Arboriculture & Urban Forestry 32(1): January 2006 3 GROWTH AND PHENOLOGY OF ESTABLISHED TILIA CORDATA STREET TREES IN RESPONSE TO DIFFERENT IRRIGATION REGIMES Oliver Bühler, Christian Nørgård Nielsen, and Palle Kristoffersen Abstract. The effect of different irrigation regimes on growth and phenology of 100 established Tilia cordata street trees was investigated during the growth period of 2004. Relative stem increment increased significantly when irrigating with 280, 320, or 640 L (74, 84.5, and 169 gal) of water throughout the growth period. The length of the growth period was affected by irrigation regimes in regard to termination of growth, as the trees receiving 640 L (169 gal) had a growth period significantly longer than the control trees. These results indicate that growth and growth period of street trees are augmented by an increase in the amount of water available to them. In addition, block effects concerning the start of the growth period were observed, as trees situated on the northern side of the street initiated stem growth 7 days earlier than the trees on the southern side. Key Words. Irrigation; phenology; stem growth; street trees; Tilia cordata; tree management; urban climate; water balance. Inadequate supply of water is considered to be a critical factor concerning establishment and vitality of street trees (Hampel 1893; Gilbertson and Bradshaw 1985; Clark and Kjelgren 1990; Balder et al. 1997; Sæbø et al. 2003). Water deficiencies on urban sites may occur because of lack of precipitation, a limited soil water reservoir due to restricted infiltration of precipitation, limited soil volumes, or inappro- priate soil characteristics—in particular, due to soil compac- tion (Impens 1999). At the same time, temperature in urban areas is generally higher and relative humidity is lower, increasing the atmospheric demand (Whitlow et al. 1992; Kjelgren and Clark 1993). These specific urban site conditions may, separately or in combination, result in water deficiencies eventually expressed as a depression of growth and vitality (Montague et al. 2000) or even increased mortality, particularly in the case of recently planted street trees (Gilbertson and Bradshaw 1990). So far, irrigation is generally restricted to the first years of newly planted trees. Previous studies report somewhat inconsistent results of additional irrigation. Whereas some provide evidence of growth responses to an additional supply of water (Bellet-Travers and Ireland 1999), others find no significant growth effects (Whitlow et al. 1992; Costello et al. 2005). Many irrigation studies, however, were carried out in field plots and may therefore not necessarily reflect the special situation of a street tree. In a European survey, Pauleit et al. (2002) report that drought is recog- nized as a problem for establishment and maintenance of urban trees but found that it is difficult to quantify the extent of the problem. The present study investigated the irrigation responses of a widely used street tree species (Tilia cordata [littleleaf linden]) and was carried out on an existing street plantation consisting of 100 trees. Its main purpose was to assess whether irrigation of well- established street trees yields additional, measurable growth responses and, thus, whether lack of water is a growth reducing factor. Furthermore, effects of irrigation on phenological parameters were investigated. MATERIALS AND METHODS Test Plants and Test Sites The experiments were conducted in 2004 on a plantation of 100 Tilia cordata ‘Greenspire’ and T. cordata ‘Erecta’ situated on both sides of Frederikssundsvej in Copenhagen, Denmark. Frederikssundsvej is an arterial road leading from east to west, surrounded by a variety of building structures. The trees were planted in 1996–97 and are today of uniform appearance; average stem circumference at a height of 1 m (3.3 ft) is 358 mm (14.1 in) (S.D. ± 48 mm [1.89 in]). The planting pits comprise a surface area of 6.4 m2 (68.9 ft2 ) and are surrounded by elevated curbstones founded in a concrete base reaching to a depth of 50 cm (1.6 ft). The surface of the pit is covered with a layer of coarse bark mulch. To a depth of 60 cm (2 ft), the pit is filled with an artificial tree planting substrate of sandy texture with little silt and no clay. Humus content is low, too, except for the very top layer where bark mulch is decomposed. Beyond a depth of 60 cm (2 ft), the soil consists of loamy clay. Excavations revealed that main root growth is restricted to the upper 60 cm (2 ft), with extensive fine root growth just below the bark mulch layer. Very few roots have been observed to grow under the curbstones into adjacent areas, and virtually no root growth was observed deeper than 60 cm (2 ft). No technical applications for improved drainage or aeration had been installed on the test site. An additional 20 Tilia spp. park trees situated in a nearby park were incorporated in the test as nontreated reference trees. The park trees were approximately the same age; ©2006 International Society of Arboriculture
January 2006
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