Arboriculture & Urban Forestry 47(4): July 2021 whereas southern European cities appear to be better adapted (Ward et al. 2016). All this influences tree growth and vitality, leading to a higher mortality and reduced maximum age of urban trees compared to trees in rural surroundings or forest stands (Roman and Scatena 2011). For several species, the pool of planted trees in cities is mostly younger than 50 years (Vaz Monteiro et al. 2017). As Sanders et al. (2013) stated, expectations on tree per- formance over time is often only expressed as estab- lishment success. However, good vitality and high growth rates for older trees also are important criteria for the urban tree stock of a city. Besides these performance criteria regarding vitality and growth, climate mitigation effects of trees are often named as important features of urban trees, especially in view of the UHI effect and climate change (Millen- nium Ecosystem Assessment 2005). These benefits for humans provided by urban trees are called ecosystem services. Ecosystem services that are especially import- ant for human thermal comfort in cities and city climate regulation include carbon storage and sequestration (Davies et al. 2011; Liu and Li 2012), cooling by shading and evapotranspiration (Konarska et al. 2016; Rahman et al. 2020b), regulation of water flow (Vil- larreal and Bengtsson 2005), air pollutant filtration (Sæbø et al. 2012), noise buffering (Roy et al. 2012), as well as promotion of human recreation and health (Gómez-Baggethun and Barton 2013). In contrast, sev- eral disservices of urban trees exist as well, e.g., aller- genicity to pollen, litter fall, and damage to streets and pavement (Escobedo et al. 2011; Moser et al. 2018). Several studies have analyzed the ecosystem service provisions of urban trees. Rahman et al. (2020a) for example studied the cooling effects of Robinia pseu- doacacia and Tilia cordata, two common urban tree species in Germany, and found clear species differ- ences. Nowak et al. (2013) assessed citywide carbon sequestration of urban trees in US cities, and Berland et al. (2017) dealt with the water regulation capacity of urban trees. Moreover, simulation studies based on growth models such as i-Tree (2019) or CityTree (Rötzer et al. 2019) give insight into the ecosystem ser- vice provisions of different tree species of various ages. However, the magnitude of provided ecosystem services by urban trees depends on various aspects. Tree species, age, and structures like crown height need to be considered when modeling services of sin- gle trees, tree stands, or on a citywide scale (Xiao et al. 151 2000; Bayer et al. 2018; Rötzer et al. 2019; Rahman et al. 2020b). Therefore, allometric studies on the struc- tural development of urban tree species over time, as published by McPherson et al. (2016) for several US cities, Stoffberg et al. (2008) for South African tree species, Semenzato et al. (2011) for urban trees in Italy, or Dahlhausen et al. (2016) for several tree spe- cies worldwide, provide the basis for accurate eco- system service modeling. Regarding the vast numbers of available urban tree species and different climate conditions worldwide, great knowledge gaps for many species, growing sites within cities, and climate regions are obvious. As McHale et al. (2009) state, growth equations of urban trees of a certain city and climate region cannot simply be transferred to other growth conditions. More region-, site-, and species-specific research on tree development over time is necessary to provide an adequate basis for ecosystem service modeling. In summary, the lack of regional and site-specific data on urban tree development is a major restriction in applying accurate modeling approaches to quantify urban tree growth and service provision. Here we present allometric relationships for 4 common urban tree species in central Europe (small-leaved lime [Tilia cordata], horse chestnut [Aesculus hippocasta- num], black locust [Robinia pseudoacacia], and London plane tree [Platanus × hispanica]), which differ in their ecological and morphological character. Differences in tree growth between species, cities, and growing sites within the city were analyzed. With this study, we try to close existing knowledge gaps in the growth relationships of urban trees for a South German city, making growth predictions for future space planning possible and providing a basis for ecosystem service calculation of urban tree species. In detail, we pose the following research questions: • What are the growth characteristics of T. cordata, R. pseudoacacia, A. hippocastanum, and P. × hispanica in South German cities? • What are the specific allometric relationships and differences of these species? • Are there differences in the growth relationships between the analyzed tree species, cities, and growing site categories (street, square, park) within the city? • How do above- and belowground obstacles, such as planting pit size, close-growing trees, or pres- ent buildings, influence tree growth? ©2021 International Society of Arboriculture
July 2021
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