332 Bühler et al.: Growth of Street Trees in Copenhagen urban trees is therefore considered a valuable tool for tree managers to create functioning plantings at reasonable costs. MATERIAL AND METHODS Selection of Street Tree Plantations To counterbalance the variation expected to be met under urban conditions, the survey was designed to include a high number of sites and trees. Plantations were selected based on information from the Copenhagen tree inventory. The follow- ing selection criteria had to be met by each plantation in- cluded in the survey: 1. Time of establishment between 1990 and Spring 2001, thus guaranteeing at least five growth seasons after es- tablishment; 2. Each plantation should consist of a minimum of 10 trees; 3. Information regarding tree size at establishment and planting method used had to be available from the tree inventory; and 4. Cultivars of the following species were included in the survey: Acer (A. platanoides and A. pseudoplatanus), Fraxinus excelsior, Platanus × acerifolia, Populus can- densis, Quercus (Q. palustris, Q. petraea, Q. robur, and Q. rubra), Robinia pseudoacacia, Sorbus (S. aria, S. intermedia, and S. latifolia), Tilia (T. cordata, T. euro- paea, and T. platyphyllos), Prunus avium and Cratae- gus lavallei. As a result of these criteria, the final data set contained in- formation on 2164 trees. Measurements For each tree, stem circumference was measured at a height of 1 m. In addition, a vitality score was assessed on a scale from 0 to 5 (Table 2). To assess growth rates, information on tree size at time of establishment was obtained in cooperation with the munici- Table 2. Vitality scores assigned to the street trees. Score Description 0 Tree dead or dying 1 2 3 4 5 Tree growth seriously affected, dead branches, discolored foliage, sparse crown Tree growth visibly affected, no shoot growth, discolored foliage Tree appears healthy but stagnating, little or no shoot growth, crown appears largely dense and without discolored foliage Tree appears healthy and in growth, annual shoot growth less than 30 cm (12 in) Tree appears healthy and growing vigorously, annual shoot growth exceeding 30 cm (12 in), high crown density and no signs of discolored foliage ©2007 International Society of Arboriculture palities of Copenhagen (Vej & Park). Because available in- formation only denoted size intervals [e.g., stem circumfer- ence of 18 to 20 cm (7.2 to 8 in), 20 to 25 cm (8 to 10 in) instead of accurate sizes), size was assumed to correspond to the average of the range minima and maxima, i.e., trees planted with a stem circumference of 18 to 20 cm (7.2 to 8 in) were assumed to have an average stem circumference of 19 cm (7.6 in). As a reference, stem increment of most species and culti- vars represented in the evaluation was measured at the Urban Tree Arboretum in Hørsholm, Denmark. Here, many urban tree species are represented established with six replicates in 2001 on a formerly afforested area. Statistics and Data Analysis Annual increment rates were calculated for each tree as dif- ferences between stem diameter at the time of establishment and measured stem diameter in 2005 divided by number of growing seasons. Analysis of variance (ANOVA) using the GLM procedure in SAS, version 8.02 (SAS Institute, Cary, NC) was used to compare yearly stem diameter increment means of street trees in response to the different establishment methods. ANOVA was first performed on the whole data set with stem diameter increment and vitality score as response variables and estab- lishment method and genus as explanatory variables. Because significant interactions between genus and establishment method were determined in regard to stem increment, ANOVA was subsequently performed on data sets consisting of only Platanus, Tilia, and Acer, respectively, i.e., genera whose data set contained plantings with different establish- ment methods. In regard to vitality, no significant interactions were determined and the reported results originate from the initial ANOVA. When ANOVA results were significant (P< 0.05), differ- ences between means were determined by Tukey’s studen- tized range tests for P < 0.05. RESULTS Stem Growth The overall mean stem diameter increment of all 2164 trees was 0.96 cm (0.38 in). With 1.99 cm (0.80 in), Populus had by far the highest mean annual stem diameter increment. Prunus (1.35 cm; 0.54 in), Platanus (1.01 cm; 0.4 in), and Fraxinus (0.98 cm; 0.39 in) had high increment rates, too, also compared with growth rates of the same genera at the urban tree arboretum. Platanus, Fraxinus, and Prunus achieve at least 80% of the growth at the arboretum (Table 3). The distribution of observation pr. genus reveals a high number of Tilia (632), Platanus (690), and Acer (256) street trees. Of all genera in this investigation, these three genera were established with different methods.
September 2007
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