Forests and vegetation provide biodiversity protection that preserves the soil cover and balances the hydrological cycle, ecosystem, air temperatures, humidity, and rainfall which helps to mitigate climate change impacts (Zheng et al. 2019). But, due to population growth and urban expansion, human beings are destructing the forest-cover falling under the greater dominion of land surface vegetation-cover (Amera and Tefera, 2013). Due to massive Land Use/Land Cover (LULC) transformation, carbon emissions are increasing and carbon sinks are decreasing and accelerating climate change. In this sense, urbanization is one of the key factors impacting forest cover, which has profound effects on both national and global climate (Hunt et al. 2017). Land cover around the world is transforming, influenced by both natural and artificial factors (Hassan, 2017). Vegetation, one of the most important land covers of the earth's ecosystem, offers an extensive variety of social, ecological, and environmental services and benefits habitats for sustainability (Wu, 2010; Zheng et al. 2019; Robinson & Lundholm, 2012). It helps to maintain sustainability between environment and ecology through carbon sequestration (Gratani & Bonito, 2016) improving air quality (Escobedo & Nowak, 2009), conserving soil and water, preserving biodiversity (Dana & Mota, 2002), regulating microclimates (Jonsson, 2004) and mitigating disasters (Jenerette et al. 2011). However, palaeoenvironmental and palaeoecological records show that human activities and climate change have been disrupting ecosystems as well as vegetation (Marignani et al. 2016; Mercuri et al. 2015; Li & Zhou, 2010). Over the past few decades, vegetation coverage has undergone a remarkable transformation which is greatly affecting environmental and habitat sustainability (Jin et al. 2018; Grimm et al. 2008). Understanding the relationship between climate change, dynamics of vegetation, and the services offered to humanity by ecosystems is one of the main research challenges in the 21st century.
Environmental concern has contracted as one of the major worldwide attention that distinctly as well as jointly affect all countries (Amera and Tefera, 2013). With the expansion of population and spontaneous extension of cities, land use patterns and biological systems have changed, prompting the arrangement of metropolitan situated natural difficulties around the globe (Arsanjani et al. 2013). Land use and land cover (LULC) type has been changing quickly because of the many main impetuses. Thus, carbon emissions, environmental change, the shift of biological systems, ecological corruption, and the random condition have been expanding which making the climate of any region inadmissible for human home (Ameen and Mourshed, 2017; Zheng et al. 2017). Therefore, Bangladesh is one of the world's exceptionally populated nations, where the metropolitan population has developed after some time because of relocation from rustic to mechanical or support areas looking for work openings and driving qualified everyday environments (Zaman et al. 2010). Rapid population growth may have beneficial economic development influences but have detrimental effects on LULC change and sustainable development (Kafy et al. 2021). The urban areas of Bangladesh have been confronting the issue of spontaneous metropolitan extension like other creating urban communities (Zaman et al. 2010). The LULC management strategy is now one of the most important obstacles to the mitigation of the adverse environmental and climatic effects by limiting unplanned urban growth and promoting green coverage.
Climate is considered as the most perilous environmental factor which not only affects the ecosystem and the environment but also LULC and humans (Hunt et al. 2017). Generally, researchers use remote-sensing techniques to illustrate land use/ land cover (LUCC) changes and vegetation variations at the city level or regional level. LULC classification, net primary productivity (NPP), and normalized difference vegetation index (NDVI) are utilized for illustrating vegetation activities (Wang et al. 2008; Kafy et al. 2020). The NDVI derived from satellite data is an important indicator for evaluating the state of living green vegetation and showing vegetation dynamics reacting to climate change (Kalisa et al. 2019; Philippon et al. 2007; Zhang et al. 2013). Using the supervised image classification method, Rai et al. (2017) showed that Bangladesh has undergone a total loss of 6.2% (9054 sq km) of vegetated areas during 1976–2014. Hasan et al. (2013) have shown that total vegetation was 12.11% in 1976, 9.02% in 2000, and 9.84% in 2010. Fu et al. (2013) have shown the NDVI changing trend exhibited spatial differences at a significant level over land surfaces around the world. Long-term climate change or interannual climatic variations can affect photosynthesis of vegetation, respiration, and organic carbon decomposition (Fu et al. 2013) and have a profound influence on vegetation cover change (VCC) (Schreider et al. 1996; Kalisa et al. 2019).
Precipitation and temperature were shown to be the main climate factors for the growth of vegetation in previous literature (Blok et al. 2011; Gessner et al. 2013). Zhou et al. (2001) showed that increase in precipitation influences to increase NDVI while temperature change causes the decrease of NDVI. Sun et al. (2011) demonstrated that rise of rainfall influences to improve vegetation cover, while in arid and semi-arid regions in China the temperature was not the limiting factor. However, several studies have demonstrated the consequences of vegetative land cover transformation on the environment in the previous literature (Kafy et al. 2021; Escobedo & Nowak, 2009; Dana & Mota, 2002; Jenerette et al. 2011; Marignani et al. 2016). There have been many studies on LULC change in Bangladesh (Hasan et al. 2017; Rai et al. 2017; Mukhopadhyay et al. 2018; Hasan et al. 2013) including in city-level LULC change (Ahmed, 2015; Ahmed, 2011; Ahmed et al. 2013; Kafy et al. 2020; Kafy et al. 2021). The change in temperature and rainfall pattern in Bangladesh is shown in the literature (Basak et al. 2013; Rahman & Lateh, 2017; Hossain et al. 2019; Hossain, 2014; Kamruzzaman et al. 2019). Hossain et al. (2019) and Biswas, (2013) analyzed the impacts of rainfall on crop production. So far yet, no previous studies have assessed the responses of VCC to climatic variables in the context of Bangladesh and this important topic has been focused on insufficient studies especially the researches that have been conducted in the south Asian region.
This study detailed the interannual growing-season NDVI (GNDVI), temperature (GT), precipitation (GP), and proportion of urban land (PUL) in Bangladesh over the past three decades. And then performed a sensitivity analysis, analyzed the spatiotemporal changing trend of GNDVI, GP, GT, and PUL, examined the correlation between GNDVI and GT, GP, and PUL change to assess the response of VCC to climatic variables. Finally, the impacts of urban expansion on GNDVI dynamics are quantified through the regression model. In general, the long-term VCC should be studied in light of their history, but used recent observation records (1990–2018) to perform studies in an important 'window' of human history. The changes in vegetation, the landscape, and the climate could also represent long-term environmental changes in Bangladesh.