Arboriculture & Urban Forestry 39(4): July 2013 West canopy spread was particularly important, as this would have the greatest effect upon the width of the shade during the sample period when the sun was south of the sample trees. Total tree height was measured using a Suunto Clinometer and total canopy height and area were calculated from these mea- surements. LAI was recorded using an AccuPAR model LP-80 PAR/LAI Ceptometer (Decagon Devices, Washington, U.S.). Shade Area Analysis In order to assess the impact of each tree on the local environment, it was first necessary to calculate the amount of shade it produced. It was not possible to directly measure this, due to time constraints and because the busy roads restricted access. Therefore, the area of shade was calculated using the morphological measurements. Total shade area produced by each of the sample trees was calculated using Monteith and Unsworth’s (1990) equation for shade area produced by an ellipsoi- dal canopy, using the angle of the sun at each sample time. [1] where a is the vertical canopy radius, b is the hori- zontal canopy radius, and β is the angle of the sun to the horizontal. During the sample periods in early sum- mer the sun angle at midday was between 57.4 and 58.7 degrees, and in mid-summer between 56.2 and 58.2 degrees. To assess if the shape of the canopy had any effect upon the shade area produced or the temperatures below the canopy, aspect ratio was also calculated from the morphological results: [2] Air Temperature At each of the sample trees, two air temperature readings were taken at a height of 1.1 m using a Digitron 2084T platinum resistance thermometer with a PT100 air probe and radiation shield attached. The first reading was taken five meters east of the center of the shade cast by the tree canopy, ensuring that the air temperature probe was at least two meters away from the edge of the shade area. The second reading was taken with the temperature probe located within the center of the tree shade. Both readings had an acclimation period of two minutes before the reading was taken, increasing the accuracy of the result. Mean Radiant Temperatures At each of the tree sample locations, two mean radiant temper- ature readings were taken at a height of 1.1 m using a HOBO U30 data logger with two globe thermometers attached. The globe thermometers were constructed following the design of Thorsson et al (2007) and comprised a hollow 38 mm matte gray acrylic sphere with a 12-bit temperature smart sensor fixed at the center. One globe thermometer was placed five meters east of the center of the shade cast by the tree canopy, ensur- ing that the globe thermometer was at least two meters away 159 from the edge of the shade area. The second globe thermometer was placed so that the globe thermometer was within the center of the tree shade along the line between the center of the tree canopy and the center of the ground shade area. Both sensors were allowed an acclimation period of five minutes before start- ing a five minute period in which temperature readings were taken every five seconds. The readings were averaged over the five minute period to give a mean radiant temperature to improve the accuracy of the result as suggested by Thorsson et al.(2007). Surface Temperature Readings At each of the tree sample locations, two surface tempera- ture readings were taken using a Fluke 572 infrared ther- mometer. The first reading was taken five meters east of the center of the shade cast by the tree canopy, ensuring that the surface temperature was taken at least two meters away from the edge of the shade area and that the area had never been in shade. The second reading was taken close to the western edge (trailing edge) of the tree canopy shade area to ensure that the surface had as much time in the shade as possible. In both cases, the sampled region was instan- taneously shaded by the researcher during the reading to remove the calibration error that would have been caused by direct sunlight. Care was taken to ensure that the sample surface type was the same as the surface taken in full sun, and any asphalt where repairs had been made was avoided to ensure there was no difference in the sampled surface type. As no acclimatization period was necessary, surface readings were recorded within thirty seconds of each other. Analysis One-way ANOVA was used to assess whether there were significant differences between the tree species in their morphological characteristics and the area of shade they produced. The LAI was subjected to two-way ANOVA to additionally determine whether it was affected by the sample period. Air, globe, and surface temperatures in sun and shade were first tested using two-way ANOVA to investigate if tree shading and sample period had a sig- nificant effect on the temperatures. If shade had a sig- nificant effect on the temperatures, then the size of the effects produced by the different tree species were com- pared using one-way ANOVA with Tukey post hoc tests. The relationships between the morphological fea- tures of trees and their effects of temperatures were also investigated using correlation conducted using SPSS analysis. All tests were V16 software and differences between groups were considered significant at P < 0.05. RESULTS Tree Morphology The five species had quite different overall morphology (Table 2), one-way ANOVA showing significant differences between the species in canopy height (F4, 46 = 6.280, P ≤ 0.005), east and west = 6.994, P ≤ 0.005) (Table 2). Post hoc analysis shows that of all the species, Prunus ‘Umineko’ had the tallest but narrowest canopy spread (F4, 46 = 8.311, P ≤ 0.005) and canopy area (F4, 46 ©2013 International Society of Arboriculture
July 2013
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