A microclimate regulation study of Ficus altissima under winter conditions in lower subtropical China

As a widespread practice in urban landscape design, tree planting plays a vital role in improving the ecological environment and microclimate. This study obtained the physical, physiological, and meteorological data of Ficus altissima, a typical tree species in lower subtropical China, through eld measurement, and analyzed its functional performance in microclimate regulation. Its results indicated that: (1) the leaf area index (LAI), sky visible factor (SVF), ground cover (GC), and other indicators of Ficus altissima had essential relationships with radiation attenuation, temperature, and humidity regulation under winter conditions in lower subtropical China; (2) there were signicant differences in leaf surface temperature and transpiration between east, west, north, and south during daytime; and, (3) thermal comfort represented by physiological equivalent temperature(PET)in the shade could be expressed as functions of solar radiation (SR), mean radiation temperature (MRT), air temperature (Ta), air humidity (RH), globe temperature (Tg), and wind speed (V). Based on these results, the following were the suggestions: rstly, Ficus altissima with higher LAI values should be selected for planting; secondly, trees must be planted on the east side of the site should solitary planting be undertaken to obtain maximum thermal comfort; and nally, activities under the canopy of Ficus altissima should be prioritized at 11:00– 16:00 during winter.


Introduction
Vegetation design is an essential method of urban landscape design, which has a vital effect on improving urban microclimate (Alchapar et al., 2017). It has a signi cant impact on reducing air temperature, increasing air humidity, and regulating wind elds (Crum et al., 2017). The primary body of vegetation design is trees, which have more substantial environmental improvement than shrubs and ground cover (Yang, 1994). Studying their microclimate regulation function provides designers with appropriate urban vegetation solutions (Park et al., 2012) and plays a vital function in improving urban microclimate and improving the life quality of residents (Martelli and Santos Jr, 2015). Researchers have come to varying conclusions regarding the effects of trees on microclimates from various perspective, either through measurements or simulations. Different tree types impact radiation ux in the environment due to their varying size and canopy characteristics. Different temperature gradients form in and around vegetation, and their performance varies during the day and at night. Rahman et al. (2020) compared Purple satin and Robinia pseudoacacia, and found that tree species with a higher leaf area index indicated better below-canopy surface cooling. Shahidan and Jones (2008) examined forest canopy attributes based on tropical climate conditions, and determined that a denser foliage cover and branching habit would make trees exhibit a more signi cant thermal radiation lter. Massetti et al. (2019) investigated the effects of a deciduous tree species (Tilia x europaea L) on surface temperature over various ground materials and on human thermal comfort, with an emphasis on tree shade variations due to leaf fall. They found that the said species had demonstrated a two-fold bene t in terms of conditions. Sabrin et al. (2021) assessed the cooling bene ts of street trees in Philadelphia City, primarily considering morphological elements. They evaluated summertime thermal comfort for a city with humid subtropical climate (i.e., Philadelphia) by examining various human-biometeorological indicators like Mean-radiant temperature (Tmrt) and Physically Equivalent Temperature (PET), utilizing the Rayman model. Feng et al. (2021) found that mango tree transpiration consumed 40% (sunny) to 60% (cloudy) of a canopy's total daily energy and continued to play a role in cloudy days, making vegetation absorb the heat of surrounding air. Liu et al. (2018) conducted ENVI-MET simulation on four kinds of trees (Michelia alba, Mangifera indica, Ficus microcarpa, and Bauhinia blakeana), and identi ed that the model data were more stable than the measured data. Leaf surface temperature, steam ux, air temperature, and humidity were lower than the observed values.
Most domestic (China) studies have focused on the quantitative in uence of certain design elements on microclimate factors, but their in uence and mechanism are not su ciently profound. There are inadequate single and multiple regression equations to depict the speci c correlations among the variable (Zhuang et al., 2017). To further clarify the impact of trees on climate and human behavior, the researchers assessed ve adult Ficus altissima trees in winter. The microclimate instrument measured the local microclimate data, while the principle of transpiration measured transpiration intensity in four directions. By comparing the data of measuring points in the shade and the contrast points (as exposed to the sun), the principle of Ficus altissima on the surrounding environment based on local climatic conditions was determined. Finally, the suggestions of Ficus altissima for adapting to site planting were proposed.

Methodology Study objects
This study focused on trees' performance in regulating microclimate. It selected Ficus trees, which thrive on the open grassland of the campus, as the primary research object. Evergreen all year round, the Ficus altissima, moraceae, Ficus genus, is a typical urban green tree species in lower subtropical China. With its thick leathery leaves, developed root system, broad crown, large tree size, and capacity for isolated planting, it can manifest an independent scenery and has a good shading effect. To study its microclimate effect, the canopy distribution, branch, leaf orientation, transpiration, shading effect, temperature, and humidity regulation of this species were evaluated.

Study area and time
Researchers took Guangdong Ocean University (110°18'32" E, 21°9'11" N) in Zhanjiang (a city in China) as the study site. The site had a typical lower subtropical monsoon climate, with its most comfortable period from November to March (Luo et al., 2017). The research time was January, which typically had cloudy and sunny days, and the average daily temperature was 14-20°C. Field measurements were performed on January 10 (cloudy day), 12 (sunny day), 13 (sunny day), and 14 (sunny day). The physical and physiological data of the plants were measured on the rst two days, and the microclimate on the succeeding two days.

Measurement items
The researchers measured various physical indicators (Table 1), including diameter at breast height (DBH), crown width, and tree height, which could help re ect the three dimensions of the trees and analyze the trees' impact on the environment and human behavior. In this experiment, the physical indicators of Ficus altissima were primarily obtained using a laser range nder (Vertex Laser Geo, Sweden), a tree girth tape (GCRAFT, China), and other instruments. Since Ficus altissima has well-developed roots attached to its trunk, the researchers applied estimation and difference methods to determine the pure DBH value at 1 m above the ground. The DBH of Ficus altissima was 41-68cm, and the under-branch height (>2 m) met the space use requirements of the active population. The crown width was 9.9-16.1 m, and the tree height was 7.64-13.65 m. Meanwhile, to assess the in uence of tree shade on the underlying surface temperature, the planting sites (i.e., the range of bare land at the base of the tree trunks) were also measured.
The sky geometry, LAI, and transpiration rate of the trees were measured using the Hemi digital plant canopy analysis system (Hemi View, UK) and 1/10,000 balance scales (FA324C, China). Sky geometry and LAI were helpful to study the cover properties of plant canopy and accomplish some simple calculations. Transpiration rate aimed to reveal the absorption and release of the canopy to the environmental microclimate. The LAIs of trees 1-5 were within 1.520-1.989; different degrees of LAI indicated signi cant differences in crown leaf densities of the ve Ficus. Sky visible factor (SVF) was inversely related to LAI, with a measurement range of 0.137-0.219. LAIDev was used to interpret the density of leaf distribution within the canopy, which could evaluate the trees' growth. Ground cover (GC) intended to measure the canopy coverage to the ground, which was complementary to SVF. The combined value of the two factors should be equal to 1; together, they could describe the shading degree of the canopy to the ground (Table 1). Transpiration rate aimed to reveal the absorption and release of the canopy to the environmental microclimate. The dynamic changes of the leaves in four directions of Trees 3 and 4 from 8:00 to 18:00 were also measured at a one-hour sampling interval (refer to Section 3.2).
Measuring microclimatic indicators is an essential step of the experiment. Air temperature (Ta), relative humidity (RH), soil temperature (Ts), wind speed (V), and global radiation were readily acquired by an urban multi-factor climate data acquisition instrument (HQZDZ-7, China). Globe temperature, dry bulb temperature, wet bulb temperature, and blade surface temperature were recorded using a globe thermometer, a mechanically ventilated psychrometer, and an infrared high-precision thermometer. All instruments were installed at measuring points 1.8 m above the ground. Regarding the trees' planting mode and distance, measuring points 1, 2, and 6 in group A and 3, 4, 5, and 7 in group B were arranged under the shade of ve Ficus altissima. Measuring points 6 and 7 were made the contrast points of the two groups ( and represents the 1000-m³ air column (Yang, 1994).
There are two ways to calculate λ in Eq.   2018). In this study, PET was selected as an objective evaluation index of the impact of Ficus altissima on outdoor thermal comfort.

Radiation attenuation and related indicators
Comparison points 6 (a, Fig. 2) and 7 (b, Fig. 2) exhibited the highest radiation values during daytime because they were exposed to the sun. The variation rule of the total radiation values of the measurement points (1-5) under the shade was consistent with the comparison points. The overall trend gradually rose from morning to noon, reaching its peak at 12:00-14:00, then gradually decreased. By comparing the radiation values of the measurement points under the shade, when LAI was not differentiated, the smaller the tree, the better the protection from the sun. Thus, the group A measurement point 1 was smaller than point 2. However, when there were signi cant differences in LAI of group B, measurement point 3 with higher LAI values had signi cant shade and presented lower radiation, while measurement point 4 with lower LAI values exhibited poor shade and manifest higher radiation.
Canopy occlusion is closely related to LAI. Higher LAI values and lower transmissivity percentage value corresponds to a higher radiation ltration percentage. The higher the LAI value, the lower the transmittance percentage value and the higher the corresponding radiation ltering percentage. It implies that LAI values that are greater than ve can approximately lter more than 90% of radiation, while approximately 70-90% ltration occurs when LAI is below ve (Shahidan and Jones, 2008). This study found that the ltered radiation percentages of the ve trees were 90.7%, 87.9%, 90%, 82.2%, and 85%, respectively, indicating that Ficus altissima canopies could effectively lter solar radiation, and that radiation blocking was in direct proportion to LAI. The relationship between the variables is as follows: ΔI = 927 LAI -1377, R² = 0.979 (5) Where ΔI is the canopy ltering radiation value, in W/m 2 .
The canopy lters solar radiation waves and lowers the average radiation temperature below the canopy (Kotzen, 2003 Canopy SVF values were obtained from the elevation angle, indicating the proportion of the sky seen through the canopy. The value ranges from 0 to 1; 1 represents a complete sky, while 0 represents complete occlusion. The smaller the value, the thicker the canopy, hence less radiation transmitted and greater radiation attenuation. GC is the opposite of SVF, where the value was obtained from the overlooking angle, and the sum would equal to 1. The measured results indicated that the radiation attenuation of the Ficus altissima canopy had the following linear relationship with SVF and GC: ΔI = -5196.21SVF + 1163.649, R² = 0.921 (7) ΔI = 5196.21GC -4032.561, R² = 0.921 (8) LAIDev was used to re ect whether the canopy distribution was uniform and whether the thickness of each direction was consistent, requiring high quality of the pictures shot. Photos were taken in the morning, evening, or cloudy days when the light was mild; the images were preprocessed through a canopy analysis software. The threshold was set at around 150. The LAIDev value is signi cantly in uenced by manual operation, making it easy to produce errors. To avoid excessive errors, ve photos for each tree were taken at different periods. The obtained LAIDev value has a speci c correlation with radiation attenuation: ΔI = -1294.795LAIDev 2 +2856.625LAIDev-1127.705, R² = 0.795 (9) Transpiration and cooling Due to variations of solar altitude angle, there were signi cant differences in the temperature data of the four directions of the tree crown (Fig. 3). Under the in uence of climatic conditions in the lower subtropical region, the highest daytime temperature (12.3-33.6°C) and maximum temperature difference (21.3°C) were found in the southern leaves of Ficus altissima in winter, while the northerly leaf temperature was at its lowest in daytime (12.1-19.4°C) and the temperature difference was the lowest (6.7°C). The eastern and western temperature ranges were in the middle, but there was a difference in time dimension. The temperature of the eastern leaf began to rise sharply at 10:00 and sustained its peak at 17:00. On the other hand, the leaf temperature on the west side signi cantly rose at 13:00 and sustained its peak at 17:00. It indicated that the leaf temperature on the east side reached its peak earlier, while the peak time was longer than that on the west side. Additionally, the temperature variations of the leaves in the four directions were affected by solar radiation, consistent with the distribution trend of radiation. Several peaks (10:00, 13:00, and 16:00) and troughs (12:00, 15:00) emerged in the leaf temperatures at four directions of Tree 3. Meanwhile, the measured total radiation values at comparison point 6 increased and decreased correspondingly. The peak at Tree 4 appeared from 13:00 to 15:00, while the measured total radiation value at comparison point 7 likewise reached the maximum range.
Regarding calculating transpiration rate per unit leaf area (Fig. 4) Leaf transpiration rate at all directions reached a low point at 13:00, mainly due to stomatal closure as a consequence of heat. LAI and the four directions were considered to calculate the transpiration rate of the entire plant; the results indicated that the transpiration rate of Ficus altissima with higher LAI was relatively stable, while that with lower LAI uctuated violently due to the in uence of solar radiation; moreover, the transpiration rate in the afternoon was signi cantly lower than that in the morning. The standard deviation and variance on the west side were the largest, indicating that the latent heat of hourly evaporation was signi cantly different from the mean value; that is, data uctuation was the most severe. The standard deviation and variance of the north side were the smallest, signifying that the hourly latent heat of evaporation on this side was closest to the mean value; that is, the data change was most stable. To summarize, the effect of heat evaporation and cooling on leaves of Ficus altissima was the most obvious on the west side, followed by the south, east, and north sides.
Adjusting ambient humidity Transpiration from the tree canopy moves water into the air, thus increasing ambient humidity (Kántor et al., 2016). During sunny days in winter, the average humidity at the measurement points under the shade from point 1 to 5 was 36.4%, 35.2%, 32.38%, 31.63%, and 32.3%, respectively, while those of comparison points 6-7 were 33.59% and 31.13%. Ficus altissima enhanced air humidity, effectively humidifying 3% (group A) to 5% (group B). The relative humidity at measurement points 1 to 5 was low at noon and high in the morning and evening, but compared with comparison points 6 and 7, the most apparent humidi cation period was at 12:00-15:00 (Fig. 5). The difference in air humidity in group A was more prominent than in group B, primarily caused by the canopy with higher LAI. Measurement point 1 with the highest LAI was most capable of improving environmental humidity, while measurement point 4 with the lowest LAI was the weakest, indicating that LAI was directly proportional to environmental humidity. The variation ranges of daytime humidity at measuring points 1 and 2 in group A were 40.3% and 45.7%, respectively, while those in group B were 41.3%, 46.2%, and 42.9%, respectively. The measurement points with stable humidity changes had larger LAI, GC, canopy, tree height, and smaller SVF values; the higher the tree shape, the thicker the shade, the wider the canopy, and the more stable the shade air humidity.
Affected by canopy shade, the underlying surface-soil moisture of the shade measuring points was more stable and slightly changed the daytime humidity value. Unlike the variation rule of air humidity, the soil humidity of the measuring points under the shade would slightly rise at noon, and the degree of rise was less than that of the exposed soil. Changes in soil moisture were related to certain physiological and physical indicators of trees. The internal comparison results of groups A and B indicated that larger LAI, GC, canopy, tree height, and smaller SVF were conducive to moisture preservation of the underlying surface. Fig. 6 illustrates that the variation range of soil moisture in the shade measuring point of group A was 1.3-1.7%, and that of soil moisture underexposure was 2.4%.
Moreover, the variation range of soil moisture in the shade measuring points of group B was 1.7-2.0%, and the variation range of soil moisture underexposure was 2.3%. Compared to points 6 and 7, the underlying surface humidity of group A was greater than that of group B when the soil moisture changes were almost the same, primarily because group A had a thicker canopy to shade the bare land below. Likewise, humidity uctuation at all measuring points in group A was smaller than that in group B, proving that tall and vigorous trees were more conducive to sustaining the humidity of the underlying surface.
Thermal comfort analysis PET was used to study the effect of Ficus altissima on thermal comfort and explored the relationship between PET and multiple climate factors (e.g., Sr, Tmrt, and Ta). Fig. 7 indicated that the maximum PET value at shade measuring points 1-5 was lower than that at comparison points 6-7 under sunny weather in winter (the maximum difference was 16.3); moreover, the variation range of PET in the shade measuring point was relatively gentle. From 8:00 to 9:00, it was in a cold and cool state under the shade.
From 9:00 to 10:00, it was generally in a slightly cool state. From 10:00, PET sharply increased at all measuring points, and some measuring points crossed three intervals. Since 16:00, the PET of all measuring points sharply decreased, and some of the measuring points decreased by three intervals. By 18:00, all measuring points returned to a cold and cool state. The optimum range for being or receiving comfortable primarily focused on 11:00-16:00. For group A with a low SVF and high LAI, PET was primarily located in a slightly cool (close to comfortable) interval; for group B with a relatively high SVF and low LAI, PET was primarily located in a comfortable interval. All shade measuring points were two to three intervals lower than the comparison points. The comparison points were in a slightly warm or warm range for most of the time (six to seven hours), while the shade measuring points were within or close to the comfortable range for most of the time ( ve hours). The results showed that the shade of Ficus altissima in winter could impact the human body's comfort and make people exercise for most of the day in a relatively comfortable thermal environment.
The correlation between PET and climate factors could be tested using Pearson correlation. The results indicated that PET was signi cantly correlated with total solar radiation (

Conclusions
Considering the growth properties and site effects of Ficus altissima in lower subtropical China, this paper implemented climate measurement work and combined the microclimate principle with the growth rule of trees. The fundamental ndings are as provided below.
Firstly, physical and physiological indicators are essential indicators for quantifying the effects of trees on the environment, and they are associated with radiation attenuation, transpiration, temperature change, humidity regulation, and PET improvement. LAI, SVF, GC, and LAIDev of different values signi cantly correlated with radiation attenuation, as expressed by Formulae 5, 7, 8, and 9. Studies have shown that a larger LAI, GC, canopy, and tree height denote a smaller SVF value; hence, the more air humidity can be improved (humidi cation was 3-5%), as well as the moisture of the underlying surface (humidity variation was less than 2%). There was no noticeable difference in LAI. The smaller the tree height, the better it was to block solar radiation. Moreover, the higher the LAI, the more stable the transpiration performance. A higher LAI value could cause a signi cant transpiration cooling on the west side of the tree crown, which could reach 1.7°C. When LAI was between 1.520-1.989, the air could be effectively humidi ed by 3-5%. In addition, when SVF was high and LAI was low, PET tended to develop in the comfort zone.
Secondly, the four directions of trees were affected by solar radiation to varying degrees, such that there were signi cant differences in leaf surface temperature and transpiration in different directions. For leaf surface temperature, the northerly leaf exhibited the most stability, while the southerly average temperature and temperature difference altered the most; the leaf temperature in the east reached the peak earlier, while its peak time was longer than that in the west. For transpiration rate, the transpiration rates in the west and south directions were better than those in the east and north directions. In uenced by LAI, the side with the maximum transpiration rate was located in the middle and afternoon, which could change (for Thirdly, in terms of thermal comfort, the differences between the shade measuring points and the open location points were compared, and signi cant correlations between several indicators and PET were explored (Fig. 8). In winter, the shade of Ficus altissima can improve human comfort and make people in motion in a relatively relaxed state.
Combined with the in uence of thermal comfort(expressed by PET) and other climate factors, the following are the landscape design suggestions. (1) In terms of seedling selection, Ficus altissima with signi cant LAI value, high volume, and lush growth should be selected for planting as far as possible due to its relatively stable climate regulation function. (2) Ficus altissima has a noticeable transpiration effect and has the most signi cant in uence on the west side. Therefore, it must be planted on the east side of the activity site in isolation so that the canopy can portray its best thermal comfort regulation function.
(3) During winter in lower subtropical China, the function of Ficus altissima leaves in regulating temperature at noon and afternoon is not as good as in the morning and evening, but they can still maintain a comfortable microclimate under the tree canopy. Moreover, the activities under the shade should be arranged at noon and afternoon (11:00-16:00) in the design.
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper.   Changes of air humidity in groups A and B, the data was measured on January 13, 2021 Figure 6 Changes of soil moisture in groups A and B, the data was measured on January 13, 2021 Thermal comfort analysis of measurement points 1-7. The vertical strip represents the hourly change of physiological equivalent temperature (PET) at measurement points 1-7, while the horizontal strip denotes the de ned interval of PET, seen in combination with Table 3