Arboriculture & Urban Forestry 33(5): September 2007 335 more uniform growth than the other establishment methods. Especially the conventional and the sand mix methods appear to cover a fairly large variation. Concerning the sand mixes, some of the problems encountered may be caused by drainage difficulties as experienced by Kristoffersen (1999) or by a soil compaction level too high for root growth. However, that sand mixes are indeed an alternative to structural soils is indicated by a vigorously growing planting of Platanus es- tablished in sand mix [mean annual stem diameter increment of 1.84 cm (0.72 in)]. Interpreting the results of this survey, it should be taken into account that even when establishing a tree using struc- tural soil or sand mix, the actual planting pit will often be filled with normal tree planting substrate also used for con- ventional plantings. Thus, in the first years after establish- ment, this soil volume will constitute the main source of water and nutrients for the tree. With the roots growing out of the actual planting pit, root processes will move into the structural soils, which will then gain in importance. Differ- ences between growth reactions in response to the investi- gated establishment methods are therefore supposed to de- velop with the course of time and continuing tree (root) growth. In addition to a mere extension of the rooting zone, ame- lioration of the soil composition or soil amendments may be considered as possibilities to optimize root growth conditions and/or direct root growth into desired soil volumes (Braun and Fluckiger 1998; Ferrini et al. 2005). Although tree growth generally is considered acceptable for all establishment methods, differences appear when trees are planted with different methods on the same site. There- fore, if the ambition is to achieve a uniform plantation, the same establishment method should be applied to all trees. Experiences from France indicate that roots indeed are ac- tively growing into the structural soils applied (Lemaire and Sorin 1997), and Dutch experiences report similar findings for sand mixes (Couenberg 1994). However, quantifications of root growth in load-bearing materials on actual street sites are currently not available, and as surfaces above those types of materials typically are paved, root growth studies are somewhat complicated. Although it is now established that the aboveground parts of trees established in root-friendly base materials actually grow, evidence of roots actually growing out of the planting pit and into the base materials is still sparse, and further research on root development in load- bearing soils is required. Because this survey was carried out on real street tree plantations in the city of Copenhagen, many potentially un- known influences on tree growth performance entailed a high variation of the obtained data. This variation was to be ex- pected and counterbalanced by a large data set. Although statistically a challenge, a study of street tree growth in a real urban context is considered to reveal valuable information. A certain degree of uncertainty concerning the estimation of tree size at the time of establishment may have affected the growth calculations. Because the majority of the trees incor- porated in the survey were planted as 18 to 20 cm (7.2 to 8 in) stem circumference, the risk of false estimates was reduced. All included genera of street trees achieved less growth on the actual street site compared with the urban tree arboretum. However, site conditions on the arboretum must be consid- ered to be superior to urban sites. Therefore, based on the presented data, growth and vitality of most street tree species established in Copenhagen during the last 5 to 15 years may be characterized as acceptable or even good. Contrasting former studies as, for example, by Gilbertson and Bradshaw (1990), stating a generally poor street tree establishment praxis, this finding accentuates that deliberate planning and establishment procedures result in functioning street tree plantations. Because many of the trees involved have not yet reached their desired size, and because they eventually will reach limits set by the available soil volume and other constraints, further studies will need to establish potential maximum sizes and life expectancies of street trees planted in accordance to the different establishment concepts. In theory, super planting pits providing the largest soil volumes in combination with a large surface area should supply both more water and nutri- tion than the other concepts. CONCLUSIONS As a general conclusion based on the presented data, growth and vitality of most of the street trees in Copenhagen planted during the last 5 to 10 years appears to be good. The alternative tree establishment methods have proven to result in at least comparable tree growth as the conventional establishment method without rooting space enhancements. The super planting pits have proven to be superior to the other planting methods. On the basis of this survey, the use of root-friendly base materials can therefore be recommended for sites where tree establishment is wished for but consid- ered difficult or impossible in a conventional planting pit. However, as the example of the super planting pits illus- trates, extra rooting space in depth combined with a large open surface area results in high growth rates and high uni- formity of the plantation. Of the four planting methods, the super planting pits are considered the safest way of establish- ing trees in the urban environments. Thus, wherever space is available, this method is recommended. In regard to more specific reactions to the different estab- lishment methods, it was determined that Platanus had ex- traordinary high growth rates in sand-based mixtures but did not react with higher growth rates to establishment in super planting pits. Tilia had significantly higher growth rates in super planting pits. Acer expressed difficulties when estab- lished in root-friendly base materials and thrived best when ©2007 International Society of Arboriculture
September 2007
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