Arboriculture & Urban Forestry 32(1): January 2006 7 13.3 L (3.5 gal) and 26.6 L (7 gal) weekly. However, because the cited studies were conducted on field plots, the same species may have reacted differently if situated under street tree conditions. Presumably, a year with less precipitation would have widened the gap between irrigated and non irrigated trees further, as well as widening the gap between the park trees and the street trees in general. As it was, the nonirrigated control street trees grew less than some of the irrigated trees and the park trees, but a relative stem area increment of 10.88%, corresponding to an annual ring of 3 mm (0.12 in), appears to be quite acceptable growth. Irrigation, however, should not be considered as the only way of providing additional amounts of water to street trees. In 2004, for instance, an increase of the surface of the planting pit by 2 m2 (21.5 ft2 ) would have allowed approxi- mately 640 L (169 gal) of additional precipitation to enter the planting pit, corresponding to the amount presented by the highest irrigation treatment. For further discussion of management options, see, for example, Nielsen et al. (2005). In addition to limiting stem increment, water deficiencies also limited the length of the growth period, and between those two parameters a certain correlation has been proven. The reported prolongation of growth periods in urban areas (Zhang et al. 2004) due to a warmer climate can, at least in regard to street trees, not be confirmed because lack of water had the opposite effect. Interestingly, growth initiation of street trees situated less than 10 m (33 ft) apart has been shown to vary signifi- cantly. This phenomenon is probably explained by the influences of temperature on springtime phenology. The southern side of the test street was subjected to more pronounced shading by buildings, while the northern side received more solar radiation, resulting in higher tempera- tures of soil and plant tissue. This again resulted in earlier initiation of growth, as shown by analysis of growth models, but, in contrast to the differences regarding cessation of growth, the differences in growth initiation were neverthe- less perceivable by visual inspection. The reported earlier growth initiation in urban areas compared to rural areas reported by Kramer (1995), White et al. (2002), and Roetzer et al. (2000), for example, thus proves to be subjected to considerable variations also within urban perimeters. CONCLUSION Although the test was carried out in a year with above- average precipitation during the growth period, irrigation yielded significant results in the form of an increase of relative stem increment and a longer period of stem growth. Those two parameters furthermore were shown to be interdependent to a certain degree. This finding indicates clearly that growth of the tested street trees is suboptimal due to lack of water and can be optimized by management approaches—be it by means of irrigation or other arrange- ments that improve the water supply during the growth period. Microclimatic conditions turned out to delay growth initiation for trees situated on the shady side of the street, emphasizing the very heterogeneous site conditions of street trees of otherwise very similar appearance. The heterogenic site conditions are further illustrated by the pronounced spread of the collected data. LITERATURE CITED Balder, H., K. Ehlebracht, and E. Mahler. 1997. Strassenbäume: Planen, Pflanzen, Pflegen am Beispiel Berlin. Patzer Verlag, Berlin, Germany. 240 pp. Bellet-Travers, D.M., and C.R. Ireland. 1999. A comparison of the growth responses and physiology of two amenity tree species subjected to water Stress. Acta Horticulturae 496:339–345. Clark, J.R., and R. Kjelgren.1990. Water as a limiting factor in the development of urban trees. Journal of Arboriculture 16:203–208. Costello, L.R., K.S. Jones, and D.D. McCreary. 2005. Irrigation effects on the growth of newly planted oaks (Quercus spp.). Journal of Arboriculture 31:83–87. Danish Meteorological Institute (DMI). 2004. Weekly reports on climate in the capital area. Copenhagen, Denmark. Devitt, D.A., R.L. Morris, and D.S. Neumann. 1994. Evapotranspiration and growth response of three woody ornamental species placed under varying irrigation regimes. Journal of the American Society of Horticultural Science 119:452–457. Gilbertson, P., and A.D. Bradshaw. 1985. Tree survival in cities: The extent and nature of the problem. Arboricultural Journal 9:131–142 . ———. 1990. The survival of newly planted trees in inner cities. Arboricultural Journal 14:287–309. Hampel, C. 1893. Stadtbäume—Anleitung zum Pflanzen und Pflegen der Bäume in Städten, Vororten und auf Landstraßen. Verlag von Paul Parey, Berlin, Germany. 73 pp. Impens, R.A. 1999. Life conditions and stress for urban trees: Example of Brussels city. Acta Horticulturae 496:253–259. Kjelgren, R.K., and J.R. Clark. 1993. Growth and water relations of Liquidambar styraciflua L. in an urban park and plaza. Trees 7:195–201. Kramer, K. 1995. Phenotypic plasticity of the phenology of seven European tree species in relation to climatic warming. Plant, Cell, and Environment 18:93–104. Montague, T., R. Kjelgren, and L. Rupp. 2000. Surface energy balance affects gas exchange and growth of two irrigated landscape tree species in an arid climate. . Journal of the American Society of Horticultural Science 125:299–309. ©2006 International Society of Arboriculture
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