Arboriculture & Urban Forestry 39(4): July 2013 161 temperatures by a greater amount than those with lower values. There was also a significant negative correlation between aspect ratio and surface temperature in early summer (rs(49) (rs(49) = 0.549, P ≤ 0.005). Trees with higher LAI reduced surface = -0.313, P = 0.025) but not mid-summer (rs(49) = 0.045, P = 0.745). Trees with more elliptical canopies reduced surface temperatures less. DISCUSSION Figure 2. The mean reduction of mean radiant temperature (a) and surface temperature (b) in tree shade during early summer and mid-summer in Manchester, UK, below five different street tree species: C. laevigata (n = 12), S. arnoldiana (n = 10), Prunus ‘Umineko’ (n = 10), P. calleryana (n = 10), and Malus ‘Rudolph’ (n = 9). Lowercase letters indicate significant differences in early summer while uppercase indicate significant differences in mid- summer; species with the same letters are not significantly differ- ent from each other. 239.583, P ≤ 0.005), these differences were significant. The temperature reductions due to shading by the five tree species are shown in Figure 2b. One-way ANOVA showed that there were significant = 129.440, P ≤ 0.005) and mid-summer (F1, 100 differences between the spe- cies for both early summer (F4, 46 = 3.959, P = 0.008) and mid-summer (F4, 46 = 136.779, P = 0.005). Post-hoc analy- temperature significantly more than S. arnoldi- sis showed that during early summer, C. laevigata reduced surface ana, while in July, C. laevigata and P. calleryana both reduced the surface temperatures significantly more than Prunus ‘Umineko’, S. arnoldiana, and Malus ‘Rudolph’. Surface Temperatures The mean surface temperatures were 40.1°C (StD ±6.6) in sun and 27.3°C (StD ±4.5) in shade in early summer and 39.2°C (StD ±4.4) in sun and 27.2°C (StD ±3.38) in shade in mid-summer, reductions of 12.8°C and 12°C due to shade re- spectively. One-way ANOVA showed that in both early sum- mer (F1, 100 = Experimental Limitations Unfortunately, this research was limited to young trees of small species because they were the only ones for which the study authors had adequate planting data. Because of the lack of his- torical records for trees in Manchester it was not possible to investigate the shading benefits of larger, more mature trees, which might be expected to have greater effects. It must be remembered, also, that species must also be chosen so that they are capable coping with the harsh growing conditions found in urban areas, which are only likely to get worse with future climate change. Because it was carried out on real streets, the research presented here necessarily has limitations in experi- mental design and sampling size. In an ideal experiment, trees would be randomly placed within identical streets with identi- cal surfaces and growing conditions. Fortunately, though the trees were not evenly spread, the different streets had similar morphology and asphalt paving, and they were located within a small area, so the soil conditions were likely to be similar. Neither was it possible to continually monitor the trees because of problems with vandalism, and so the research was limited to two visits during periods of warm, sunny weather, when the shading effects of the trees was likely at its maximum. The poor summer meant that even during these visits conditions were not ideal, but despite the relatively low air temperatures, the visits should have recorded the maximum effects of shading because on the sampling days the trees were being heated by full sun. Despite the fact that sampling was carried out only twice, the large number of trees sampled meant that researchers obtained reliable and statistically analyzable data on both occasions, and despite its limitations, the research showed clear trends in the shading effects of street trees that can be extrapolated to provide recommendations about how to maximize tree shade benefits. Tree Morphology and Shade The sampled tree species showed significant differences in their canopy size and shape, but these differences did not result in significant differences between the shade areas produced by the tree canopies. This was no doubt because the taller trees, such as Prunus ‘Umineko’ had narrower canopies, while the shorter trees, such as C. laevigata and Malus ‘Rudolph’ had wider cano- pies. This difference in shape is quantified by the widely differing aspect ratios of the canopies of the five species, which ranged from 1.5 to 2.5. The tree species with most elliptical canopies, ©2013 International Society of Arboriculture Relationship Between Morphology and Surface Temperature The morphology of individual trees also affected their performance at reducing surface temperature. There was a significant positive correlation between LAI and surface temperature reduction in both early summer (rs(49) = 0.735, P ≤ 0.005) and mid-summer
July 2013
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
