Arboriculture & Urban Forestry 47(4): July 2021 chosen for data collection in the 6 cities. Tilia cordata is a native, very common urban tree in central Europe. It is shade-tolerant, can grow up to 30 m tall, has a medium to high leaf area index (LAI)(Rauner 1976), and a diffuse-porous wood anatomy (Larsen and Kristoffersen 2002; Radoglou et al. 2009). Robinia pseudoacacia, on the other hand, was introduced from the northern US; however, it is now a very common urban tree in central Europe. It is light-demanding, grows up to 30 m, has a low LAI, and has a ring-porous wood anatomy (Moser et al. 2016). Platanus × hispan- ica is a crossing of P. orientalis with P. occidentalis and is also common in central Europe. It is light- demanding, can reach heights of 35 m, and has a diffuse- porous wood anatomy as well as a low LAI. Aesculus hippocastanum can be found commonly in central Europe, has a high LAI, can grow up to 30 m, and is diffuse-porous (Roloff 2013). The sampled trees were selected to represent 3 types of open spaces with their different growth con- ditions: park trees, trees at public squares, or street trees. If possible, at least 30 individual trees were collected per species in each site category in each city. Park trees were growing in a green area without buildings, while street trees were trees planted in a street can- yon. Only trees growing between the sidewalk and the street were selected; other trees, such as those on front lawns, were excluded. Hereby, public square trees are defined as freestanding trees with open, detached crowns growing in smaller, mostly paved spaces freely accessible to the public. Additionally, crowns of public square trees did not reach over streets. Damaged, pruned, or low-forking trees were excluded from data collection. For each tree, the fol- lowing information was recorded: diameter at breast height (DBH), tree height (TH), crown base height (CS), crown length (CL), crown radii, tree pit, vital- ity, coordinates and altitude, and distance to adjacent buildings and trees (Figure 2). DBH of all trees was measured with a diameter measurement tape at a height of 1.3 m. Tree height and crown length, defined as the distance from the top of the crown to the beginning of the trunk and to the lowest primary branch, respectively, were measured using a Forestor Vertex IV (Haglöf, Langsle, Sweden). Crown radii and tree pit were measured in 8 intercar- dinal directions (N, NE, etc.) along the ground surface with a measuring tape from the center of the trunk to the tip of the most remote downward-projecting shoot 153 Figure 2. Illustration of measured and calculated tree struc- tural variables: diameter at breast height (DBH), tree height (TH), crown base height (CS), crown diameter (CD), crown length (CL), crown projection area (CPA), crown volume (CV), and tree pit. and to the last visible, open, non-asphalted surface of the soil. The distances to the neighboring trees and buildings were estimated in 8 intercardinal directions. From the measured tree data, further tree variables were calculated (Figure 2). With the following equa- tions, average crown radius (CR) and crown diameter (CD), crown volume (CV), crown projection area (CPA), and average tree pit were derived: with RN as the widest measured crown extension in the northern direction, RNE the widest crown exten- sion in the northeast direction, etc. Moreover, the unsealed area of the tree pit in each intercardinal direction was calculated from the maxi- mum influence area of the tree crown in each inter- cardinal direction: With influencing areaX = ([1.5 × CRX ]² × π)/8 and DX as the longest distance of open surface in each direc- tion (N, NE, etc.). ©2021 International Society of Arboriculture
July 2021
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