Arboriculture & Urban Forestry 35(5): September 2009 CONCLUSION Norway maple establishment was promoted by irrigation for two years after transplant. After this period, irrigation is probably not needed anymore. Linden establishment was promoted by irriga- tion also in the third growing season after transplant, while more research is needed to determine for how long Tilia should be ir- rigated after planting in the landscape. Only in T. platyphyllos and Acer ‘Summershade’ did water shortage cause a significant increase in F0 and decline in the primary yield of PSII. In T. platy- phyllos, Fv/Fm was significantly decreased by drought. These ef- fects on chlorophyll fluorescence parameters have been related to the occurrence of water or other environmental and chemical stresses. On this basis, these two species were considered the least tolerant to water stress during the establishment phase. Different responses to water shortage were found in the different species and cultivars. T. platyphyllos had lower gas exchange than the other species, even in well-watered conditions and stomatal con- ductance, carbon assimilation and transpiration were unaffected by water shortage. This strategy was also found in beech seed- lings from dry (rainfall 580–680 mm/year) provenances (Pueke et al. 2002), but in the case of linden, it failed to prevent disruption of PSII under nonirrigated conditions. T. cordata, T. × europaea, Acer ‘Deborah’ and ‘Emerald Queen’ showed, in 2007, higher gas exchange which was little affected by water shortage. This “water spending” strategy may depend on a better access to soil water thanks to a more developed root system (Levitt 1972; Mediavilla and Escudero 2004). This is also confirmed by the fact that before full establishment (2006, two years from transplant) effects of water shortage on leaf gas exchange were observed—especially in T. × europaea and Acer platanoides ‘Deborah’. T. tomentosa— showed high gas exchange, but, in response to water shortage, stomatal conductance and transpiration were reduced. A similar strategy was found on Quercus coccinea and classified as “drought avoidance based on water saving” by Sakcali and Ozturk (2004). The results obtained in this experiment may be a useful tool for species/cultivar selection for urban forestry. Among lindens, T. tomentosa seems particularly adapted to those situations where root development and root access to water is limited by small planting pits (e.g., street trees). T. cordata, which showed the highest gas exchange and water use efficiency in the year fol- lowing transplant, seems well suited to perform well where ac- cess to underground water is not restricted. Differences among Norway maple cultivars seem smaller than those observed among lindens (no significant differences in Fv/Fm). Regardless, in the establishment phase, A. platanoides ‘Deborah’ and ‘Emer- ald Queen’ seem better performers than Acer ‘Summershade’. Acknowledgments. This work has been undertaken as a research project with the title “Tecniche eco-compatibili di gestione del vivaismo e del verde ornamentale” financed by Regione Lombardia - Agricultural De- partment, according to the Plan of Research and Development 2004. LITERATURE CITED Abrams, M.D., and M.E. Kubiske. 1990. Photosynthesis and water rela- tions during drought in Acer rubrum L. genotypes from contrasting sites in central Pennsylvania. Functional Ecology 4:727–733. Akbari, H. 2002. Shade trees reduce building energy use and CO2 sions from power plants. Environmental Pollution 116:S119–S126. emis- 249 Angelopulos, K., B. Dichio, and C. Xiloyannis. 1996. Inhibition of pho- tosynthesis in olive trees (Olea europaea L.) during water stress and rewatering. Journal of Experimental Botany 47(301):1093–1100. Armond, P.A., O. 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