116 Gillner et al.: Leaf-Gas Exchange of Five Tree Species at Urban Tree Sites sured to compare soil water conditions at the sites dur- ing physiological measurements. Two access tubes were installed vertically into the soil at every tree pit to measure Θ with PR2 soil moisture profile probes at six depths 10, 20, 30, 40, 60, and 100 cm (Delta-T Devices Ltd, United Kingdom). For Ψsoil tensiome- ters (T1S; UP GmbH, Germany) with shaſts lengths of 20, 40, 60, 80 cm were installed in the ground. Gas Exchange Gas exchange rates were measured between 8:00 am and 6:00 pm (CEST, Central European Summer Time) at fixed temperature of 25°C with a relative hu- midity within the chamber of about 40%–75%, and at constant photosynthetic photon flux density (PPFD) of 1600 μmol m-2 s-1 . Researchers investigated fully ex- To assess efficiency of water use, the wue was cal- culated from the ratio of Amax panded, healthy leaves on exposed peripheral twigs in full sun canopy sections in the top one-third of the crowns using a liſt. Leaves with signs of fungi and insect pests as well as senescent leaves were excluded from examination. Several leaves of one species were measured during a measuring day. For every species, several replicates were conducted, with a minimum of four. The first replicate was done aſter all five species had been measured once; the second aſter all species had been measured two times, and so on. Measure- ments were conducted during June, July, and August in the years 2009 and 2010 using the portable infrared gas analyzer HCM-1000 (Heinz Walz GmbH, Germa- ny). Before each measurement started, a porometer was calibrated in accordance with the user guide- lines (Heinz Walz GmbH, Germany). Under these conditions, net photosynthesis was defined as the maxima or light-saturated net photosynthesis (Amax ). and E (Larcher 2001). Statistics Statistical analysis was performed with the R soſt- ware (R Development Core Team 2008). Data sets were tested for normal distribution using the Sha- piro-Wilk-Test (P ≤ 0.05) (Shapiro and Wilk 1965). Since data showed no normal distribution, the non- parametric method of Kruskal–Wallis was used to test if distribution of data sets differs. In case of sig- ©2015 International Society of Arboriculture Volumetric Soil Water Content and Soil Water Potential For the year 2010 volumetric soil water content (Θ, Vol.-%) and soil water potential (Ψsoil , hPa) were mea- nificant differences Pairwise Mann–Whitney U rank sum test with a Bonferroni correction was applied to localize the differences. Due to the non-normal distri- bution of datasets, box-and-whisker plots were used to describe gas-exchange data and enabled a clear comparison between species and dates. Plots were created with Xact (Version 8.03, SciLab, Germany). RESULTS Leaf-Gas Exchange Figure 2 shows the distribution of transpiration and stomatal conductance in the years 2009 and 2010. Species with high rates of stomatal con- ductance also have high rates of transpiration. In both years, maxima rates for transpiration up to 2.3 mmol m-2 s-1 for Platanus × hispanica and minima values close to zero for Acer pseudoplata- nus were measured. Comparing the medians, Acer platanoides had only 33%, Acer pseudoplatanus 47%, and Tilia platyphyllos 53% of the transpira- tion rate of Platanus × hispanica in the year 2010. These differences were significant at P < 0.05. In 2010, the medians of transpiration as well as stomatal conductance are higher than in 2009 for all species. However, medians of transpiration rates of Acer pseudoplatanus, Platanus × hispanica, Quercus rubra, and Tilia platyphyllos ranging from 0.80 to 1.25 mmol m-2 s-1 don’t show any significant differ- ences among the species in 2010. Transpiration rate of Acer platanoides with only 37% of Platanus × his- panica differed significantly from all other species. Corresponding to the transpiration rates and sto- matal conductance, researchers measured the highest medians of net-photosynthesis for Platanus × hispanica with 4.7 μmol m-2 4.5 μmol m-2 s-1 s-1 with maxima of more than 8 μmol m-2 and for Quercus rubra with s-1 (Figure 2). In contrast to these species showing high rates of net photosynthesis, Acer platanoides has only 21%, Acer pseudoplatanus 37%, and Tilia platyphyllos 64% of net photosynthesis of Platanus × hispanica in 2010. In the year 2009, these differences are even more pronounced with rates of net photosynthesis of only 14% for Acer platanoides, 23% for Acer pseu- doplatanus, and 25% for Tilia platyphyllos compared with Platanus × hispanica. Significant differences of net photosynthesis at P < 0.05 therefore can be found between the group of Platanus × hispanica and Quercus rubra and the other trees species (Figure 2).
May 2015
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