An analysis of relationships among plant community phenology and seasonal metrics of Normalized Difference Vegetation Index in the northern part of the monsoon region of China

This study focuses on relationships between the phenological growing season of plant communities and the seasonal metrics of Normalized Difference Veg etation Index (NDVI) at sample stations and pixels overlying them, and expl ores the procedure for determining the growing season of terrestrial vegeta tion at the regional scale. using threshold NDVI values obtained by surface -satellite analysis at individual stations/pixels. The cumulative frequency of phenophases has been calculated for each plant community and each year in order to determine the growing season at the three sample stations from 1982 to 1993. The precise thresholds were arbitrarily set as the dates on w hich the phenological cumulative frequency reached 5% and 10% (for the begi nning) and 90% and 95% (for the end). The beginning and end dates of the gr owing season were then applied each year as time thresholds. to determine t he corresponding 10-day peak greenness values from NDVI curves for 8-km(2) pixels overlying the phenological stations. According to a trend analysis, a lengthening of the growing seasons and an increase of the integrated grow ing season NDVI have been detected in the central part of the research regi on. The correlation between the beginning dates of the growing season and t he a threshold NDVI values is very low, which indicates that the satellite- sensor-derived greenness is independent of the beginning time of the growin g season of local plant communities. Other than in spring, the correlation between the end dates of the growing season and the corresponding threshold NDVI values is highly significant. The negative correlation shows that the earlier the growing season terminates. the larger the corresponding thresh old NDVI value, and vice versa. In order to estimate the beginning and end dates of the growing season using the threshold NDVI values at sites withou t phenological data from 1982 to 1993, we calculated the spatial correlatio n coefficients between NDVI time-series at each sample station and other co ntiguous sites year by year. The results provide the spatial extrapolation area of the growing season for each sample station. Thus, we can use the th reshold NDVI value obtained at one sample station/pixel for a year to deter mine the growing season at the extrapolation sites with a similar vegetatio n type for the same year.