330 Bühler et al.: Growth of Street Trees in Copenhagen Arboriculture & Urban Forestry 2007. 33(5):330–337. Growth of Street Trees in Copenhagen With Emphasis on the Effect of Different Establishment Concepts Oliver Bühler, Palle Kristoffersen, and Søren Ugilt Larsen Abstract. Limited rooting space is considered a major problem for growth of street trees. Different approaches to extend the soil volume accessible to roots have been implemented in Copenhagen, Denmark, during the last 15 years. The presented survey investigates growth and vitality of trees planted in 1) structural, load-bearing soil; 2) sand-based load- bearing soil; and 3) so-called super planting pits and trees planted in conventional planting pits. The trees were either street trees or situated on paved squares. The results provide evidence that vitality of recently established street trees in Copenhagen generally is on an acceptable level. Furthermore, tree growth in both of the load-bearing materials allowing for root growth was found to be comparable to tree growth in conventional planting pits, and those methods are therefore considered applicable at sites where conventional pits cannot be established. However, tree growth in super planting pits was superior to any of the other methods both in regard to growth rates and vitality. This method is therefore recommended wherever space for large planting pits with open surfaces can be made available. Growth of trees planted in conventional pits varied considerably, reflecting the broad range of different conditions the term “conventional planting” pit covers. Key Words. Acer; load-bearing soils; planting pit design; Platanus; Populus; road foundations; rooting volume; sand mix; stem increment; stone matrix; structural soils; Tilia; tree growth; urban trees. The majority of street trees are exposed to a challenging environment for tree growth. As a result, growth, vitality, and ultimately the lifespan of street trees may be reduced consid- erably compared with trees growing in their natural habitats. Gilbertson and Bradshaw (1990), for example, reported that 23% of newly planted trees in Liverpool had died after three growing seasons, and estimates on the average lifespan of urban trees are far lower than what would be expected on more favorable, natural sites (Balder et al. 1997). The reasons for this are complex, but limited soil volumes and increased soil compaction are some of the major explanations for the poor performance of many urban trees. Although the planting pit of a tree may be connected to surrounding soil volumes within reach for the tree roots, these soil volumes are mostly load-bearing layers for pavements or buildings and, consequently, highly compacted. One of the parameters that indicate whether a soil is com- pacted is soil bulk density. Soil bulk density is, to some degree and in combination with other soil characteristics, reflective of the mechanical resistance roots meet in the soil. Root growth of Quercus robur was impeded when soil bulk density exceeded 1.5 g/cm3 (Grabosky and Bassuk 1996), and Malus domestica seedlings also exhibited reduced shoot length, leaf area, leaf size, and dry weight of leaves, shoots, and roots when grown in soils compacted to bulk densities of 1.5 g/cm3 (Ferree et al. 2004). Mullins (1991) states that for ©2007 International Society of Arboriculture many soil types, root growth is limited seriously when bulk densities exceed 1.6 g/cm3. Road foundations in Denmark are typically compacted to bulk densities exceeding 2 g/cm3 (Kristoffersen 1999). Although the compaction of base materials is intended and a prerequisite for the performance of load-bearing materials, compaction may also occur unintended in the course of con- struction works. An investigation by Randrup (1997) reveals soil bulk densities on construction sites in Denmark averag- ing at 1.94 g/cm3. Because of the high level of compaction of the load- bearing layers of many urban structures, the root zone of street trees is often confined to the rather limited soil volume of the planting pit (Kristoffersen 1999). In an effort to extend rooting volumes and thus enhance the success rates of urban tree plantations, the city of Copenha- gen has from the beginning of the 1990s implemented dif- ferent establishment methods: 1) conventional establishment, 2) structural soils, 3) sand-mix, and 4) super planting pits. These establishment methods are described in detail. Conventional Establishment of Trees in Copenhagen Typically, trees are established in planting pits of varying sizes dependant on site possibilities and limitations. Surface area varies between 1.5 and 6 m2 (16.2 and 64.8 ft2), aver-
September 2007
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