## 5 Conclusions  
NPP is an important ecosystem variable which provides a metric for the rate of carbon fixed in the terrestrial ecosystems. It is also an indicator of the health of ecosystems. We used AVHRR-derived NPP [8] to analyze the linear trends and variability over India for the period 1982-2006. For this period, the mean and standard deviation of NPP are estimated at 0.83 PgC\(\cdot\)y\({}^{-1}\) and 0.03 PgC\(\cdot\)y\({}^{-1}\), respectively. Our regression analysis shows a linear trend of 3.9% per decade (r = 0.72,R\({}^{2}\) = 0.61) which indicates that the rate of carbon-fixing by terrestrial ecosystems has increased in the past two decades. This result is in close agreement with other recent studies which also found an increasing trend [17,18]. Studies based on long-term observations plots also conclude that observations \"over a period of 25 years indicates that the forests are in the process of recuperation, sequestering atmospheric carbon and providing environmental service\" [45].  
Spatial maps of the trends show large increasing trends in the Western Ghats, Himalayan forests in the north, northeast India, peninsular and central-east India and near-zero or decreasing trends over northern and northwest India. A multivariate linear regression of NPP with temperature, precipitation and global-mean atmospheric CO\({}_{2}\) concentration (r = 0.80, R\({}^{2}\) = 0.64) indicates that the increasing trend is likely driven by CO\({}_{2}\) fertilization of the terrestrial ecosystems. There are many modeling and observational studies in the literature that suggest enhanced carbon uptake due to CO\({}_{2}\) fertilization. However, we also find a major influence of irrigation (r = 0.77, R\({}^{2}\) = 0.59) and fertilizer use (r = 0.73, R\({}^{2}\) = 0.53) on the non-forest areas of the country. India initiated a large-scale forest conservation, afforestation and reforestation program starting in the early 1980s. We find that this forest management could have also contributed to the NPP increase over the forested areas in the country (r = 0.75, R\({}^{2}\) = 0.56).  
A multivariate regression of NPP anomaly with anomalies in temperature, precipitation, soil water, solar radiation and global mean CO\({}_{2}\) suggests that the interannual variability in NPP is primarily driven by precipitation and temperature variability (Table 1 and Figure 4). In general, we find that the interannual variations in NPP are positively and strongly correlated with precipitation, and negatively but weakly with temperature and global-mean CO\({}_{2}\). Mean seasonal NPP is highest during post-monsoon and lowest during the pre-monsoon period. Though climatic conditions (in terms of rainfall, temperature and solar radiation) are most conducive for plant growth during the monsoon period, NPP does not peak during that period. We find that the seasonal maximum of NPP is associated with the seasonal maximum of soil moisture that occurs during the post-monsoon period. This indicates the dominant role of soil moisture for vegetation productivity. Our multivariate regression analysis shows a strong correlation of 0.95 (R\({}^{2}\) = 0.91) between observed seasonal cycle of NPP and multivariate NPP that is generated using soil water content and solar radiation as independent variables.