West-east contrast of phenology and climate in northern Asia revealed using a remotely sensed vegetation index

The phenology of the vegetation covering north Asia (mainly Siberia) and its spatial characterstics were investigated using remotely sensed normalized difference vegetation index (NDVI) data. The analysis used the weekly averaged NDVI over 5 years (1987-1991) using the second-generation weekly global vegetation index dataset (0.144degreesx 0.144degrees spatial resolution). In the seasonal NDVI cycle, three phenological events were defined for each pixel: green-up week (NDVI exceeds 0.2), maximum week, and senescence week (NDVI drops below 0.2). Generally there was a west-early/east-late gradient in the three events in north Asia. In the zonal transect between 45degrees and 50degreesN, the timing of green-up, maximum, and senescence near 60degreesE (Kazakh) was about 3.4, 8.7, and 13.4 weeks earlier than near 110degreesE (Mongolia) respectively. It has been suggested that vegetation near Kazakh only flourishes during a short period when water from snow melt is available from late spring to early summer. In Mongolia, abundant water is available for the vegetation, even in midsummer, because of precipitation. In the 50-60degreesN zonal transect, the green-up and maximum near 40degreesE were about 3.8 and 3.9 weeks earlier than near 115degreesE, respectively. As for the week of senescence, there was no clear west-east trend. This west-to-east phenological gradient was related to the weekly cumulative temperature (over 0 degreesC). Weeks in which the cumalative temperature exceeded 40 degreesC and 140 degreesC had a similar west-east distribution to green-up and maximum NDVI.