162 Moser-Reischl et al: Urban Tree Growth Characteristics in South Germany surprising though, since the chosen cities vary in their climate and provide different growing conditions for trees. Kempten, for example, receives double the amount of precipitation of Würzburg, but this does not result in markedly faster tree growth. The tree height and crown diameter growth of the studied tree species in the 6 cities was not uniform. Small differences between cities seem to be highly influenced by species characteristics, as well as by different above- and belowground growing conditions. In detail, the results fit well with the different growth strategies of the analyzed tree species. Light-demanding, fast-growing species such as P. × hispanica and R. pseudoacacia showed in all cities and at all grow- ing sites the highest growth rates, e.g., fastest tree height or crown diameter development. Slower grow- ing species such as T. cordata and A. hippocastanum were, on the other hand, characterized by lower growth rates. As was proven for several urban tree species (see e.g., Rhoades and Stipes 1999; Pretzsch et al. 2017; Vaz Monteiro et al. 2017), urban trees often experi- ence higher growth rates due to growing more iso- lated, or experiencing warmer temperatures and longer growing periods. However, as our study could show, especially at street sites and public squares, the tree pit size can be tremendously reduced, which clearly resulted in reduced growth. Also, aboveground com- petition occurs at all growing sites. It could be expected that light-demanding species react with faster height growth to aboveground limitations. However, this was not found for P. × hispanica and R. pseudoacacia in the city of Nuremberg, where short- est distances to obstacles like buildings and other trees were observed. For example, at a DBH of 60 cm, P. × hispanica had in Nuremberg a 23% reduced tree height and 11% reduced crown diameter, while in Hof, tree height was reduced by 22% and crown diameter by 40% compared to the average tree height and crown diameter in all cities of 60 cm DBH. Sim- ilarly, R. pseudoacacia had at the same DBH a reduced crown diameter of 6% in Hof and a 12% reduced crown diameter in Nuremberg, while tree height was similar to the average tree height in all cities. So it seems that both species, and in particular P. × hispan- ica, are sensitive to belowground restrictions, as in the city of Hof, where the tree pit was smallest and high growth reductions of up to 40% were measured. For R. pseudoacacia, these results might also be biased by a low number of sampled trees, since in ©2021 International Society of Arboriculture Hof less than 20 individuals could be included in our study. Moreover, T. cordata and A. hippocastanum also showed in Hof (city with smallest tree pits) a slower crown diameter development than in other cit- ies with bigger tree pits. At a DBH of 60 cm, crown diameter reductions of 5% compared to the average were found. In parks, tree pits were significantly larger than tree pits of streets and squares. However, due to close plantings with other trees, aboveground space was also found to be markedly reduced in parks. This was reflected by the average tree height of the analyzed tree species, where park trees had greater DBH and tree height on average. However, as has been found before by Moser et al. (2015), trees in parks are also often older than trees in streets and on squares, which could bias the results. The light-demanding species P. × hispanica showed at streets a fast tree-height development, which may have been influenced by limited aboveground space. Trees at streets and pub- lic squares often showed smaller DBH and tree height, especially for R. pseudoacacia and A. hippo- castanum, while tree structures were more similar for T. cordata and P. × hispanica. In a similar study on the growth of urban trees in street canyons, squares, and parks, a likewise better tree growth of park trees compared to trees in streets and at squares was found (Kjelgren and Clark 1992). Moreover, highest shoot growth was found for street trees, possibly due to light obstructions, as has been found for P. × hispanica as well in our study. Neal and Whitlow (1997) also found in a study on urban tree growth of Quercus phellos in Washington, DC (USA) a higher growth of trees growing in semi-natural, park-like settings. In their conclusions, the authors stated that when urban trees experience proper design and installation, high growth rates can be achieved in unfavorable settings, like at streets or squares (Neal and Whitlow 1997). CONCLUSION The study described the structural development of 4 urban tree species in detail, providing a database for the space requirements of common tree species over time, as well as for further modeling of ecosystem services such as carbon storage, cooling, or shading. Differences found in structural development were mostly related to different species characteristics and growing conditions within the city, while the city itself and its climate seemed to have minor influence. With certain limitations, a transfer of the provided
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