Spatio-temporal variability of net ecosystem productivity over India and its relationship to climatic variables

The net ecosystem productivity (NEP) represents net carbon exchange between the terrestrial ecosystem and the atmosphere that plays a crucial role on the control of the atmospheric CO2 in different time scales. The present study examined the spatial and temporal variability of NEP over India during 1981-2006 in relation to the climatic variables using the Carnegie-Ames-Stanford Approach (CASA) terrestrial ecosystem model and regional databases on the land surface-vegetation characteristics and the climatic parameters. At national scale the NEP exhibits semi-annual cycle with primary positive values up to 80 TgC month(-1) during August-December, secondary positive values up to 15 TgC month(-1) during January-March and negative values up to -70 TgC month(-1) during April-July. The estimated long-term NEP budget for the country is 10 TgC year(-1). It had undergone substantial inter-annual change in response to the climate variability. In the early 1980s, the Indian terrestrial biosphere remained a source and later became a sink of carbon during four pentad periods. The NEP budgets are positive for all the extreme years with severe flood and drought conditions except 1982. The normal years have either positive or negative NEP budgets. The precipitation-induced reduction of the net primary production (NPP) dominates the NEP variability in dry years, whereas in good monsoon years the precipitation-induced enhancement of the soil respiration (Rh) dominates the NEP variability.