Vegetation Phenology

In natural vegetation systems, monitoring changes in vegetation over time is of fundamental interest for identifying and quantifying impacts of management and natural processes. There is an increasing scientific consensus that natural vegetation system is experiencing high risk due to anthropogenic activities, which modified the environment to the extent that most common landscape depicts mosaic of human settlements, agricultural land and scattered fragments of natural ecosystem (Midha & Mathur. 2010). Decrease in forest vigour and deforestation has large impacts on carbon stocks, biodiversity, ecosystem services and livelihoods. Net carbon emissions from deforestation during the past decade are estimated at 3.3 billion tonnes of CO2 annually

The potential indicators of forest degradation include: forest area affected by human driven fire, fragmentation, decline in species population, forest canopy cover, dominance of invasive alien species and soil fertility. Subtle changes in forest vigour can be identified by analysing changes in its seasonal greenness using vegetation phenology. Vegetation phenology is a study of recurring pattern of crop growth and development. Changes of vegetation phenology are recognized as being possibly the most important early indicator of impact of climate change on ecosystem (White et al., 1997). Variation of vegetation phenology has broad impacts on terrestrial ecosystems and human societies by altering global carbon, water and nitrogen cycles, crop production, duration of pollination season and distribution of diseases etc (Penuelas and Fiella, 2001; Soudani et al., 2008). Therefore, phenological study has recently become an important focus for ecological and climatic research (Menzel et al., 2001; Cleland et al., 2007). Monitoring the variations of vegetation phenological events can be performed by three possible ways i.e. In situ observations, bioclimatic models and remote sensing techniques (Schaber and Badech, 2003; Fisher et al., 2007). Field based

Vegetation phenology is usually addressed through temporal monitoring of the satellite derived Normalized Difference Vegetation Index (NDVI) time series, available since the launch of the earth observation satellites. The NDVI is calculated as the normalized difference between near-infrared and red bands, and quantifies the photosynthetic capacity of plant canopies, thus indicating the amount of vegetation present in one place. Coarse resolution sensor cannot provide directly the phenological events, rather different phenology analogues such as start of the growing season (SGS), end of the growing season (EGS), seasonal NDVI amplitude (AMP), seasonally integrated NDVI etc could be derived. Time series NDVI had been used to detect positive trends of LGS over North America and Eurasia (Zhou et al., 2001), changes of vegetation activities over Inner Mongolian steppe (Lee et al., 2002), Europe (Stockli and Vidal, 2004), and China (Piao et al., 2006. Fang et al., 2003, Qiu & Cao 2011), Central Asia (de Beurs and Henebry, 2004), Sahel & Soudan (Heumann et al.