Impact of Accumulated Temperature on Wetland Vegetation Area in燩oyang燣ake

Accumulated temperature, which is now widely used in agronomy, is an important ecological factor to the growth of plants, but few relative studies have been found on the vegetation area of floodplain grasslands in Poyang Lake. This research used the classification and regression tree (CART) to classify normalized vegetation area index derived from MODIS LAI (Moderate Resolution Imaging Spectroradiometer Leaf Area Index) images from 2008 to 2014, according to different climate indexes, such as mean daily air temperature (n), accumulated temperature (jw), daily maximum temperature (g), daily minimum temperature (d), accumulative precipitation (j), water level (s) and average water level for 20 days preceding (a). The results showed that: (1) The accumulated temperature and the 20-day average WL (water level) were found to have the greatest impact on variation in wetland vegetation area, and they both dominated the classification process twice; (2) Two classification thresholds of accumulated temperature were 790°C and 1784°C, approximately corresponding to the beginning of April and mid-May; (3) 790°C could also be used as a threshold to select remote sensing images to analysis the annual variability of vegetation, i.e. while accumulated temperature is lower than 790°C, remote sensing images of similar accumulated temperature rather than similar date should be selected from different years for comparison. We also found that, effects of different hydrological factors on area of floodplain grasslands showed stage characteristics: (1) From January to March, water level changes slowly with less rainfall, as a result, the 20-day average WL which can interpret the hydrologic characteristics smoothly showed significant importance in this stage; (2) While entering April, intense rainfall make accumulative precipitation to be the dominating factor of classification; (3) From late April to mid-May, in condition of accumulative precipitation higher than 405 mm, daily water level is of most importance, because to the flood recession process as well as rapid water level fluctuations.