Declined trend in herbaceous plant green-up dates on the Qinghai-Tibetan Plateau caused by spring warming slowdown

There has been much debate on the temporal change trend and existence of a turning point in spring green-up date (GUD) of plants on the Qinghai-Tibetan Plateau (QTP). Most previous studies on the QTP used remote sensing data, which have large uncertainties. In this study, using a large amount of long-term ground observation data at 27 phenological stations across the QTP (1694 GUD records), we showed that on the whole, QTP herbaceous plant GUD insignificantly advanced during 1982-2017. Although the direction of the GUD trend did not change from 1982 to 2017, the magnitude of the advancing trend greatly weakened after 1999. According to our estimated results from 28 paired GUD time series, the overall GUD trend shifted from -2.70 clays/decade during 1982-1999 to -0.56 days/decade during 2000-2017. This finding contrasts with the conclusions of previous satellite-based studies, which either reported a continuous significant advancement of GUD or a turning point in the mid-to-late 1990s. Through partial correlation analysis and partial least squares regression, we found that winter and spring air temperatures were the primary climatic factors that influenced the temporal change in GUD, and both had negative effects on GUD. The decreased GUD trend was mainly attributable to the warming slowdown in spring. On average, the spring warming rate decreased by 52.43% after 1999, whereas the winter warming rate displayed no obvious change. This study also found that the GUD of (orbs showed stronger sensitivity to air temperature change than that of sedges and grasses. This indicates that (orbs are more competitive in adaptation to climate warming, which might shift plant community structure and affect ecosystem service function. Moreover, the declined advancement in GUD implies that the spring phenologically driven increase in carbon uptake may have also slowed in the past two decades. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Based on long-term ground observation data, this study found that the spring green-up date of herbaceous plants on the Qinghai-Tibetan Plateau did not significantly advance from 1982 to 2017, with the advancing trend weakening after 1999. Winter and spring air temperatures were identified as the primary climatic factors affecting the change in green-up date trend, with a significant decrease in the spring warming rate after 1999. Additionally, the study highlights the differential sensitivity of different plant types to temperature change, suggesting potential shifts in plant community structure and ecosystem functions.