334 Bühler et al.: Growth of Street Trees in Copenhagen planted conventionally [1.53 cm (0.61 in), n14] compared with trees established in structural soil [0.86 cm (0.34 in), n26]. A plantation of Tilia europaea ‘Pallida’ established partly in super planting pits [1.38 cm (0.55 in), n 16] and partly in structural soil [0.89 cm (0.36 in), n4] showed the same tendency, however, without being statistically sig- nificant. Growth differences of species belonging to the same genus were small and insignificant for Quercus, Acer, and Tilia, except for Tilia platyphyllos, which achieved a mean stem increment 1.08 cm (0.43 in), significantly higher than T. eu- ropaea (0.85 cm; 0.34 in) and T. cordata (0.81 cm; 0.32 in). Significant differences of mean stem increment was also de- termined between all species of Sorbus with S. latifolia achieving 1.02 cm (0.41 in), S. intermedia 0.77 cm (0.31 in), and S. aria 0.64 cm (0.26 in). Fraxinus, Platanus, Populus, Robinia, Prunus, and Crataegus are only represented by one species each. Vitality With 1733 of the 2164 trees in this survey achieving vitality score 4 or 5, 80% of all trees are within the vitality range describing functioning street trees. Only Quercus and Cra- taegus had mean vitality scores ranging below 3.5. Mean vitality scores above 4.5 were obtained by Populus, Prunus, and Robinia (Table 3). Vitality of trees established in super planting pits differed significantly from trees established in structural soil, sand mix, and conventional planting pits (Table 5). DISCUSSION Comparing the different establishment methods over the whole range of observations, neither significant nor relevant differences were determined when comparing stem growth in response to conventional establishment, establishment in structural soils, and establishment in sand mixes, respec- tively. It is thus evident that establishment of trees in base materials allowing root growth is possible and allows for acceptable tree development at least for the first 15 years after establishment. However, super planting pits providing a large open surface area and large soil volume result in significantly higher stem growth and vitality score. Table 5. Mean vitality score of all 2164 trees in response to establishment method.z N Conventional Sand mix Structural soil Super planting pit 1461 94 536 73 Mean vitality score 4.3 a 4.2 a 4.1 a 4.9 b Std 0.03 0.13 0.05 0.03 zMean values in the same column differ significantly when followed by different letters (Tukey test, P < 0.05). ©2007 International Society of Arboriculture Populus, Platanus, and Fraxinus achieved good growth results in this survey and may thus be regarded as good street tree candidates. Populus in particular may be regarded as a promising option for locations, where expensive approaches are impractical, because it achieves good growth rates even at utterly unprepared sites. On the other hand, possible damage to paved surfaces as well as pipelines by the vigorously grow- ing root system has to be taken into consideration when es- tablishing poplars (see e.g., Kopinga 1994; Gilman 2006). Platanus achieved particularly good results in sand mix and is the only species in this survey that actually achieves better results in sand mix compared with the other establish- ment methods. Although this is only documented for 17 trees in one single plantation, the combination of plane trees and sand mix appears promising. Growth rates in super planting pits are comparable to growth rates of trees established con- ventionally, suggesting that plane trees thrive even when es- tablished under less favorable conditions and that super plant- ing pits may be reserved for other, more demanding species. Tilia clearly grows best in super planting pits. A compari- son with growth of Tilia at the urban tree arboretum also indicates that growth conditions for Tilia in the urban envi- ronment leave room for improvement. Based on this survey, super planting pits are a potent option to enhance the perfor- mance of Tilia street trees. A closer look on the growth of Acer reveals that this genus achieves high growth rates only if planted in conventional systems. However, this result may be colored by the fact that the one site representing Norway maple in sand mixes is situated at an especially heavily trafficked bus terminal and experienced problems during establishment. Nevertheless, based on the presented data, the tree manager should not rely on designed soils to establish A. platanoides and A. pseudo- platanus on difficult sites but instead use this genera on lo- cations offering naturally good conditions, as, for example, connected planting strips with permeable surface situated at some distance to the road to avoid deposition of deicing salt (Pedersen et al. 2000). Because establishment of trees in these alternative road base materials is more expensive than conventional establish- ment, their adoption should be preceded by careful consider- ation of alternatives. Their cardinal use should be restricted to sites that are unsuitable for conventional establishment, but where tree establishment is absolutely desired. Wherever the urban structures allow for it, super planting pits and compa- rable methods offering large open surface areas and large soil volumes will provide superior growth conditions for street trees, as shown in other studies, too (Grabosky and Gilman 2004; Smiley et al. 2006). When comparing the different establishment methods in regard to data variation (e.g., standard deviation in Table 4), it becomes obvious that the super planting pits result in a
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