has been studied for many years and is known to be very productive in culture in southern Québec (Labrecque et al. 1993, 1994). Two thousand willow stems were used. The stems harvested were 4 years old but were developed on 7-year-old roots. Straight, nonramified stems with a minimal length of 3.5 m (11.5 ft) and a diameter at the base between 4 and 6 cm (1.6 and 2.3 in.) were cut during March 2002 (before budbreak) about 10 cm (4 in.) above ground level. Following their harvesting, stems were packed and stored in the field under a white plastic cover for 1 month until their transportation to the site of construction. During outdoor storage, temperatures stayed around 0°C (32°F), and only a very limited number of the stems started to bud. Construction of the Wall The construction was set up in April, as soon as the ground had thawed out. Following transport to the construction site, stems were quickly assembled tightly in the wooden frame and installed upright in two 1 m (3.3 ft) deep trenches set 1.2 m (4 ft) apart. The two willow walls were solidly held together by wood pieces and steel rods (Figures 1 and 2). A permeable, 2.5 m (8.2 ft) wide, thin geotextile was fastened inside over the entire length of the wall. The trenches were filled with a sandy-textured soil. A tractor equipped with a front loader was used to fill the space (with the same soil) between the two walls up to a height of 2.5 m (8.2 ft). At the end, a perforated irrigation hose connected to the municipal aqueduct was fixed at the base of the stems (20 cm [8 in.] above ground level) throughout the two sides of the wall. Once finished, the wall was 30 m (98 ft) long, 1.2 m (4 ft) wide, and 2.5 m (8.2 ft) high. More details about the construction of the wall are given in the patent description (Freitag 2001). ©2005 International Society of Arboriculture
March 2005
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