114 Gillner et al.: Leaf-Gas Exchange of Five Tree Species at Urban Tree Sites 2011a; Aasamaa and Sõber 2011b). This approach is also gaining increasing importance for urban trees (Kjelgren and Montague 1998; Chunxia et al. 2008; Fini et al. 2009; Leuzinger et al. 2010), in particular to answer questions of species-specific drought tolerance and to use this information for water management, for example (Bush et al. 2008). For planners and managers of urban woodlands and trees, knowledge of species stomatal conduc- tance and transpiration rates, as well as water-use efficiency (wue), may provide essential support for the selection of water-conserving species. Until now there are only few studies assessing gas exchange parameters of mature urban trees, since it is diffi- cult to quantify any physiological reaction of trees to a challenging environment like complex urban conditions. This may be attributed to the difficul- ties in comparing sites with diverging environmen- tal conditions, as well as how it is linked to specific growth rates, life spans, and management regimes. The objective of this research was to study five commonly distributed urban tree species concerning leaf-gas exchange at urban sites, as well as species sensitivity to vapor pressure deficit (VPD) and soil moisture of mature trees. In detail, researchers used seasonal measure- ments of stomatal conductance, transpiration, net photosynthesis, and the water-use efficiency measured during the vegetation periods of two consecutive years for species-specific sensitiv- ity ranking at highly urban street tree sites. MATERIALS AND METHODS Study Site and Species The study took place in the city of Dresden, Sax- ony, Germany, during the summer months in 2009 and 2010. Leaf-gas exchange was measured for the following five species: Acer platanoides L., Acer pseudoplatanus L., Platanus × hispanica Münchh., Quercus rubra L., and Tilia platyphyllos Scop. Table 1 indicates the number of measured trees per site, the DBH, the height, and the year planted. Acer species were planted in 1914 and all other species in 1920. All trees had been care- fully managed over the last decades and did not show any symptoms of damages, injuries, pest, and diseases on trunk and crown (Landeshauptstadt Dresden 2008). This means that only in case of ©2015 International Society of Arboriculture imminent danger for pedestrians, traffic, or other targets, damaged parts of the trees were removed. All sites are located in residential areas with a building stock of the 19th and early 20th century arranged in classical block development (Lande- shauptstadt Dresden 2008). Building heights were comparable, reaching maximum values of 15 m. The trees were planted in strips on the sidewalk along both sides of the street, surrounded by sections of impervious surface and paved areas. The soil types show sandy (loam) to clay-loam substrate with a minimum distance to the mean groundwater table of 3 m (Table 1). Soil pH-values ranged from 5.8 to 6.9. Since compaction is a major stress on urban trees (Blume 2000; Bühler et al. 2007), researchers used data and classification of the soil compaction used by Dresden city administration (less compacted, heavily compacted, and not compacted). For this assessment soil structure, bulk density and shear- ing resistance in 40 to 60 cm soil depth soil depth in the tree pit was used (Landeshauptstadt Dresden 2008). Except for trees of Acer pseudoplatanus and Tilia platyphyllos with less compacted soil, heav- ily compacted soil was observed in all other tree pits (Landeshauptstadt Dresden 2008) (Table 1). Modification of microclimate, runoff, infiltra- tion, soil compaction groundcover, and condi- tions of pavement strongly affect leaf-gas exchange (Mueller and Day 2005; Roberts et al. 2006). On this account, porosity index of the surface type, according to Blume (2000), was calculated, which describes the permeability of the surface type in crown projection area. The percentage of every surface type (asphalt, grass, gravel-sand mixture, big cobblestone, small cobblestone) in crown pro- jection area was obtained, and the surface-specific porosity index according to Blume (2000) was multiplied with the respective percentage in crown projection area to obtain the mean porosity of the surface under every crown. Mean porosity of the selected sites is comparable and ranges from 0.15 (Quercus rubra) to 0.50 (Tilia platyphyllos). Climatic Conditions The Elbe Valley within the city of Dresden (lati- tude 51°02'; longitude 13°49') is a metropolitan area located in eastern Germany. The climate is humid-continental, and the longtime averages of the meteorological station Dresden-Klotzsche in-
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