uptake rates, Acer platanoides has only 25% and Acer pseudoplatanus has only 52% of the rates of Platanus × hispanica. In the selected periods of low soil mois- ture condition, species nevertheless showed very similar values of wue between 2.5 to 2.9 µmol mmol-1 without significant differences among the species. 120 as high as rate of Acer platanoides. Considering CO2 Gillner et al.: Leaf-Gas Exchange of Five Tree Species at Urban Tree Sites - studied sites did not differ in building heights and building structure, the relative low leaf-gas exchange rates can be regarded as species-specific response to the environmental conditions at the urban sites (Kjelgren and Montague 1998; Cregg and Dix 2001; Montague and Kjelgren 2004; Bartens et al. 2009). Forrai et al. (2012) reported just as low values of photosynthesis rates (3.68–13.03 μmol m-2 s-1 s-1 ) and correspondingly limited transpiration rates (0.85–2.65 mmol m-2 ) for ten different urban Figure 5. Seasonal scale of soil water content (Θ) and soil water potential (Ψsoil 21 measurements). Indicated are the means of the measure- ments of the site. Vertical light gray bars indicate periods with VPD > 1.0 kPa, and dark gray bars periods of lowest soil moisture conditions. DISCUSSION Site and Environmental Conditions During Measurements Homogeneous microclimatic, edaphic and growth environments are prerequisites for comparing species-specific physiological traits (Larcher 2001; Ferrini and Baietto 2006; Fini et al. 2009; Stokes et al. 2010; Aasamaa and Sõber 2011a;). The tree pits showed comparable degree of impervious surface and pH values, indicating a slightly acidic to neu- tral soil (Table 1). Except the site for Acer platanoi- des and Tilia platyphyllos, which had less compacted soil, all other sites showed heavily compacted soil condition. Despite of the more favorable conditions for root development and water uptake (Roberts et al. 2006), both species did not take advantage of the lower soil compaction and even showed lower leaf- gas exchange rates compared with Platanus × his- panica and Quercus rubra. Soil texture and volumet- ric soil water contents show similar values except for the site for Tilia platyphyllos, which had sandier and drier soil conditions (Table 1; Figure 4). Since the ©2015 International Society of Arboriculture ) from January 6 to August 31, 2010 (n = tree species, like Acer pseudoplatanus, Acer plat- anoides, Tilia cordata, and others. However, the low level of leaf-gas exchange rates may be a result of the high value of PPFD resulting in photoinhi- bition (Pollastrini et al. 2013), and reducing the capacity to convert solar energy to electron trans- port (Takahashi and Murata 2008). The variable light regime in mixed forests leads to a wide range of light adaptation, and low values of PPFD may already induce photoinhibition (Pollastrini et al. 2013). At the urban tree sites, exposure to light is very high and crowns are more sun-exposed com- pared to forest trees. As a consequence, a higher amount of sun-adapted leaves form (Lichtenthaler et al. 1981). Sun-adapted leaves are characterized by higher leaf thickness, leaf area, stomatal density, and differ in chlorophyll a and b content leading to a higher light saturation, and light compensa- tion point compared to shade leaves and show maximum rates of net photosynthesis at a high level of PPFD (Lichtenthaler et al. 1981). How- ever, in further studies the species-specific level of photoinhibition should be taken into account. Seasonal changes with higher leaf-gas exchange rates in spring and declining rates through sum- mer drought can be observed in regions with a typical drought period during summer—e.g., in regions with Mediterranean climate, like Califor- nia, U.S. (Goulden 1996). In the current study, leaf-gas exchange rates showed only slightly changing values over the growing season (data not shown). That may be due to balanced climatic conditions during the summer months of the years 2009 and 2010 without long-lasting periods of a high levels of VPD or low levels of soil moisture. Although a decline in soil moisture conditions can be observed over the measuring period for all sites in 2010 (c.f. Figure 5), the values of Θ in 100 cm soil depth never dropped down to less than
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