3.1. Distribution of NDVI values in Medan Baru and Selayang Subdistricts
The NDVI compares the total amount of visible red light absorbed with the amount of reflected near-infrared light by a surface (Fusami et al. 2020). It basically uses a mathematical ratio to compare the amount of absorbed visible red light and the reflected near-infrared light.
Furthermore, Fusami et al. (2020) explained that negative values indicate areas with water, marshy surfaces, man-made structures, rocks, clouds, snow; bare land usually gives values that fall within 0.1–0.2 while plants always have positive values ranging between 0.2 and 1. For healthy, dense vegetation canopy, the values are above 0.5 while sparse vegetation has its values ranging from 0.2 to 0.5. Generally, the NDVI values are between 0.2 and 0.4 for sparse vegetation areas and 0.4 and 0.6 for moderate vegetation, and anything above 0.6 indicates the highest possible green density. The distribution of the NDVI values in Medan Baru and Medan Selayang sub-districts in 1999 and 2019 can be seen in tables 1-2.
In the year 1999, the Medan Baru and Medan Selayang sub-districts had the highest NDVI value in the range > 0.4, which is 609.66 ha (29.96%) of the total area, while the smallest in the range of NDVI < 0.1, which is 151.94 ha (7.47%) of the total area (Table 1). These are quite different from the year 2019 (Table 2), which shows the NDVI of the Medan Baru and Medan Selayang subdistricts, where they had the highest NDVI value in the range of 0.1–0.2. It comprised the area of 768.09 ha (37.73%). The smallest NDVI range > 0.4 covered 226.04 ha (11.1%) of the total area. A comparison of each NDVI area class is illustrated in Figure 2.
Table 1. Distribution of the NDVI values in Medan Baru and Medan Selayang in 1999
No
|
NDVI
|
Area (Ha)
|
% Area
|
1
|
< 0.1
|
151.94
|
7.47
|
2
|
0.1 – 0.2
|
346.11
|
17.01
|
3
|
0.2 – 0.3
|
447.15
|
21.97
|
4
|
0.3 – 0.4
|
479.97
|
23.59
|
5
|
> 0.4
|
609.66
|
29.96
|
Total
|
|
2,034.84
|
100.00
|
Table 2. Distribution of NDVI values in Medan Baru and Medan Selayang in 2019
No
|
NDVI
|
Area (Ha)
|
Area (%)
|
1
|
< 0.1
|
243.47
|
11.96
|
2
|
0,1 - 0,2
|
768.09
|
37.73
|
3
|
0,2 - 0,3
|
468.48
|
23.02
|
4
|
0,3 - 0,4
|
329.43
|
16.18
|
5
|
> 0.4
|
226.04
|
11.10
|
Total
|
|
2,035.50
|
100.00
|
Figure 3 shows the comparison of the NDVI values’ distribution of the years 1999 and 2019. There are increasing areas with lower NDVI values in the whole area. Definitely, there is a decrease in areas with high values of NDVI. This means that there are prominent changes in vegetated areas to less or no vegetated areas.
Based on field checks, there are 9 land cover classes, namely buildings, roads, mixed gardens, oil palm, settlements, trees, grass, rice fields, and shrubs. The division of objects and the range of NDVI values can be seen in Table 3.
Table 3. NDVI values range of each land cover validated by ground check
№
|
Class
|
NDVI Range
|
1
|
Settlements
|
0.04-0.28
|
2
|
Road
|
0.04-0.29
|
3
|
Buildings
|
0.06-0.29
|
4
|
Mixed Garden
|
0.24-0.50
|
5
|
Grass
|
0.26-0.44
|
6
|
Trees
|
0.26-0.52
|
7
|
Rice fields
|
0.30-0.46
|
8
|
Shrubs
|
0.30-0.52
|
9
|
Oilpalm
|
0.45-0.49
|
Human activities have led to land-use change with a strong influence on the existence of vegetated land. Land-use changes driven by human activities are a consequence of any development. Wang et al. (2020) mentioned farmland, forest land, and grassland are converted to construction land due to urban expansion along with economic and social development. Furthermore, Wang et al. (2020) highlighted the contribution of the NDVI in studying the impacts of land-use change on vegetation.
In the sub-district of Medan Baru and Medan Selayang, the NDVI value range lies between 0.04–0.49. Based on ground check, there are land covers in each range of the NDVI. Land cover with the NDVI value less than 0.1 to 0.2 are building areas, roads, and settlements while land cover with the NDVI value 0.2–0.4 are mixed gardens, land with trees, grass, rice fields and shrubs. There is more dense vegetation in the area with NDVI values higher than 0.4, including oil palm, trees, and shrubs. Visualization of each land cover can be seen in figures 4.
The denser the vegetation, the higher the NDVI value. The low the NDVI value will represent the low vegetation density. There are variations of the NDVI values in each land cover class due to the variation of objects found in the field; for example, there are trees alongside the road and trees in between houses in settlement areas. Some trees can also be found near buildings, and some near rice fields, grass land, and shrub. The higher the NDVI values found in areas with trees, shrub and oil palm. The lower value is found in settlements and roads.
The spatial resolution of Landsat images contributes to the variation of NDVI values of each land cover classes as there are many objects found in areas of 30 m. There is also due to existence of vegetation in each land cover classes as mentioned above.
3.2. Vegetation Existence in Medan Baru and Selayang Subdistricts
Based on the finding in the field and some references in the classification of NDVI (Lufilah et al (2017), Mukhoriyah et al (2019), Irawan and Sirait (2018)), the vegetation density class was divided into five classes: the non-vegetation class with an NDVI value < 0.1, a low dense class with an NDVI value of 0.1–0.2, medium dense with NDVI values of 0.2–0.3, dense class NDVI value of 0.3–0.4, and high dense class with NDVI values of > 0.4.
Non-Vegetation Class
Non-vegetation classes are roads, dense settlements, and tall buildings. There is little or no vegetation in this class. The non-vegetation class consists of residential areas, business areas represented by tall buildings near the main road, and congested streets in the flyover area. Some campus areas of the University of North Sumatra, which is located in Medan Baru, show non-vegetation density classes. The library, faculty building, and administration office center building belong to this class. Areas that belong to the non-vegetation class illustrated in Figure 5.
Low Dense Class
In the low dense class, the vegetation density is low. Areas that belong to this class are roads, buildings, settlements, and bare lands with less vegetation. Low dense class is found in some areas in the city surrounded by buildings and existing vegetation. There are also trees along the path of the road and around houses. The species found here are Plumeria rubra, Mangifera indica, Pterocarpus indicus, Gnetum gnemon, Polyalthia longifolia, and Areca catechu. In the campus area of the Universitas Sumatera Utara (USU), there are some buildings with trees around them. The species found here are Terminalia catappa, Casuarina equisetifolia, Mangifera indica, and Ficus benjamina. Areas that belong to the low dense class can be seen in Figure 6.
Medium Dense Class
In the medium dense class, the density of vegetation is rather high. There are roads, buildings, and settlements, where those covers have vegetation; for example, individual trees near those covers.
Medium dense class is found in settlements with a large yard with various trees. There are also gardens in the area of settlement. The research identified Leucaena leucocephala, Solanum melongena, Vigna unguiculata, Musa paradisiaca, Manihot esculenta, Mangifera indica, Theobroma cacao, Morinda citrifolia, and Artocarpus heterophyllus. It also includes a city park with trees such as Swietenia mahagoni, Pterocarpus indicus, Leucaena leucocephala, Adenanthera pavonina, Polyalthia longifolia, Mangifera indica, Mimusops elengi, Artocarpus altilis, Plumeria rubra, Nephelium lappaceum, Delonix regia), Gnetum gnemon, Cocos nucifera, and Annona muricata. In the campus area of the USU, library parks and some faculties have areas with trees. The species found here are Swietenia mahagoni, Polyalthia longifolia, Filicium decipiens, Pinus merkusi, Mimusops elengi, Alstonia scholaris, and Syzygium aqueum. Areas that belong to the medium dense class can be seen in Figure 7.
Dense Class
In the dense class, rice fields, shrubs, trees, and mixed gardens were found. People grows cassava, sugar cane, corn, banana and oranges. Trees are found around the areas. The dense class includes shrubs, grasses, and trees such as Gnetum gnemon, Durio zibethinus, Swietenia mahagoni, Ficus benjamina, Tectona grandis, Mangifera indica, and Tamarindus indica. In some areas, community-owned gardens belong to this class with species such as Musa paradisiaca, Zea mays, Saccharum ficinarum, Carica papaya, Vigna unguiculataArachis hypogaea, and Cocos nucifera. There is a mini-stadium, saga park, auditorium park, and research farmland in the USU campus area. The research found many tree species. The tree species are Annona muricata, Durio zibethinus, Artocarpus heterophyllus, Persea americana, Psidium guajava, Tamarindus indica, Mangifera indica, Ficus benjamina, Mimusops elengi, Leucaena leucocephala, Polyalthia longifolia, Terminalia catappa, Areca catechu, Adenanthera pavonina, Pinus merkusii, Swietenia mahagoni, Aleurites molucana, Pometia pinnata, Syzygium aqueum, Hylocereus undatus, Carica papaya, and Nephelium lappaceum. Areas that belong to the dense class can be seen in Figure 8.
High Dense Class
While in high dense, there are dense vegetation found including trees, oil palm, rice fields, very dense shrubs, and mixed gardens.High dense class includes land overgrowing with trees accompanied by shrubs, land planted with oil palm, vast rice fields, and mixed gardens owned by the community planted with various plants such as Citrus nobilis, Zea mays, Cymbopogon citratus, Solanum torvum, Artocarpus altilis, Saccharum officinarum, Cocos nucifera, Etlingera elatior, Theobroma cacao, Carica papaya, Manihot esculenta, Musa paradisiaca, and Psidium guajava. The high dense class was found in the campus forest which is consisting of some species. The species are Swietenia mahagoni, Artocarpus heterophyllus, Psidium guajava, Paraserianthes falcataria. In USU administration center park is also found Terminalia catappa, Terminalia mantaly, Ficus elastica, Swietenia mahagoni, Syzygium oleana, Ficus benjamina, Durio zibethinus, and Pterocarpus indicus. Areas that belong to the high dense class can be seen in Figure 9.
The increase of population growth and development of the Medan areas as a big city has increased built-up areas and settlements. It decreases vegetated land. Changes in rice fields and gardens into housing areas are examples of conversion. The ground check shows dense settlements and multi-storey buildings are found more than vegetated lands such as trees, rice fields, and community-owned gardens. The decrease in vegetation areas could have a bad impact on the quality of the environment, i.e., problems of flood due to a decrease in water absorption areas.
Many vegetated lands were converted into housing and buildings. Moreover, some vegetation areas were replaced by roads and public facilities. Vegetation cover changes could lead to environmental problems, such as the incident of flood in the rainy season, bad air quality due to pollution, and health problems. In urban contexts, vegetation surfaces are very important for the well-being and health of the urban population. The NDVI is often correlated with socioeconomic and/or sociodemographic data to demonstrate the inequality in environmental settings that themselves influence individual health and questions of environmental justice (Juergens and Meyer-Heß 2020).
Medan has faced some flood incidents, especially in the areas near the river. Some of the areas have no or less vegetation on the side of the river. This situation can worsen unless there is an improvement of areas by replanting trees and better planning the areas. This requires preparing both the community and land condition. Land-use planning process was used to provide alternative and possible land uses as well as management activities to ensure land-use sustainability (Samsuri et al. 2019). It is necessary to initiate efforts and actions to preserve the environment to prevent further damage. Four criteria were used for determining suitable plants that could be planted as well as cultivated by the community: plant species found in natural forests near the city, plant species meeting physical and soil characteristics, plant species cultivated by the community, and plant species based on community perceptions (Samsuri et al. 2019).
The fast population growth and the significant exploitation of natural resources imply great modifications in the environment, causing environmental impacts, including the average temperature increase of the Earth. Understanding the effects of changing land cover allows greater agility in public managers’ decision to mitigate the effects of urban heat and the formation of heat islands by vegetation insertion in densely occupied areas (Pessi et al. 2020).
Many problems in urban areas need a serious attention such as pollution (Arsovski et al 2018, Jorquera et al 2019, Zhou et al 2018a, Yang et al 2020, Twisa et al 2020, Dardouri and Sghaier 2019, Qu et al 2019), environmental pollution and health (Yang 2020), urban water security (Hoekstra et al 2018, Twisa et al 2020).
Urban vegetation has an important role in ensuring the ecological security of cities. Rapid urbanization in China has radically changed the urban vegetation cover. Isolation of the contributions of human activities from the observed vegetation can help understand the effects of human activities on urban ecosystems (Wan and Gao 2020).
The research shows big changes in vegetated areas into other land uses as the field check found the existence of built areas in the city. Settlements and buildings with no or less vegetation are prominent. NDVI analysis shows the change of dense vegetation class into the lower class, especially the change into non-vegetation and the low density class.
A big part of the urban land occupation is composed of a highly dense buildings and large paved and impermeable areas, which support the high increase in the air temperature in urban centers, which creates microclimates in different zones of the city, causing heat islands (Giacomelli et al. 2020).
In that case, the modeling of the urban sprawl effect on vegetation-cover is realized by the NDVI) After observing and characterizing the areas altering by the urban expansion, the results displayed that in 17 years, the urban growth of Annaba decreased the vegetation cover by 28.50 % (Saouli et al. 2021).
Urban studies shows wide scope of discussion with the same aim that is to overcome problems and built a good environment for man and nature. Such studies including study on urban resilience (Ribeiro et al 2019, Bush and Doyon (2019). Issues on building a smart sustainable cities are also among those studies (Garau et al 2018, Kociuba and Maj (2020), (Tang and Lee (2016). There are also studies on urbanization strategy and planning (Han et al 2018, Huayi et al 2020, Guzal-Dec et al 2020), insights for the future of urban ecosystem services research (Kremer et al 2016).
Paying attention to the construction of green space and green roads in cities, reducing urban land use, rationally distributing land resources, and strengthening environmental management and protection are important ways to improve the ecological environment level of vegetation in Zaozhuang City (Wen and Xhang 2020). The research confirmed the importance of vegetation structures in the process of mitigating urban climate extremes and improving environmental quality (Rozova et al. 2020). The development of fringe areas outside the urban core areas should be avoided as much as possible to mitigate vegetation degradation effectively, and the positive influences of urbanization on vegetation growth through effective urban landscape planning should be encouraged (Li et al. 2020). The research provides information on vegetation's condition by giving information on the location and the density of vegetation within the area. It is a valuable information for urban planning to achieve the highest quality of the environment and supporting the health of the community.