Start of vegetation growing season on the Tibetan Plateau inferred from multiple methods based on GIMMS and SPOT NDVI data

In this study, we have used four methods to investigate the start of the growing season （SGS） on the Tibetan Plateau （TP） from 1982 to 2012, using Normalized Difference Vegetation Index （NDVI） data obtained from Global Inventory Modeling and Mapping Studies （GIMSS, 1982-2006） and SPOT VEGETATION （SPOT-VGT, 1999-2012）. SGS values esti- mated using the four methods show similar spatial patterns along latitudinal or altitudinal gradients, but with significant variations in the SGS dates. The largest discrepancies are mainly found in the regions with the highest or the lowest vegetation coverage. Between 1982 and 1998, the SGS values derived from the four methods all display an advancing trend, however, according to the more recent SPOT VGT data （1999-2012）, there is no continu- ously advancing trend of SGS on the TP. Analysis of the correlation between the SGS values derived from GIMMS and SPOT between 1999 and 2006 demonstrates consistency in the tendency with regard both to the data sources and to the four analysis methods used. Com- pared with other methods, the greatest consistency between the in situ data and the SGS values retrieved is obtained with Method 3 （Threshold of NDVI ratio）. To avoid error, in a vast region with diverse vegetation types and physical environments, it is critical to know the seasonal change characteristics of the different vegetation types, particularly in areas with sparse grassland or evergreen forest.

Assessment of Heavy Metals Control from Soil and Vegetable Plants in Different Growing Systems

Heavy metals contents of soils and vegetable edible parts were studied at three crop systems （conventional, in conversion and organic） from seven traditional sites for vegetable growing in North-eastern Romania, during 2010. The main goal of the research was to find a correlation between the heavy metals contents and the crop systems. Determination of heavy metals was performed by atomic absorption spectrometry （AAS）, using a Shimadzu spectrophotometer, 6300 version, equipped with graphite furnace and autosampler. Heavy metals contents in the soils varied with the metals and locations, according to the crop system. The following experimental average contents were determined： Mn-262.03 mg kg1, Zn-45.60 mg kgq, Cu-28.53 mg kgl, Pb-5.20 mg kg1 and Cd-1.84 mg kg1. In vegetable edible parts, heavy metal varied with metals, sites and plant species, without a significant correlation with crop system. The trend of heavy metals in conventional and organic crops was Zn 〉 Cu 〉 Mn = Pb 〉 Cd, but in conversion crops it was Zn 〉 Mn = Pb 〉 Cu 〉 Cd. The highest content was of Zn （4.85 mg kg＇t）, at a cabbage crop in conversion. Cd was not detected in vegetable products and Pb was detected only in some vegetable samples. Research emphasized that heavy metals contents from soil and vegetable edible parts did not exceed the admitted maximum limits, according to Romanian and European Regulations. Final conclusion of research is that no correlations exist between crop systems and heavy metals contents from soil and plant.

Variations of Vegetation Phenology Extracted from Remote Sensing Data over the Tibetan Plateau Hinterland during 2000–2014

How vegetation phenology responds to climate change is a key to the understanding of the mechanisms driving historic and future changes in regional terrestrial ecosystem productivity. Based on the 250-m and 8-day moderate resolution imaging spectroradiometer(MODIS) normalized difference vegetation index(NDVI) data for 2000-2014 in the Three-River Source Region(TRSR) of Qinghai Province, China, i.e., the hinterland of the Tibetan Plateau, we extracted relevant vegetation phenological information(e.g., start, end, and length of growing season) and analyzed the changes in the TRSR vegetation in response to climate change. The results reveal that, under the increasingly warm and humid climate, the start of vegetation growing season(SOS) advanced 1.03 day yr-1 while the end of vegetation growing season(EOS) exhibited no significant changes, which led to extended growing season length. It is found that the SOS was greatly affected by the preceding winter precipitation, with progressively enhanced precipitation facilitating an earlier SOS. Moreover, as the variations of SOS and its trend depended strongly on topography, we estimated the elevation break-points for SOS. The lower the elevations were, the earlier the SOS started. In the areas below 3095-m elevation, the SOS delay changed rapidly with increasing elevation;whereas above that, the SOS changes were relatively minor. The SOS trend had three elevation break-points at 2660, 3880, and 5240 m.