Exploring the relationship between vegetation greenness and precipitation on the Tibetan plateau during the growing season.

The Tibetan Plateau (TP), one of the most climate-sensitive regions in the world, has experienced significant warming and wetting in recent decades, which is widely recognized has promoted vegetation greening. However, a paradoxical phenomenon has been observed: during the growing seasons, vegetation greenness negatively correlates with precipitation across large areas of the TP. The underlying causes of this counterintuitive relationship remain unclear. In this study, we investigated this unexpected correlation relationship, using remotely sensed normalized difference vegetation index (NDVI) data, meteorological station observations, and several hydrometeorological datasets. First, we explored inter-annual variations in NDVI and precipitation on the TP during the growing seasons over the last four decades. Second, we examined the correlations between NDVI and precipitation in the growing seasons. Our analyses showed that from June to September, NDVI exhibited a significant (p < 0.05) positive correlation with precipitation in 3.04% - 10.9% of the vegetated area of the TP, whereas a significant negative correlation was observed in 3.02-6.03% of the vegetated area. Over half of the vegetated area showed negative correlations in July-September. Focusing on negatively correlated regions, we employed the structural equation model to explore the mechanisms causing this paradoxical relationship. Our findings suggest that the negative relationship between vegetation greenness and precipitation is primarily driven by a reduction in solar radiation associated with increased precipitation. During the growing season, higher precipitation levels led to lower solar radiation, which negatively impacted vegetation growth. These findings improve our understanding of vegetation-climate interactions in this climate-sensitive region.