©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(6): November 2023 307 samples to support assessments of growth variability and climate signals over the past 50 to 100 years, instead of only the past 1 to 2 decades covered by ≤ 3 cm microcores. Additional information about locally vary- ing groundwater access and root barriers would be beneficial, though we acknowledge that such data are difficult to generate in urban environments. It appears instructive to replicate this study in another, perhaps Mediterranean, climate zone, and to differentiate between Platanus orientalis, Platanus occidentalis, and their hybrid, Platanus × acerifolia, using genetic analyses. Projects considering these approaches will demonstrate whether the conclusion established here on the dominance of climate forcing over site differ- ences on plane growth and bark exfoliation will remain. LITERATURE CITED Adamska I. 2019. Epidemic of Erysiphe platani in urban areas on the example of the Szczecin (NW Poland). Ecological Questions. 30(4):71-81. https://doi.org/10.12775/EQ.2019.029 Bowden JD, Bauerle WL. 2008. Measuring and modeling the variation in species-specific transpiration in temperate decid- uous hardwoods. Tree Physiology. 28(11):1675-1683. https:// doi.org/10.1093/treephys/28.11.1675 Browicz K. 1964. On the geographical distribution of Platanus orientalis L. in Bulgaria. Arboretum Kórnickie. 9:37-58. https://rcin.org.pl/Content/141966/KOR001_145864.pdf Büntgen U, Franke J, Frank D, Wilson R, González-Rouco F, Esper J. 2010. Assessing the spatial signature of European climate reconstructions. Climate Research. 41(2):125-130. https://doi.org/10.3354/cr00848 Büntgen U, Krusic PJ, Verstege A, Sangüesa-Barreda G, Wagner S, Camarero JJ, Ljungqvist FC, Zorita E, Oppenheimer C, Konter O, Tegel W, Gartner H, Cherubini P, Reinig F, Esper J. 2017. New tree-ring evidence from the Pyrenees reveals Western Mediterranean climate variability since medieval times. Journal of Climate. 30(14):5295-5318. https://doi.org/ 10.1175/JCLI-D-16-0526.1 Cedro A, Nowak G. 2006. Effects of climatic conditions on annual tree ring growth of the Platanus × hispanica “Acerifolia” under urban conditions of Szczecin. Dendrobiology. 55:11-17. https:// www.idpan.poznan.pl/images/stories/dendrobiology/vol55/55 _11_17.pdf Cheung HN, Keenlyside N, Koenigk T, Yang S, Tian T, Xu Z, Gao Y, Ogawa F, Omrani NE, Qiao S, Zhou W. 2022. Assessing the influence of sea surface temperature and arctic sea ice cover on the uncertainty in the boreal winter future climate projections. Climate Dynamics. 59:433-454. https:// doi.org/10.1007/s00382-022-06136-0 Deslauriers A, Rossi S, Anfodillo T. 2007. Dendrometer and intra-annual tree growth: What kind of information can be inferred? Dendrochronologia. 25(2):113-124. https://doi.org/ 10.1016/j.dendro.2007.05.003 that rainfall and drought conditions, even during the currently dry-hot springs and summers, had no mea- surable impact on plane growth in Mainz. Instead, the urban trees appear to benefit from warmer cold-season temperatures, which are projected to increase in upcoming decades (Cheung et al. 2022). Whether the decoupling from growing season drought conditions might additionally be forced by access to (likely highly variable) groundwater levels across the city remains unclear, as the plane root systems are not accessible and spatially resolved water table and root barrier data unavailable (Gilman 2006). While the climate-growth comparisons support hypothesis 3 on the suitability of tree-ring data to assess potential drought and temperature signals, the expected forcing differences between SR1 through SR3 and SR4 (parks and playgrounds), based on varying chronology and Rbar results (Figures 6 and 7), are not validated by the calibration statistics that appear highly uniform among the sites (Figure 8b). This finding, together with the spatially invariable exfoliation data, led us to conclude that a common climate forcing synchronizes growth variability across the urban structures of Mainz, and that the highly variable site characteristics within the city have no detectable impact on both plane growth and bark exfoliation. CONCLUSIONS An unprecedented network of monitoring and TRW data from 349 plane trees has been established to evaluate spatial patterns of drought-induced bark exfoliation across the urban structures of a central European city. Unlike highly variable site conditions including changes in impervious cover, vegetation cover, and building heights, no systematic differences in plane bark exfoliation are detected. This conclu- sion is in line with supplementary TRW analyses revealing largely coherent growth variations among urban plane sites forced by cold season temperatures. The significant and spatially invariable temperature signal as well as the consistent exfoliation rates sup- port the conclusion that climate rather than local site conditions control London plane stem growth and drought-induced bark peeling, limiting potential efforts to practically influence these variables in urban environments. The conclusions from this pioneering study could be substantiated by including classic 40+ cm core
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