Abstract
Recent patterns of land cover and vegetation dynamics on the Euasian continent have been linked to changes in the global carbon cycle. Our study was conducted to evaluate patterns in a 19-yr record of global satellite observations of terrestrial vegetation from the Advanced Very High Resolution Radiometer (AVHRR) as a means to characterize major trends in both vegetation egreenness and ecosystem disturbance. The fraction absorbed of photosynthetically active radiation (FPAR) by vegetation canopies worldwide has been computed from the AVHRR at a monthly time interval from 1982 to 2000 and gridded at a spatial resolution of 8 km globally. Unlike previous studies of the AVHRR multiyear time-series of vegetation dynamics, the 8-km spatial resolution makes it possible to compare disturbance events and greenness trends at the same level of spatial detail. Positive trends in FPAR were detected throughout a major greenbelt of central-eastern Europe starting in the mid-1980s. This Eurasian greenbelt extended in a wide swath over the Urals, into the vicinity of Lake Baykal south of the central Siberian plateau, mainly along a latitude belt from 55°.N to 65°N. There was also significantly positive greening in relatively large areas of Great Britain, Italy, Greece, Turkey, the Caucasus and southern India. Nonetheless, a strong downward trend in the FPAR time-series over most of Eurasia was observed by the end of the 1990s. Throughout the 19-yr time period, Eurasia was also impacted by many notable droughts and other disturbance events that could have substantially offset decadal carbon gains attributed to satellite-observed greening. Large-scale ecosystems disturbance events were identified in the FPAR time-series by locating anomalously low values (FPAR-LO) that lasted longer than 12 consecutive months at any 8-km pixel. We find verifiable evidence of numerous disturbance types across Eurasia, including regional patterns of severe droughts, forest fires and insect outbreaks.
Highlights
Europe’s terrestrial sink for atmospheric CO2 has been estimated as a net uptake flux of between 0.14 and 0.2 Pg C per year (Janssens et al, 2003). Potter et al (2003a) estimated the potential terrestrial sink for atmospheric CO2 over the larger Eurasian continental area at between 0.3 and 0.6 Pg C per year since 1988, except during relatively cool temperature periods such as 1991–1992 and 1995–1996 when the Eurasian continental carbon sink was predicted to vary between 0.1 and 0.25 Pg C per year
Like the Normalized Difference Vegetation Index (NDVI), higher fraction absorbed of photosynthetically active radiation (FPAR) levels observed over the course of a seasonal plant growing cycle indicates denser green leaf cover and less disturbance of the vegetation cover, and/or a longer the time period has elapsed since the last major disturbance
It is plausible that any significant and sustained decline in vegetation FPAR observed from satellites represents a disturbance event, a hypothesis we evaluated here using independent records of such disturbance events throughout Eurasia
Summary
Europe’s terrestrial sink for atmospheric CO2 has been estimated as a net uptake flux of between 0.14 and 0.2 Pg C per year (Janssens et al, 2003). Potter et al (2003a) estimated the potential terrestrial sink for atmospheric CO2 over the larger Eurasian continental area at between 0.3 and 0.6 Pg C per year since 1988, except during relatively cool temperature periods such as 1991–1992 and 1995–1996 when the Eurasian continental carbon sink was predicted to vary between 0.1 and 0.25 Pg C per year. Potter et al (2003a) estimated the potential terrestrial sink for atmospheric CO2 over the larger Eurasian continental area at between 0.3 and 0.6 Pg C per year since 1988, except during relatively cool temperature periods such as 1991–1992 and 1995–1996 when the Eurasian continental carbon sink was predicted to vary between 0.1 and 0.25 Pg C per year. Potter et al (2003b) characterized a large scale ecological disturbance as an event that results in a sustained disruption of ecosystem structure and function, generally with effects that last for time periods longer than a single seasonal growth cycle for native vegetation. The present study was conducted to evaluate patterns in a 19-year (1982-2000) record of satellite AVHRR observations of vegetation greenness over Eurasia as a means to characterize major trends in land cover and ecosystem disturbance events for the first time at a common spatial resolution of 8-km (image pixel size of about 64 km). Like the Normalized Difference Vegetation Index (NDVI), higher FPAR levels observed over the course of a seasonal plant growing cycle indicates denser green leaf cover and (presumably, on average) less disturbance of the vegetation cover, and/or a longer the time period has elapsed since the last major disturbance
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