Arboriculture & Urban Forestry 47(4): July 2021 were measured in 6 cities. To generate a broad picture of the structural development of urban trees, from planting to old age, our approach includes the mea- surement of trees of a broad range of DBH classes. With trees ranging in DBH from 10 cm up to 150 cm, we provide a large database for further studies, such as on the ecosystem services of urban trees, as has been illustrated by Rötzer et al. (2019). The allometric relationships show, with some exceptions, high correlations between DBH and all other structural parameters (R² > 0.7). Only the rela- tionships of R. pseudoacacia for DBH with tree height and crown length are weaker, possibly due to the light-demanding character of this species, which leads to different tree structures depending on site conditions (Moser et al. 2015). All in all, the strong correlations demonstrate that the tree structures can be predicted rather well by DBH. Recently, many studies on the allometric relationships of urban trees have been published (e.g., Stoffberg et al. 2008; Semenzato et al. 2011; Rijal et al. 2012; Sanders and Grabosky 2014). They provide information on the structural development of different urban tree species over time. However, the vast number of urban tree species makes it hard to compare studies, since these studies also have been conducted in other climate regions (e.g., Acadian Region in North America, Cal- ifornia in USA, Italy, and South Africa). An earlier study in Germany (Eastern, Western, and Central regions) also analyzed tree-height development over age and DBH for A. hippocastanum, T. cordata, and P. × hispanica (Rust 2014). In that study, a more sim- ilar tree height growth was found for all species, while in our study, a faster growth of P. × hispanica compared to the other studied species was obvious. Also, differences between the cities were marked, which was explained by the different influence of wind in the cities, leading to a varying slenderness of the crown (Rust 2014). On the other hand, another study on A. hippocastanum in Munich and T. cordata in Berlin, Germany, as well as P. × hispanica in Paris, France, found the lowest tree-height development for A. hippocastanum and fastest tree height growth for P. × hispanica (Dahlhausen et al. 2016), similar to our study. Likewise, the results showed the widest crown diameter development for P. × hispanica, with T. cor- data being intermediate, and A. hippocastanum the smallest growing species (Dahlhausen et al. 2016). As McHale et al. (2009) stated, due to the various growing conditions, the great number of urban tree 161 species, and the different climate regions, the transfer of allometric studies to other cities may not be possi- ble. Pretzsch et al. (2015) assigned tree species to allometric types in their study, which can be used as a first approach to generalize allometric studies for similar climate and growing conditions (Dahlhausen et al. 2016). These types were based on the growing size of the tree species and the growth speed (devel- opment of tree height over DBH) and are, for instance, medium-size trees with medium growth, like Fraxi- nus excelsior, or large-size trees with medium growth. The 4 species of this study, T. cordata, P. × hispanica, R. pseudoacacia, and A. hippocastanum, are catego- rized as large-size trees with medium growth (Pretzsch et al. 2015). The allometric equations in our study were derived from a large data set including a great range of growth conditions, making them robust enough for a transfer to similar urban situations at least within southern Germany, but likely also more widely for urban areas where climatic conditions and geology are not strongly different. In particular, if tree inventories or cadastres are missing, such a transfer of allometric relationships to similar species types or functional groups can generate important information on tree structural development and ecosystem service provision. However, the environmental conditions of urban ecosystems (e.g., climate, air pollution) or even the small-scale conditions of the site (e.g., soil char- acteristics, soil sealing, light reduction) can change tree growth, carbon allocation, and ecosystem service provision significantly. For example, substantial soil sealing and/or a low soil water-storage capacity might induce a stronger root growth and therefore change the root-shoot allometry (Rötzer et al. 2009). Such a behavior is of course species dependent and may alter the growth strategy of urban trees along with other species-specific physiological characteristics, such as the stomatal reaction to drought (aniso- or isohydric behavior; see e.g., Roman et al. 2015; Rötzer et al. 2017). This assumption on the transferability of allome- tric relationships is also supported by the small differ- ences in the growth patterns between the analyzed cities. Up to an age of 100 (with exception of the height development of R. pseudoacacia), trees showed a very uniform growth, regardless of the city. Differences found for the tree-height development in particular for old R. pseudoacacia and P. × hispanica can be explained by their high light demand, which leads to faster shoot development. These small differences are ©2021 International Society of Arboriculture
July 2021
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