The study of morphological variables of urban buildings and urban tree canopy cover is essential for sustainable urban development in arid areas. Access or masking of solar radiation in highly forested urban environments with dry climates and their interrelationships influence energy consumption and environmental conditions. The problems of the environment and energy have been debated intensely by scientific sectors, and mucظh has been discussed about the relationship between energy consumption and urban morphology [(Givoni, 1998); (Breheny, 1996); (Oke, 1987); (Moreno Garćıa, 1993)].

On the level of urban configurations, urban green spaces provide great social and environmental benefits that benefit the quality of life in cities. The contribution of urban trees in improving the microclimate, air quality, and living in cities is well-documented [(Bernatzky, 1982); (Rowntree, 1986); (Mcpherson et al., 1989); (Mcpherson, 1992); (Mcpherson et al., 2011); (Santamouris et al., 2011); (Santamouris et al., 2001); (Simpson & Mcpherson, 1998); (Santamouris et al., 2001); (Mascaró, 1996)]. Urban trees improve air temperature and humidity, evapo-transpiration, absorption of pollutants, and wind speed [(Akbari & Konopacki, 2005); (Block et al., 2012); (Simpson, 2002); (Tooke et al., 2012); (Tooke et al., 2009); (Tooke et al., 2011); (Armson et al., 2012); (Hamada & Ohta, 2010); (Loughner et al., 2012); (Morakinyo et al., 2013); (Shashua-Bar & Hoffman, 2000); (Shashua-Bar et al., 2009)].

Some authors have evaluated the reduction of solar radiation on horizontal surfaces [(Gómez-Muñoz et al., 2010); (Heisler, 1986)] and on the surrounding surfaces that radiate heat [(H, 1983); (Papadakis et al., 2001)]. To date, some studies have focused on the ceiling masking woodland, [(Tooke et al., 2009); (Tooke et al., 2011); (Tooke et al., 2012); (Wang et al., 2016)] but the shading trees’ effect on the walls and windows has been unevaluated.

The present work-studies available regarding solar radiation on the north facades of highly forested urban environments typical of the Mendoza Metropolitan Area (AMM), Argentina. This is part of the environmental and energy-sustainable development issues regarding the construction industry in cities of the arid sub-Andean mesothermal region of the West-Central Argentina. Its population is about one million inhabitants, and its geographical coordinates are latitude -32.85, longitude 63.85 and altitude 746 m. The main climatic characteristics are the following: in comfort for 21.5% of yearly hours, heating needed for 70% of yearly hours, and cooling needed for 8.5% of yearly hours. Heating DD (base 18◦C) is 1384 and cooling DD (base 23◦C) is 163. Yearly mean horizontal global solar radiation is 18.06 MJ/m² day.

The AMM is no stranger to the intense population growth of cities in developing countries and the unsustainability of current development has inexorably deteriorated through time due to a lack of knowledge, management and planning actions.

Given this scenario, the following question arises: what are the impacts of both urban-building morphologies and the different tree species that predominate in the urban environment in the availability of the solar energy resource?

In the AMM, several specific studies have been developed, considering the representative groups of the urban-building morphology, which were determined by the solar potential in low and high density environments, highly-forested areas and its foreseeable evolution [(Rosa, 1988); (L. et al., 2004); (Fernández et al., 2003); (Mesa et al., 2010); (Basso et al., 2003); (Arboit, 2013); (Mesa et al., 2010)]. Studies have been developed with the objectives of: i. preserving the physiognomy of the forested city, maintaining the scale and homogeneity of the constructions and the aesthetic contribution and environmental services of urban forest; ii. enabling the maximum use of solar resource for space and sanitary water heating, through the control of urban morphology and of the forest; iii. Improving habitability conditions of the building area; iv. contributing to the sustainability of local development, by enabling the recycling of existing constructions that were well maintained and of good quality. In this way, extractive processes and solid waste emission (rubble) in the ecosystem are reduced.

In order to complete these studies, a solar irradiance measurement was performed on the north facades of seven selected cases, according to previous results. The intention of the study was to offer a contribution, both conceptual and operative, so that, through the transfer channel in the future, the official sector will become aware of the seriousness of the situation and soon begin to implement new urban and building norms. These could also reduce the consumption of natural gas and other non-renewable energy in urban buildings.

In previous work (Arboit et al., 2008), the Mean Insolation Factor (MIF) indicator has been defined. The Mean Insolation Factor (MIF) provides a measure of the potentially collecting north facing walls, not masked by neighboring buildings and trees, calculated as: the ratio of the sum of insolated collecting areas of north facing walls, times the sum of the energy received at each considered hour, during a heating season, to the sum of the total areas of the same surfaces, free of all masking, times the sum of the hourly impinging radiation during the considered heating season, as a percentage. It is defined in Figure 1.

TPCA: Total potential collecting area of the North facade (m²) SMA: Solid Masking Area (constructions):

Potential collecting facade affected by shade projected by constructions of close buildings (m²). (Figure 1)

TMA: Transmissible Masking Area (trees): Potential collecting facade affected by shade projected by urban trees (W/m²).

P: Monthly Transmissibility Factor. Solar Transmissibility percentages of each vegetable species and for each month of the heating season (%).⁴²

R(m-d-h): Energy by surface unit available in the North facades for each hour, day and month of the heating season (Wh/m²). Hourly impinging radiation on North facing walls for each month (Wh/m²).

Sub-indicators:

m: Month of heating. It varies between April and September. Number of months to which the heating season is extended (n).

d. Day of the month. It varies between 1and 30. h: Hour. It varies between 9:00am and 6:00pm.

(1)

(2)

Considering the Mean Insolation Factor (MIF) on north facades and using a Multiple Linear Regression Model, the main variables that influence the access of the sun were determined (Source not found, n.d.). These variables are: Building Morphology (B. Morp.), Street width between construction lines (St. Wi), Fullness of trees (F. Trees) - relationship of the existing (healthy) trees around a city block to the total number of trees that could fit around the city block, considering the dominant species and their corresponding distance between individuals, as percentages- and Total Occupation Factor (TOF)-Total built-up area to total buildable area of corresponding parcels, as percentages-. (eq.2).

The easiest way to note the influence of the urban-building variables with access to the sun is by measuring global solar irradiance over the vertical plane of the completely sunny north facade and measuring the same variable of the facade that is partially sunny. The latter is affected by the urban-building morphology (building morphology, street width, fullness of trees, and building height).

The measurements were carried out with portable irradiance data-measuring systems, equipped with Eppley 8-48 pyranometers.

The measurement period was established according to the latitude and longitude of the place of study, for a given day. Considering the local time, the solar noon was defined (» 1:30pm), and four and a half hours before and after the solar noon were taken into consideration. This approach left nine definite solar hours to measure from 9am-6pm. Data was registered every minute during the autumn-winter and spring-summer seasons.

Based on what has been presented, the corresponding measurements were selected during a clear day for each urban environment. Clear sky conditions allow the evaluation, in its complete magnitude, of the influence of the trees and the urban morphology.

Selection of case studies

For the monitoring of global solar irradiance on north facades affected by solid and tree masking, seven cases were selected, in which different measuring conditions were present. For the selection of case studies, the urban-building and forest morphology of the MMA urban environments were considered based on the Mean Insolation Factor (MIF). In Table 1, the case studies and the characteristics of urban and building variables can be observed.

Figure 2 and Figure 3 show the diversity of energy situations according to the analyzed urban environment, and they show a preponderance of each of the variables in equation 1. Measurements of shaded surfaces registered a reduction in the autumn-winter season from 2% to 66% of the total energy received on the plain sunlight sections, and in the spring-summer season from 10% to 83%.

This said percentage is dependent on the urban building environment. Cases 3, 4 and 5 are those with the most favorable ratio between maximum insolation in winter and maximum masking in summer. The irradiance measured on the vertical plane and oriented towards the north of Case 3, in partial sunlight condition, presents a relationship for the summer months of a masking effect of 68%, and for the winter months of an available net energy of 78%.

While in Case 1 the percentage of net energy available on north facades in winter is 98% (favorable from the point of view of the solar energy use), in summer, the energy masked is 10% (the lowest value of solar masking of the cases analyzed). The environment involved has a street width of 30 meters and homogeneous building morphologies at a height of 10mts (with scarce influence of forest at the crown canopy level and little influence on surfaces shaded by neighboring buildings).

In the autumn-winter season, the maximum reduction percentage, 66%, corresponds to Case 7, which is characterized by a wide street channel of 14.20 m, and the measurement surface is partially shaded by trees at a second magnitude Morus alba and first magnitude Platanus acerifolia, with the consequential reduction of the available energy.

In the spring-summer season, the maximum reduction percentage, 83%, corresponds to Case 2, which is characterized by the influence of two different species, Morus alba and Fxaxinius excelsior. In winter, masking for the same case study is 55% resulting from the urban forest, a variable first order masking solar resource in highly forested urban environments.

Cases 1 and 3 for detailed study are compared in the first instance, as they represent the extreme conditions, through the ratio between maximum sunlight in winter and maximum masking in summer. In this way, the variables of street width, tree canopy cover, and building height and morphology will be analyzed. How both will influence the quantity of the received solar energy on north facades will be observed.

The obtained results provide evidence of the impact of the principal variables that condition the access of the sun in a highly forested urban environment. The seven urban configurations that were studied in the Mendoza Metropolitan Area (AMM) present differential characteristics with respect to the urban and building variables.

In forested urban public spaces, it is necessary to consider the benefits provided by trees in summer. When planning urban environments, proper choice of species seems essential to balance the positive and negative contributions of trees. The following should be considered: 1. tree species of magnitudes consistent with the building height (1^st high-magnitude density, 2^nd medium-density magnitude, and 3rd low-magnitude density); 2. deciduous species (with maximum transmissivity values in winter, and minimum in summer); and 3. planting distance between chosen species (study of the position of and distance between the trees of each species).

The important influence that bare foliage presents in the autumn-winter season has been demonstrated. These characteristics can be accentuated in certain forested conditions and under determined solar positions, as is demonstrated by the measured values in case study 2, where afternoon hours recorded a resource availability of only 15%. However, case 2 presents the most favorable solar masking (83%) in the summer season. These results show that urban trees result in a first-order variable when capturing a solar resource in highly forested urban environments, and the choice of the optimal tree species at the moment of urban planning should be structurally considered when contemplating future design strategies.

In addition, the choice of tree species is also important in the consideration of combining high transmissivity values in winter and low values in summer. Moreover, it is necessary to reinforce the conclusion that the environmental and energy benefits of urban trees in a city with a desert climate and warm-dry summers, such as the AMM, should be considered as a first priority at the time of planning, designing and building any type of construction project, of any building density in a city.

The analysis of building height variable demonstrates the importance of establishing design strategies and legislation in accordance with a highly forested city that generates unique conditions above and below the trees. Furthermore, the influence of the surrounding buildings decreases when we consider greater building heights, as this combination of effects allows a measurement in winter uptake of solar energy of 98% of full sunlight condition in Case 1, and 90% in the summer.

Heterogeneous building morphology strongly conditions the possibilities of access to the sun, which has been demonstrated in case study 6, where during the heating season, a building with entrances and exits produces a solid masking up to 84% in the morning hours on its applicable North-facing facade. In the summer season, solar masking values of 44% were recorded. As for the vegetative presence of Cyprus (Cupressus sempervirens), its morphology and planting distance nullify the influence of the urban tree variable, when the results of case 6 in both seasons are considered.

Building morphology has been considered in this analysis in terms of heterogeneity that is generated by front and rear overhangs, which produces solid masking and compromises access to the sun. These types of heterogeneous morphologies are difficult to recover when implementing building recycling. In the same way, when planning and designing new complexes, homogeneity is a basic condition for assuring full sunlight in the winter season. In the summer season, measures for building-technology recycling, protection design, and green design could improve the present situation.

The analyzed urban determinants for solar energy collection on north facades (trees, building morphology and building height), have greater intensity when they are combined with a narrow street channel. This situation corresponds to case 7, where the minimum received solar radiation values are produced when considering all the hours of measurement. This demonstrates that the strategies recommended for narrow urban canyons should be demanding, concerning the type of permitted tree species, to the maximum height and the shape of buildings, and to the setback from the street.

In the spring-summer season, masking produced by green mass of trees generates multiple benefits: controlling the intensity of the urban heat island, the urban micro climate, the absorption of pollutants, the cooling and humidifying of the air through evapotranspiration and the reducing of thermal loads of buildings. Benefits also include improving the occupancy of public open spaces and making an invaluable contribution to the urban aesthetic.

The achieved results go towards contributing to the progressive reforming and updating of urban and building codes, in order to implement the highest levels of energy efficiency and to minimize any negative environmental impact from urban buildings.