Determining the growing season of land vegetation on the basis of plant phenology and satellite data in Northern China

The objectives of this study are to explore the relationships between plant phenology and satellite-sensor-derived measures of greenness, and to advan ce a new procedure for determining the growing season of land vegetation at the regional scale. Three phenological stations were selected as sample si tes to represent different climatic zones and vegetation types in northern China. The mixed data set consists of occurrence dates of all observed phen ophases for 50-70 kinds of trees and shrubs from 1983 to 1988. Using these data, we calculated the cumulative frequency of phenophases in every 5-day period (pentad) throughout each year, and also drew the cumulative frequenc y distribution curve for all station-years, in order to reveal the typical seasonal characteristics of these plant communities. The growing season was set as the time interval between 5% and 95% of the phenological cumulative frequency. Average lengths of the growing season varied between 188 days i n the northern, to 259 days in the southern part of the research region. Th e beginning and end dates of the surface growing season were then applied e ach year as time thresholds, to determine the corresponding 10-day peak gre enness values from normalized difference vegetation index curves for 8-km(2 ) pixels overlying the phenological stations. Our results show that, at the beginning of the growing season, the largest average greenness value occur s in the southern part, then in the northern, and finally the middle part o f the research region. In contrast, at the end of the growing season, the l argest average greenness value is measured in the northern part, next in th e middle and lastly the southern part of the research region. In future stu dies, these derived NDVI thresholds can be applied to determine the growing season of similar plant communities at other sites, which lack surface phe nological data.