Arboriculture & Urban Forestry 39(1): January 2013 35 Figure 4. Average diurnal variation in temperature (°C) and relative humidity (%) during the two experimental periods, lasting four days each, was typical for equatorial climatic conditions in Singapore. Experiment one was conducted between June 15–18 and experiment two between August 10–13, 2011. Note: error bars (± SD) illustrate the observed variability in environmental parameters at regular intervals. the image. Whereas a visible gradient of progressively cool- er temperatures was visible at the base of the stem among all remaining treatments, this feature was uniquely absent from the non-irrigated control. With the exception of these two observa- tions, the surface temperature distributions appeared visibly similar between the other treatments and the irrigated control. Temperature data extracted from the rectangular transects displayed inconsistent relationships between the void treatments and each of the four evaluated statistics, including the mean temperature, mean surface-air temperature differential, standard deviation, and skewness. However, some obvious and consistent patterns were apparent when the measurements, representing separate treatments, were sorted from least to greatest. In both experiments, the non-irrigated control and the plant with the 3.8 cm void registered the lowest mean temperature within the tran- sect among all plants measured in the evening. For D. fragrans, the average air-surface temperature differential for Plants 1 and 7 was -1.64°C (SD 0.29) and -1.62°C (SD 0.41), respectively. For S. grande, these values for Plants 1 and 8 were -0.92°C (SD 0.42) and -0.81°C (SD 0.78). The values for these two treatments were reasonably close to one another at 1830h, their values being sepa- rated by 0.02°C in the first experiment and 0.11°C in the second, while all remaining treatments registered warmer temperatures. Overall, paired t-tests of mean temperatures did not reveal sig- nificant differences between the surface temperature for the stems containing the void treatments and irrigated control (Appendix). In addition, the temperature distribution skewness within the rectangular transect on the plant with the 3.8 cm void in both experiments notably diverged from the other treatments at 1830h (Appendix). At this time of day, the surface temperature distributions above this void were skewed to the right (i.e., pos- itive values) while all remaining treatments were skewed to the left (i.e., negative values). Paired t-tests conducted with these data, however, revealed that only the non-irrigated control for both D. fragrans and S. grande was skewed significantly great- er than the irrigated control at 1830h, and S. grande Plants 3, 6, and 7 were skewed significantly greater than the irrigated control at the same time (Appendix). In contrast, the standard deviation of temperature distributions within the rectangular transects did not rank consistently when sorted from least to Table 2. Localized stem temperature deviation from the mean surface temperature in linear temperature plots. Set 1: Dracaena fragransz Time 0830h Treatment 1 2 3 4 5 6 7 8 Deviation from linear trend (°C) Mean ± SD 0.11 ± 0.07 P-value 0.361 0.06 ± 0.09 0.05 ± 0.07 0.04 ± 0.06 0.05 ± 0.10 0.04 ± 0.01 0.00 ± 0.07 − − 0.908 0.728 0.908 0.728 0.306 − z Number of repeated measures (n = 12). y Represents a significant difference (P < 0.05) from Treatment 2, the irrigated control. ©2013 International Society of Arboriculture 1830h Mean ± SD 0.17 ± 0.07 0.21 ± 0.04 0.15 ± 0.02 0.28 ± 0.04 0.19 ± 0.07 0.27 ± 0.15 0.51 ± 0.14 − P-value 0.588 − 0.751 0.162 0.821 0.188 0.001y − Set 2: Syzygium grandez Time 0830h Deviation from linear trend (°C) Mean ± SD 0.14 ± 0.09 P-value 0.389 0.09 ± 0.08 0.10 ± 0.09 0.12 ± 0.08 0.08 ± 0.09 0.14 ± 0.02 0.10 ± 0.08 0.12 ± 0.09 − 0.878 0.575 0.918 0.446 0.878 0.646 1830h Mean ± SD 0.03 ± 0.11 0.04 ± 0.06 -0.01 ± 0.01 0.08 ± 0.13 -0.04 ± 0.17 0.05 ± 0.03 0.14 ± 0.04 0.34 ± 0.16 P-value 0.851 − 0.551 0.736 0.355 1.000 0.337 0.004y
January 2013
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