122 Gillner et al.: Leaf-Gas Exchange of Five Tree Species at Urban Tree Sites Platanus × hispanica indicate a more efficient use of water at urban sites remaining also high in the selected periods of a high VPD and low soil moisture. In dry years, forest trees of Quercus rubra expe- increasing VPD as indicated by the highest slope of regression—80.4 (Figure 3). Quercus rubra also showed higher sensitivity to environmental stress- ors at urban sites, such as drought and salt, com- pared with Platanus × hispanica, but it appears to be less sensitive to drought events than Tilia cor- data and Tilia × vulgaris (Swoczyna et al. 2010). CONCLUSION The findings demonstrate clear differences in stoma- tal conductance and leaf-gas exchange of five urban tree species. Under comparable soil and microcli- matic conditions and in accordance with literature, researchers were able to classify the five selected tree species with regard to their tolerance to the specific site conditions in urban streets as well as their re- sponse of gas exchange to VPD and soil moisture. Mean water-use efficiency of Platanus × his- panica and Quercus rubra is higher compared with Acer species and Tilia platyphyllos, and allows clas- sifying them as less likely to be restricted by envi- ronmental conditions at urban sites. The other tree species show lower water-use efficiencies under the prevailing urban conditions. The species-specific differences in leaf-gas exchange are also true in the selected periods of high VPD and low soil moisture. Tilia platyphyllos differed slightly due to drier soil conditions at their site. Nevertheless, the findings enable a classification of this species, too. It can be found between the group consisting of Acer platanoides and Acer pseudoplatanus, and the group of Platanus × hispanica and Quercus rubra. This ranking should not, however, mean that greenspace management must use species adapted to the harsh urban environments exclusively, since a high biodiversity of trees and shrubs is one important long-term strategy to minimize ©2015 International Society of Arboriculture rience minimal decline in net photosynthesis rate compared with wet years in Massachusetts, U.S. (Cavander-Bares and Bazzaz 2000), indicating a high tolerance to drought. The relatively stable transpiration rates under an increasing vapor pressure deficit support these findings for urban trees in Utah, U.S. (Bush et al. 2008). In the cur- rent study, Quercus rubra gs decreased rapidly with negative biotic and abiotic effects from climate change. Rather, these results can inform consider- ation of suitable urban sites that correspond best with species habitat and climate/soil preferences. Thus, species reacting, sensitive under the pre- vailing urban conditions, in this case both Acer species, might be used for shady, north-exposed streets with balanced soil moisture regimes. Acknowledgments. The authors would like to thank Mr. Alexander Solger and Mr. Erik Fritzsche for their field assistance in sample collection, thank Dr. Juliane Vogt for kindly correcting the paper. We extend our thanks to the staff members of offices for Municipal Affairs (Amt für Stadtgrün und Abfallwirtschaſt Dresden), and in particular Mr. Steffen Löbel, for their administrative support, help- ful information, and logistic maintenance during the fieldwork. We also want to extend our acknowledgments to the three anonymous reviewers of our manuscript for their useful and helpful comments. This study was realized with the financial support by the Bun- desministerium für Bildung und Forschung (BMBF) in the project REGKLAM (Grant number: 01 LR 0802). LITERATURE CITED Aasamaa, K., and A. Sõber. 2011a. Stomatal sensitivities to changes in leaf water potential, air humidity, CO2 concentration and light intensity, and the effect of abscisic acid on the sensitivi- ties in six temperate deciduous tree species. Environmental and Experimental Botany 71:72–78. Aasamaa, K., and A. Sõber. 2011b. Responses of stomatal conduc- tance to simultaneous changes in two environmental factors. Tree Physiology 31:855–864. Arnfield, A.J. 2003. Two decades of urban climate research: A review of turbulence, exchanges of energy and water, and the urban heat island. International Journal of Climatology 23:1–26. Bartens, J., S. Day, J. Harris, T. Wynn, and J. Dove. 2009. Transpi- ration and root development of urban trees in structural soil stormwater reservoirs. Environmental Management 44:646–657. Bernhofer, C., J. Matschullat, and A. Bobeth. 2009. Das Klima in der REGKLAM-Modelregion Dresden. Regklam Publikationsreihe Heſt 1, Rhombos, Berlin, Germany. 117 pp. Bhaduri, B., M. Minner, S. Tatalovich, and J. Harbor. 2001. Long-term hydrologic impact of urbanization: A tale of two models. Journal of Water Resources Planning and Management 127:13–19. Blume, H.-P. 2000. Böden städtisch-industrieller Verdichtung- sräume. pp. 154–171. In: H-P. Blume, P. Felix-Henningsen, W.R. Fischer, H.G. Frede, R. Horn, and K. Stahr (Eds.). Handbuch der Bodenkunde. Ecomed, Landsberg, Germany. Bühler, O., P. Kristoffersen, and S.U. Larsen. 2007. Growth of street trees in Copenhagen with emphasis on the effect of different establishment concepts. Arboriculture & Urban Forestry 33:330–337. Bush, S.E., D.E. Pataki, K.R. Hultine, A.G. West, J.S. Sperry, and J.R. Ehleringer. 2008. Wood anatomy constrains stomatal responses to atmospheric vapor pressure deficit in irrigated, urban trees. Oecologia 156:13–20. Cavander-Bares, J., and F.A. Bazzaz. 2000. Changes in drought re- sponse strategies with ontogeny in Quercus rubra: Implications for scaling from seedlings to mature trees. Oecologia 124:8–18.
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