INTRODUCTION

Green spaces in urban environment aid in maintaining the biodiversity through the sustenance of ecological processes. Green spaces predominantly with vegetation plays a vital role in an urban ecosystem by moderating micro climate, apart from sequestration of greenhouse gases (CO2, etc.) and also aid in the percolation of water. Urban vegetation includes trees, shrubs and herbs in public and private lands (parks, streets, backyards) in the landscape dominated by paved surface. Enormous environmental, ecological, economic and social benefits from urban vegetation documented that include  removal of air pollutants such as Sulphur dioxide (SO2), Nitrogen oxides (NOx), Carbon dioxide (CO2), particulate matter by leaf stomata and by the leaf surfaces especially if their surfaces are waxy, spiny or hairy (Li et al., 2014;  Vailshery et al., 2013; Ghose et al., 2005; Khan and Abbasi 2000; Chaturvedi et al., 2013; Ghauri et al., 2007; Yang et al., 2005; Harris and Manning 2010; Dwivedi et al., 2009; Inkilainen et al., 2013; Secon, 2010), dust removal by leaves of Mango (Mangifera indica), Ashoka (Polyalthia longifolia), Pongamia (Derris indica) and Umbrella - Thespepsia populnea (Shetye and Chaphekar ,1980, Ghose et al., 2005). Trees act as a barrier which helps to reduce noise pollution by absorbing and blocking urban noise, reducing stress for people in the region (Zannin et al., 2006). Furthermore, urban trees function as natural ‘air conditioners’ which influences the use of local energy use (electricity and cooling) in the residential areas and offices (Akbari et al., 2001; Sudha and Ravindranath 2000).

Vegetation helps in altering the amounts of heat energy absorbed, reduction of the solar heat gains on windows, walls, roofs through shading, increase the latent cooling by addition of moisture to the air through evapotranspiration and reducing the long-wave exchange with the sky as building surface temperatures are lowered through shading (Dimoudi and Nikolopoulou 2003; Taha et al., 1988; Grimmond and Oke 1991). Urban vegetation helps in the hydrologic cycle as about 10-18% of the precipitation absorbed by leaves gets back to the atmosphere through evaporation. Significant part of precipitation gets percolated through plant roots, which help in recharging ground water aquifers. Due to microbial interactions between soil and plant roots, the soil is porous and permeable, which allows infiltration of water, aiding in recharging groundwater aquifers and sub-surface regions. Infiltration to an extent of 40-45% (of precipitation) in the landscape covered with the native vegetation also helps in mitigating flooding episodes. The extent of vegetation cools the urban climate, which however depends upon the type and composition of the vegetation. The leaf temperatures depend upon the anatomical, physical and physiological factors (Monteith and Unsworth, 1990). Studies have substantiated temperature moderation by vegetation (Myrup et al., 1993; Shashua-Bar and Hoffman, 2004; Doick and Hutchings 2013). Reports reveal unshaded suburban area is approximately 2.5○C warmer than compared to the open rural area and unshaded suburban site warmer by 1○C compared to the shaded area.

Ability to reduce the air temperature depends on the tree size and canopy characteristics.  World Health Organization recommended minimum green space of 9.5 m2/person) considering the services (oxygen, moderation of micro climate) and goods in the urban environment (Kuchelmeister, 1998). Trees play major role in accommodating arboreal species of insects, birds, etc.  Trees also have aesthetics value which adds beauty to the surrounding by adding color, softening harsh lines of building and contributes to the character of their environment.
Industrialization, coupled with unplanned urbanisation, associated deforestation and other anthropogenic activities have increased the emission of greenhouse gases such as Carbon dioxide (CO2), Methane (CH4), Nitrogen oxide (NOx), Sulphur dioxide (SO2). About 75% of CO2 is emitted due to the burning of fossil fuels during the past 20 years (Schneider, 1989) and the atmospheric carbon dioxide concentration increased (Trenberth and Kevin 2007) from 280 ppm (preindustrial) to 382 ppm, (2006), and to 390 ppm (2011). This has led to an increase in the atmospheric temperature by trapping the certain wavelength of radiation in the atmosphere. Trees, wetlands and soil in the urban area serves as sink to capture and store the atmospheric carbon dioxide (Negi and Gupta 2013; Nowak and Crane 2002). Urban vegetation plays a vital role as it moderates micro climate apart from sequestration of greenhouse gases (CO2, etc.) and also aid in the retention of water. Estimates indicate that about 6 tons of carbon is sequestered by 1 hectare of forests annually and this accounts to about 6 kg/tree/year (Secon, 2010). The per capita human respiratory carbon ranges from 192 to 328 kg/year, which means at least 8 trees per person are required to sequester respiratory CO2 in a region.

Rapid pace of urbanization is the dominant anthropogenic phenomenon worldwide (Ramachandra et al., 2012). Unplanned urban development has telling impacts on the environment in the developing countries, evident from higher pollutants in air, contamination of water and land (Gairola 2013). Population in India has increased from 63 (1950) to 127 million (2011). India’s urban population stands next to China and the demand of land has increased substantially for various human needs. The vegetation cover, wetlands and other natural ecosystem have been retreating with the expansion of cities (Gairola 2013; Zhou et al., 2011). Urbanization being a global phenomenon involving unprecedented expansion of land cover which in turn led to rapid increase in urban extent and growth of unplanned regions (Ramachandra and Bharath, 2012). Unplanned urbanization and lack of optimal management of natural resources by city officials has led to the gradual decline in the urban service such as water quality, air quality, inadequate infrastructure, poor quality of life, etc.  (Escobedo et al., 2011; Ramachandra et al., 2011). Trees have potential to moderate air temperature through shading, reduction of surface temperature and through evaporative cooling (McPherson et al., 1994). Urban heat island refers to the trend in which urban pockets are warmer than their surroundings, which is caused by anthropogenic heat discharge due to energy consumption, increased land surface coverage by artificial materials having high heat capacities and conductivities coupled with decrease in vegetation and water bodies (Ramachandra and Uttam 2010; Song 2013). Table 1 lists the efforts towards quantification of ecological services.

Trees have potential to moderate air temperature through reduction of surface temperature and evaporative cooling (McPherson et al., 1994). In order to understand the current tree cover traditional practices of tree counting has inherent problems such as requirement of extensive labour and biasedness. Remote sensing and geographic information system (GIS) makes timely unbiased spatial information accessible. Spatial information refers to extraction of various land form features (Bhatta, 2009; Ramachandra et al., 2013). Multi resolution remote sensing data aid in capturing this dynamics. Fusion of data from multiple sensors aids in delineating objects with comprehensive information due to the integration of spatial information present in the high resolution (HR) panchromatic (PAN) image and spectral information present in the low resolution (LR) Multispectral (MS) images.  Image fusion techniques integrate both PAN and MSS and can be performed at pixel, feature and decision levels.
This work optimises geospatial techniques using multi-resolution data (spatial and temporal) with open source GIS (Geographic Information system) to understand the green cover in Greater Bangalore. Remote sensing image fusion techniques are useful to integrate a lower spatial resolution MSS (Multi spectral data) spatial data with a higher spatial resolution PAN (panchromatic) data. Fusion of multi resolution data aided in taking advantage of spectral and spatial resolutions for identification of features in urban areas because the characteristic of urban objects are determined not only by their spectra but also by their structure, shape and size. The main aim of the study is to estimate the current green space through quantification of vegetation cover in Greater Bangalore. Assessment of vegetation involved (i) mapping of trees in each ward and (ii) computation of metrics such as population density, trees per person, etc. 
Table 1: Benefits of trees