Effects of Climate Change and Urbanization on Vegetation Phenology

in the Bucharest Metropolitan Area

DAN M. SAVASTRU, MARIA A. ZORAN*, ROXANA S. SAVASTRU,

MARINA N. TAUTAN, DANIEL V. TENCIU

National Institute of R&D for Optoelectronics,

Bucharest-Magurele,

ROMANIA

*Corresponding Author

Abstract: - Being an essential issue in global warming, the response of urban vegetation to climate change and

urbanization has become an increasing concern at both the local and global levels. This study aims to

investigate the effect of the urban environment on vegetation phenology for the Bucharest metropolitan area in

Romania and to identify the potential climate drivers that influence key phenology in the urban environment. In

this study, we comprehensively analyzed the response of urban vegetation phenology shifts due to climate

variability and urbanization in the Bucharest metropolitan area from a spatiotemporal perspective during the

2002- 2022 period. Through synergy use of time series of the main climate variables, Air temperature -AT, land

surface temperature (LST), and biophysical variables derived from MODIS Terra/Aqua satellite and in-situ

data, this study developed a complex statistical and spatial regression analysis. Green space was measured with

satellite-derived vegetation indicators Normalized Vegetation Index (NDVI), and Enhanced Vegetation Index

(EVI), Net Primary Production (NPP) data, which captures the combined availability of urban parks, street

trees, forest, and periurban agricultural areas. Leaf Area Index (LAI) and Photosynthetically active radiation

(FPAR) indicators have been used to characterize the effects of meteorological parameters and urbanization

impacts on vegetation phenology and their changes. The results show that the response of vegetation phenology

to urbanization level and climate parameters variability has a distinct spatiotemporal difference across the

urban/periurban gradient. The findings of this study show that the land surface temperature anomalies

associated with urbanization-induced climate warming, especially during strong summer heat waves and under

urban heat islands alter urban vegetation biophysical properties, directly impacting its phenology shifts. At the

metropolitan scale, the urban thermal environment directly impacts vegetation phenology patterns. The

quantitative findings of this study are of great importance for understanding the complex impacts of

urbanization and climate changes on vegetation phenology and for developing models to predict vegetation

phenological changes under future urbanization.

Key-Words: - climate changes, vegetation phenology, biophysical parameters, MODIS Terra/Aqua satellite

data, Bucharest, Romania.

Received: April 26, 2023. Revised: July 15, 2023. Accepted: September 10, 2023. Published: September 27, 2023.

1 Introduction

Is well known that urban vegetation absorbs carbon

dioxide - CO2 as organic compounds through the

mechanism of photosynthesis, regulating the global

carbon cycle and energy exchange, [1]. Vegetation

photosynthetic phenology is an ecologically

sensitive indicator of seasonal and interannual

changes in environmental conditions, related to the

rhythm variation of its photosynthetic activity, and

triggered by periodically changed environment, the

temporal shift of photosynthetic phenology being

responsible for the carbon balance of terrestrial

ecosystems changes, [2]. In the frame of global

warming and the increasing trends of extreme

climate events, urban vegetation is affected by the

increased levels of air temperature, atmospheric

carbon dioxide (CO2), and air pollutant

concentrations, [3], [4]. Vegetation phenology acts

as a control tool for urban cooling/warming effects

and provides helpful information for future urban

green space planning aimed at mitigating local

climate warming, [5]. Urban ecosystems have

remarkable human-induced characteristics, and

urbanization has altered the sensible and latent heat

fluxes over vegetation, causing apparent

phenological shifts. Due to the highest concentration

WSEAS TRANSACTIONS on ENVIRONMENT and DEVELOPMENT

DOI: 10.37394/232015.2023.19.90

Dan M. Savastru, Maria A. Zoran,

Roxana S. Savastru, Marina N. Tautan,

Daniel V. Tenciu

E-ISSN: 2224-3496

961

Volume 19, 2023

of human activity, causing the urban heat island

(UHI) effect, [6], [7], [8], [9], cities can be

considered ideal natural laboratories for predicting

the response of vegetation phenology to regional

and global warming. The response of vegetation

phenology to climate changes and urbanization

levels is an important issue of studies on complex

interactions between urbanization and vegetation.

Also, land surface vegetation and its spatiotemporal

changes determine the radiation balance of the

surface and affect the surface temperature and

boundary-layer structure of the atmosphere. Due to

anthropogenic and natural factors, urban vegetation

land cover changes result in the land surface albedo

changes. Recent studies highlighted that

urbanization has both direct impacts on vegetation

due to changes associated with the replacement of

vegetated areas with build-up surfaces, and indirect

impacts on vegetation related to the shifts of

vegetation characteristics resulting from changes in

climatic and environmental factors, [10]. The

increase in urban impervious land cover surfaces

can be used as a proxy for urbanization rate and

assessment of its impacts on vegetation phenology

as well as on induced impacts on long-term surface

urban heat island intensity, [11]. On the other side,

urbanization affects different climatic, especially the

urban thermal environment, [12] and environmental

factors that impact vegetation functions, [13]. The

impact of air pollutants on urban vegetation has

several aspects: with the increasing anthropogenic

CO2 emissions, the near-surface CO2 concentrations

in urban areas are enhanced, which may increase the

vegetation productivity and growth through the

stimulation of photosynthetic rates, while high

concentrations of particulate matter -PM deposition

on vegetation may have adverse effects on

photosynthesis, [14]. Synergy's use of time series-

derived satellite biophysical parameters and in-situ

monitoring data can provide helpful information for

urban vegetation phenology spatiotemporal

dynamics. The rapid advance of satellite-to-earth

observation technology provides a fast, systematic,

cost-effective, and excellent configuration for

processing large and complex spatial data. Remotely

sensed phenological observations have become

important for revealing the response and feedback

of vegetation dynamics to global climate change,

[15], [16], [17], [18], [19].

2 Materials and Methods

2.1 Study Site

The urban metropolitan region of Bucharest (Figure

1) capital of Romania is located in the South –

Eastern part of the country, and South-Eastern part

of Europe, being bounded by latitudes 44.33 ON and

44.66 ON and 25.90 OE and 26.20 OE longitudes. Its

center is situated at 44.4355381 oN Latitude and

26.100049 oE Longitude. It has about 1.8 million

inhabitants. According to the European

Commission urban vegetation land cover represents

5.6% of the total territory of the urban core, [20]. In

Bucharest, the green space is very low as compared

with other European metropolises like Paris,

London, and Brussels, where urban/periurban

vegetation land cover is placed in the range (7.37 -

20.84) m2/capita. From 1993 year till 2020 the

metropolitan vegetation land cover in Bucharest has

decreased, from 4839 ha in 1993 to 4506 ha, [21].

This large area covers multiple urban-to-rural

transition areas and consists of different vegetation

types (shrubs, grass, forest, crops, etc.).

Additionally, the metropolitan area has a diverse

landscape pattern in terms of the spatial distribution

of various land cover types, with flat plain areas.

Satellite remote sensing time series data can be used

to gather data on the urban vegetation density, the

size of the land area, and field conditions.

Fig. 1: Bucharest test site, capital of Romania

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DOI: 10.37394/232015.2023.19.90

Dan M. Savastru, Maria A. Zoran,

Roxana S. Savastru, Marina N. Tautan,

Daniel V. Tenciu

E-ISSN: 2224-3496

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The study test area includes Bucharest city and

the surrounding periurban areas with very complex

environments (built, green, and blue structures),

under a rapid urbanization process, and one of the

most air-polluted cities in Europe. Its climate is

temperate continental, with Western European

Climate influences, Mediterranean Cyclones, and

the East-European Anticyclone.

2.2 Data Sets

Daily time series of average daily meteorological

data, including air temperature at 2m height (T), air

relative humidity (RH), air pressure (p), wind speed

intensity (w), and direction, for the Bucharest

metropolitan region were collected from the

Modern-Era retrospective analysis for Research and

Applications, Version 2 (MERRA-2) at, [22], and,

Climate Change Service of Copernicus (C3S) data,

[23]. This study focused on estimating Bucharest

metropolis vegetation land cover phenology

dynamics using time series MODIS Terra data for

the 2002-2022 period. The analyzed period has

registered several heat wave periods, of which the

summers of 2003, 2007, 2010, 2012, 2017, and

2022 have been the highest. We used time series

MODIS Terra products: 8-Day L3 Global 1km SIN

Grid land surface temperature (LST)/emissivity

MOD11A2/LST_Day_1km, 16-day MODIS

13Q1/250m_16_days_NDVI/EVI composites with a

250 m spatial resolution, and MODIS Tera Leaf

Area Index (LAI) MOD15A2H MODIS/Terra Leaf

Area Index/FPAR 8-Day with 500m spatial

resolution, mainly for their capacity to detect

anthropogenic and climate impacts on urban

vegetation land cover changes. Also, we used

MODIS Terra/Aqua phenology data for both cycles

1 and 2 data. Missing values were replaced by linear

interpolation considering neighboring values within

the LST, NDVI/EVI, LAI, and FPAR time series.

Landsat ETM+ 17/07/2022 image was used for

validation and training. Have been selected 6

periurban and 6 urban test areas corresponding to

the six sectors of Bucharest city, a central Bucharest

test area, and the entire metropolis test area. In situ-

monitoring data with GER-260 spectroradiometer

additional data, as well as meteorological

observational data have been used. Statistical

analysis through Spearman rank correlation

coefficients was used. ENVI 5.7, e-cognition, and

ORIGIN 11 software have been used.

2.3 Statistical Analysis

For similarity between two-time series data of the

averaged daily air pollutants, climate observables

(air temperature and relative humidity, wind speed,

surface solar irradiance Planetary Boundary Layer

heights), in Bucharest, this study used Spearman

cross-correlation analysis and non-parametric test

coefficients as well as linear regression analysis. For

assessment of the normality of the averaged daily

time-series data sets, Kolmogorov-Smirnov Tests of

Normality were used. Because the daily climate

variables have a non-normal distribution, Spearman

rank correlation was selected to identify the linear

correlation between the important variables: (1) air

pollutants PM2.5, PM10 concentrations, climate

variables, and ORIGIN 11.0 software version 2023

for Microsoft Windows was used for data

processing.

3 Results and Discussion

Bucharest city, the capital of Romania had an

intense and rapid periurban development after 1990,

and the boundaries of the functional urban region

have shifted outwards from the urban core. The

rapid urbanization of the Bucharest metropolitan

area may be responsible for the recorded higher air

and land surface temperatures associated with UHIs

and HWS during summer periods due to vegetation

land cover reduction and increase of impervious

surfaces. According to Copernicus Urban Atlas land

use land cover (LULC) in 2018 distribution (km2)

for Bucharest metropolitan region shows: artificial

area was 33.6%, agricultural area 53%, natural areas

10.7%, wetland 0.2%, and water 2.4%. This study

used spatiotemporal analysis of time series MODIS

Terra/Aqua NDVI, EVI, LAI, FPAR, LST, NPP,

and vegetation phenology indicators for analysis of

urban vegetation phenology in the Bucharest

metropolitan area during 2002-2022 years. The

main objective of this study was to establish

whether a significant correlation exists between

these indices and other factors. The synergy of Heat

Waves and Urban Heat Islands (UHI) effects

increases the air and land surface temperature in

densely populated metropolitan areas, making urban

areas more predisposed to heat stress compared with

rural areas.

3.1 Impact of Air Temperature and Land

Surface Temperature on Urban Vegetation

During the summer season (June-August), the rank

correlation analyses at the metropolitan pixel scale

(40.5km x 40.5 km) revealed that TA and LST

present a strong positive correlation (r= 0.86%,

p<0.01). Land Surface Temperature (LST) is a

significant radiative skin parameter of the ground,

that provides essential information on surface-

WSEAS TRANSACTIONS on ENVIRONMENT and DEVELOPMENT

DOI: 10.37394/232015.2023.19.90

Dan M. Savastru, Maria A. Zoran,

Roxana S. Savastru, Marina N. Tautan,

Daniel V. Tenciu

E-ISSN: 2224-3496

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Volume 19, 2023

atmosphere interactions and energy fluxes between

the atmosphere and the ground in the

urban/periurban areas. Gross Primary Production

(GPP) and Net Primary Production (NPP) represent

vegetation productivity. Figure 2 shows the annual

rates of Net Primary Production at 500m spatial

resolution from MODIS Terra data for the 2002-

2022 period and evidences the clear decreasing

trends of urban vegetation phenology during

recorded summer HWs of years 2000, 2003, 2007,

2012, 2016 and 2022. During the recorded summer

heat wave events in Bucharest metropolitan region,

the average extracted net radiation was in the range

of 896- 999 Wm-2. At the microscale, surface albedo

and temperature should have a large variety in the

selected six urban sectors because of the large

material and structural versatilities. The storage heat

flux exceeds the sensible heat flux in urban areas,

whereas the sensible heat flux is higher than the

storage heat flux in industrial areas. In particular,

negative storage heat flux appears at a number of

industrial points, [24]. This tendency shows that

high surface temperature in the periurban industrial

areas of Bucharest is induced by mass energy

consumption because most of the anthropogenic

heat discharge is transferred to the atmosphere as

sensible heat. Figure 3 presents the temporal

distribution of NDVI and LST over the Bucharest

metropolitan area during the 2002-2022 period.

Fig. 2: Temporal pattern of Net Primary Production

of Bucharest vegetation during the 2002-2022

period

Based on the spatial and seasonal (spring and

summer) distributions of LST–NDVI relations over

Bucharest metropolitan area (40.5km x 40.5 km),

using long-term (2002–2022) MODIS Terra images,

this study found that relations between LST and

NDVI/EVI were highly diverse among the various

urban/periurban biomes and seasons throughout the

entire study period. So, during the spring season

March–May), LST-NDVI presents the dominance of

significant positive correlation (Spearman rank

correlation coefficient r=0.71; p<0.01 for metropolis

area), while during the summer season (June–

August), most of the vegetation test areas turned to

negative correlation (for metropolis areal r= -0.68,

p<0.01). For the autumn and winter seasons, LST

shows positive correlations with NDVI/EVI (r= 0.62

and p<0.01 for the metropolis area). This study

demonstrates that the drought/vegetation/stress

spectral indices, based on the prevalent hypothesis

of an inverse summer LST–NDVI correlation are

spatially and temporally dependent.

4/7/2002 4/7/2006 4/7/2010 4/7/2014 4/7/2018 4/7/2022

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

NDVI

LST_Day

Day

NDVI

BUCHAREST

250

260

270

280

290

300

310

320

LST_Day (oK)

Fig. 3: Temporal patterns of daily average MODIS

LST (oK) and NDVI during 2002-2022 for the

Bucharest metropolitan area.

Figure 4 presents the temporal distribution of

FPAR and LAI over the Bucharest metropolitan

area during the 2002-2022 period. Statistical

analysis shows positive significant correlations

between land surface temperature both day and

night with FPAR (Spearman rank correlation

BUCHAREST

WSEAS TRANSACTIONS on ENVIRONMENT and DEVELOPMENT

DOI: 10.37394/232015.2023.19.90

Dan M. Savastru, Maria A. Zoran,

Roxana S. Savastru, Marina N. Tautan,

Daniel V. Tenciu

E-ISSN: 2224-3496

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coefficient LSTDay-FPAR r=0.87; p<0.01, and

LSTNight-FPAR r=0.85; p<0.01, for metropolis

areal). Also, LST exhibits lower positive

correlations with LAI for both day and night

monitoring data (Spearman rank correlation

coefficient LSTDay-LAI r=0.20; p<0.01, and

LSTNight-LAI r=0.25; p<0.01, for metropolis areal).

Spearman rank correlation coefficients between

LST and evapotranspiration- ET present moderate

positive values (LSTDay-ET r=0.32; p<0.01, and

LSTNight-ET r=0.39; p<0.01, for metropolis areal).

7/4/2002 7/4/2006 7/4/2010 7/4/2014 7/4/2018 7/4/2022

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6 LAI 40.5x40.5 km

FPAR 40.5x40.5 km

Data

LAI_500m (MOD15A2H)

BUCHAREST

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

FPAR_500m (MOD15A2H)

Fig. 4: Temporal patterns of daily average MODIS

LAI and FPAR during 2002-2022 for the Bucharest

metropolitan area.

3.2 Urban Vegetation Phenology Evolution

Time series satellite remote sensing data provide a

useful tool for spatiotemporal observations of the

land surface, making it essential for urban

vegetation phenology study across large geographic

areas. From MODIS Terra available

MOD17A3HGF products, during analyzed 2002-

2022 period urban vegetation phenology showed

different temporal patterns per cycles 1 (spring) and

2 (autumn), the changes been triggered by climate

changes being influenced by alterations in

meteorological conditions at micro- and mesoscale

as a result of urbanization and the intensification of

anthropogenic activities in Bucharest metropolitan

region (as can be seen in Figure 5 and Figure 6).

Like other studies, [25], [26], [27], the figures in

this article found different yearly changes (advanced

or delayed) in the average start of the vegetation in

spring and autumn seasons date of selected years

during the period 2002-2022. Although differences

were also observed for the end and length of the

growing season, these patterns showed great

interannual variability of urban vegetation

phenology corresponding to different stages

(Greenup, Maturity, MidGreendown, MidGreenup,

and Peak). The analysis of more specific vegetation

classes enables a better understanding of the

phenological response of vegetation to urbanization

intensity. Also, the sensitivity of vegetation

productivity to air temperature, precipitation rate,

soil moisture, and solar surface irradiance are the

key metrics for understanding the variations in

vegetation productivity under changing climate and

predicting future changes in ecosystem functions,

[28], [29].

2000 2005 2010 2015 2020

102

126

147

168

189

207

230

253

276

105

120

135

2000 2005 2010 2015 2020

138

161

184

207

Year

Greenup (G)

Maturity (M)

MGd

MidGreendown (MGd)

MGu

MidGreenup (MGu)

Cycle 1 Phenology

Day

Peak (P)

BUCHAREST

Fig. 5:Temporal pattern of the vegetation phenology

during cycle 1 in Bucharest metropolitan region.

Is considered that solar-induced chlorophyll

fluorescence is a great proxy for vegetation

productivity in urban and all terrestrial ecosystems.

Among several factors that directly and indirectly

affect vegetation productivity the most important

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DOI: 10.37394/232015.2023.19.90

Dan M. Savastru, Maria A. Zoran,

Roxana S. Savastru, Marina N. Tautan,

Daniel V. Tenciu

E-ISSN: 2224-3496

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are: meteorological and hydrological conditions;

increasing carbon dioxide (CO2) concentrations

drive enhanced vegetation productivity) and also

lead to rising temperatures; soil nutrient availability

determines potential vegetation productivity, [30],

[31], [32].

2000 2005 2010 2015 2020

192

224

256

288

238

272

306

340

110

220

330

198

231

264

297

2000 2005 2010 2015 2020

216

252

288

324

Year

Greenup (G)

Cycle 2 Phenology

Day

Maturity (M)

MGd

MidGreendown (MGd)

MGu

MidGreenup (MGu)

Peak (P)

BUCHAREST

Fig. 6: Temporal pattern of the vegetation

phenology during cycle 2 in Bucharest metropolitan

region.

4 Conclusion

Climate warming, air pollution, and heatwaves

(HWs)-related drought events could become a major

driver of large-scale urban vegetation dieback. The

main contributions of this study consist in: (1) the

investigation of the effect of the urban environment

on vegetation phenology for the Bucharest

metropolitan region in Romania, and (2) identifying

the main potential drivers that influence key

phenology in the urban environment. Also, urban

temperate deciduous and broadleaf forests placed in

Baneasa, Cernica-Branesti, and Snagov areas have

been affected by the rise of summer HWs

temperatures through significant disturbances. In the

next decades is expected as climate change to

trigger significant changes in urban vegetation

phenology, air and land surface temperature, and

soil moisture, altering the urban ecosystems. The

novelty of this research was the use of the time

series MODIS Terra/Aqua remotely sensed

observations to assess the importance of phenology

and environmental factors on vegetation

productivity indicating that phenology was the

dominant driver during the investigated period,

despite the differences in the importance of land

surface temperature. The results of this study

support the hypothesis that urbanization produces

significant differences in plant phenology in

Bucharest metropolis, and points to contrasting

responses by different functional vegetation types.

Although differences were also observed for both

the end date and length of the growing season

between vegetation types, these patterns showed

great inter-annual variability. The analysis of

different time series remotely sensed data and

meteorological/hydrological data provide the trends

of spatial patterns of increased land surface

temperature-LST, decreased soil moisture, and

lengthened phenology.

Acknowledgments:

This work was supported by the Romanian Ministry

of Research, Innovation and Digitalization: through

Program 1- Contract no. 18PFE/30.12.2021;

National Research Development and Innovation

Plan 2022-2027, project no. PN 23 05; Grant

CNCS-UEFISCDI, project number PN-III-P4-PCE-

2021-0585; Romanian Ministry of European

Investment and Projects & Romanian Ministry of

Research, Innovation and Digitalization, Contract

no.8/1.2.1 PTI ap.2/17.02.2023

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DOI: 10.37394/232015.2023.19.90

Dan M. Savastru, Maria A. Zoran,

Roxana S. Savastru, Marina N. Tautan,

Daniel V. Tenciu

E-ISSN: 2224-3496

966

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date: 1 June 2023).

WSEAS TRANSACTIONS on ENVIRONMENT and DEVELOPMENT

DOI: 10.37394/232015.2023.19.90

Dan M. Savastru, Maria A. Zoran,

Roxana S. Savastru, Marina N. Tautan,

Daniel V. Tenciu

E-ISSN: 2224-3496

967

Volume 19, 2023

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Contribution of Individual Authors to the

Creation of a Scientific Article (Ghostwriting

Policy)

- Maria Zoran: Conceptualization; Methodology,

Supervision, Writing - review & editing.

- Roxana Savastru: Methodology, Validation,

Review.

- Dan Savastru: Methodology, Validation, Review.

Marina Tautan: Methodology, Validation.

- Daniel Tenciu: Software.

Sources of Funding for Research Presented in a

Scientific Article or Scientific Article Itself

This work was supported by the Romanian Ministry

of Research, Innovation and Digitalization: through

Program 1- Contract no. 18PFE/30.12.2021;

National Research Development and Innovation

Plan 2022-2027, project no. PN 23 05; Grant

CNCS-UEFISCDI, project number PN-III-P4-PCE-

2021-0585; Romanian Ministry of European

Investment and Projects & Romanian Ministry of

Research, Innovation and Digitalization, Contract

no.8/1.2.1 PTI ap.2/17.02.2023

Conflict of Interest

The authors have no conflict of interest to declare.

Creative Commons Attribution License 4.0

(Attribution 4.0 International , CC BY 4.0)

This article is published under the terms of the

Creative Commons Attribution License 4.0

https://creativecommons.org/licenses/by/4.0/deed.en

_US

WSEAS TRANSACTIONS on ENVIRONMENT and DEVELOPMENT

DOI: 10.37394/232015.2023.19.90

Dan M. Savastru, Maria A. Zoran,

Roxana S. Savastru, Marina N. Tautan,

Daniel V. Tenciu

E-ISSN: 2224-3496

968

Volume 19, 2023