Strong impacts of daily minimum temperature on the green-up date and summer greenness of the Tibetan Plateau

Understanding vegetation responses to climate change on the Tibetan Plateau (TP) helps in elucidating the land-atmosphere energy exchange, which affects air mass movement over and around the TP. Although the TP is one of the world's most sensitive regions in terms of climatic warming, little is known about how the vegetation responds. Here, we focus on how spring phenology and summertime greenness respond to the asymmetric warming, that is, stronger warming during nighttime than during daytime. Using both insitu and satellite observations, we found that vegetation green-up date showed a stronger negative partial correlation with daily minimum temperature (T-min) than with maximum temperature (T-max) before the growing season (preseason' henceforth). Summer vegetation greenness was strongly positively correlated with summer T-min, but negatively with T-max. A 1-K increase in preseason T-min advanced green-up date by 4days (P<0.05) and in summer enhanced greenness by 3.6% relative to the mean greenness during 2000-2004 (P<0.01). In contrast, increases in preseason T-max did not advance green-up date (P>0.10) and higher summer T-max even reduced greenness by 2.6%K-1 (P<0.05). The stimulating effects of increasing T-min were likely caused by reduced low temperature constraints, and the apparent negative effects of higher T-max on greenness were probably due to the accompanying decline in water availability. The dominant enhancing effect of nighttime warming indicates that climatic warming will probably have stronger impact on TP ecosystems than on apparently similar Arctic ecosystems where vegetation is controlled mainly by T-max. Our results are crucial for future improvements of dynamic vegetation models embedded in the Earth System Models which are being used to describe the behavior of the Asian monsoon. The results are significant because the state of the vegetation on the TP plays an important role in steering the monsoon.